Astronomy & Geophysics: February 2011

& News and Reviews in Astronomy & Geophysics February 2011 • Vol. 52 • Issue 1

Explosive transients RAS Medals and Awards Antimatter from lightning

Astrobiology special issue

The search for life

& NEWS AND REVIEWS IN ASTRONOMY & GEOPHYSICS

Contents News and Views 4 Editorial: Managing expectations • Research

Astronomy & Geophysics publishes news reviews and comment on topics of interest to astronomers and geophysicists. Topical material is preferred. Publication will be as fast as is compatible with referees’ and authors’ responses. Contact the Editor or see http://www.ras.org.uk for further information.

Editor: Sue Bowler School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK Tel: +44 (0)113 343 6672 Fax: +44 (0)113 343 3900 Email: s.bowler@leeds.ac.uk Management Board Chair: Ian Crawford Birkbeck College, Univ. of London David Elliott RAS Robert Massey RAS Paul Murdin RAS Editorial Advisors Andrew Ball ESTEC Tom Boles Coddenham Allan Chapman Oxford University Roger Davies Oxford University Mike Edmunds University of Wales, Cardiff Jane Greaves University of St Andrews Mike Hapgood Rutherford Appleton Lab. Richard Holme University of Liverpool Ian Howarth University College London David Hughes Sheffield Katherine Joy University College London Margaret Penston IoA, Cambridge Claire Parnell University of St Andrews Roberto Trotta Imperial College London Althea Wilkinson University of Manchester The Council of the RAS

Burlington House, Piccadilly, London W1J 0BQ Tel: (0)20 7734 4582 or 3307 Fax: (0)20 7494 0166 Email: info@ras.org.uk Web: http://www.ras.org.uk Opening Hours (Monday to Friday) Offices: 9.30–17.00 Library: 10.00–17.00 Staff Contacts Executive Secretary David Elliott de@ras.org.uk RAS Communications Officer Robert Massey rm@ras.org.uk Produced for the RAS by Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK Tel: +44 (0)1865 776868 Email: aag@wiley.com Blackwell Publishing is part of Wiley-Blackwell http://www.blackwellpublishing.com/AAG This journal is available online at Wiley Interscience.

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Features 10 Explosive transients: a time-variable sky

Disclaimer The contents and views expressed in A&G are the responsibility of the Editor. They do not represent the views or policies of the RAS or Blackwell Publishing Ltd, except where specifically identified as such. While great care is taken to provide accurate and helpful information and advice in the journal, the RAS, its Council and the Editor accept no responsibility for errors or omissions in this or other issues.

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D Bersier and M Bode report from a meeting that discussed GRBs, supernovae and novae.

J Horner introduces a special issue on astrobiology.

16 Which exo-Earths should we search for life?

J Horner and B W Jones work out how to select the most likely planets to search for signs of life.

21 SETI: peering into the future

A Penny describes an exciting future for the search for extraterrestrial intelligence.

Cassini reveals Rhea’s icy surface, p8

25 Biological constraints on habitability

L Dartnell discusses what extremophiles on Earth tell us about the possibility of extraterrestrial life.

29 The Astrobiology Society of Britain

T P Kee, M J Burchell and D A Waltham outline the network at the heart of this new discipline.

30 Does astrobiology include human space flight?

K Apagyi and M J Burchell argue that aspects of astronautics overlap with astrobiology.

Understanding stellar explosions, p10

34 In situ biomarkers and the Life Marker Chip

Z Martins examines some of the challenges involved in identifying and detecting biomarkers.

36 Volcano–ice interaction: a haven for life on Mars?

Volcanism and ice interactions may offer chances for microbial life on Mars, argues C Cousins.

39 Is there life on … Titan?

L H Norman and A D Fortes consider the possibilities for life on Saturn’s complex icy moon.

Obituaries 43 Allan R Sandage; Audouin Charles Dollfus.

Society News 45 RAS Awards and Prizes; RAS medal goes into space • Planetarium show sets sail • New Fellows.

A&G • February 2011 • Vol. 52

VISTA peers into the dusty Lagoon Nebula, p6

15 Astrobiology: young science, old questions

ISSN 1366-8781 ©2011 RAS and individual contributors. All rights reserved. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by the RAS for libraries and other users registered with the Copyright Clearance Center Transactional Reporting Service, provided that the base fee of $15 per copy is paid directly to CCC (http://www.copyright.com). Special requests should be addressed to the Editor.

council resource allocations • e-MERLIN up and running • First data from SDSS-III • First science from Planck • VISTA images Lagoon Nebula • Hot Jupiter gives something back • Fermi finds antimatter from thunderstorms • A little bit of Vesta goes a long way.

Mission update: Icy Rhea and Saturn’s aurora.

Cover: The young science of astrobiology, featured in this issue, seeks life off Earth using analogues of distant planets, space missions, observations of exoplanets and understanding of life at the extremes, among other methods.

The articles begin on page 1.15. (See articles for details of individual images.)

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News

Editorial

Managing expectations Sue Bowler, Editor The gloomy times in which we live and work are occasionally illuminated by a few shafts of light; one came in the form of a government Comprehensive Spending Review settlement for science that was no worse than expected, and possibly a bit better. Before the nitty-gritty of the settlement has been worked through by individual research councils, there’s a sense of relief that some of the worst outcomes foretold for UK science will not come to pass. And it is of course difficult to be sure how likely some of the very bad possibilities really were – warning of a coming catastrophe in order to make a mere disaster more bearable is part of the art of “managing expectations”.

However, it is unlikely that the shadow of worse times will distract attention from the fairly bad times we live through, with pressure on research time and money increasing. The focus of that pressure is changing, too.

“Managing demand” is coming into vogue among research councils at the moment, with the laudable goal of limiting the submission of poor quality applications for the ever smaller pot of research funding. Less time spent by research councils on assessing applications with a low chance of success means less money spent, also. But it also means more time at institutional level, assessing submissions locally. This may be good for research councils, who will be able to say that they are still funding the same proportion of applications, but I’m not sure it’s necessarily good for research.

Managing demand is a clumsy tool – just look at the railways, who do it in effect by making popular journeys expensive and uncomfortable. Managing science well, with a transparent approach and genuine two-way links with the community, would be much better for all of us.

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Research council resource allocations: managing demand The research councils discovered in December the allocation of money from the UK government’s Comprehensive Spending Review, and have set out their delivery plans outlining how they will spend it. Details and decisions will follow consultation in the coming months.

The Science and Technology Facilities Council receives resources declining from £77.3m in 2010–11 to £68.9m in 2014–15 which, although a serious cut, is in line with the existing delivery plan. The Natural Environment Research Council will reduce by 3% (excluding inflation) by the end of the spending period 2011–15.

The STFC delivery plan includes continuing support for the Large Hadron Collider at CERN, ESO telescopes, research and development for the proposed European Extremely Large Telescope and Square Kilometre Array, and grant support for other projects. The plan put in place the recommendations of the Drayson Review concerning international subscriptions and facilities operations for other research councils, so that funding will be much less affected by currency fluctuations changing international subscriptions, for example.

But there are no changes to the plans that leave UK astronomers without access to major northern hemisphere telescopes after 2012, when we withdraw from Gemini. There are also changes to fellowship and studentship support that should benefit the best young researchers.

Prof. Roger Davies, President of the RAS, said: “The newly announced scheme for early-career researchers is good news and allows researchers to continue to play a serious international role. Nonetheless, British astronomy still faces a decline in funding in the next few years, with much more limited opportunities for the postgraduate and postdoctoral researchers that are the lifeblood of research in the UK.” The NERC intends to “protect front-line science as much as possible by reinvesting efficiency savings into priority science areas”. However, the NERC capital budget will be reduced by 50% from 2012–13. Additional funds have been allocated through the BIS Large Facilities Capital Fund in order to complete three major capital projects that are already underway, including the Halley Antarctic base and replacement of

the research ship Discovery. NERC anticipates problems supporting new capital projects in coming years with additional funds.

A common theme for responsive mode funding across the research councils is demand management – RCUK is involved in developing a set of common processes to limit the numbers of research proposals submitted, with the goal of maintaining success rates. Measures already in place for NERC research include rejecting around 40% of proposals before external peer review through triage, for example, and publishing the success rates of research organizations to encourage self-management of demand and quality. Further measures may include explicit strategic agreements to manage levels of demand and boost pre-submission quality assurance, for example.

STFC wishes to continue to place a premium on critical mass in research groups and, while not imposing explicit targets for greater concentration of funding, does expect this policy of focusing resources to continue over the next four years. http://www.stfc.ac.uk http://www.nerc.ac.uk

e-MERLIN radio telescope network is up and running The first image from eMerlin, the UK’s national radio astronomy facility, shows the power of the enhanced network of radio telescopes spread over 220 km and now linked by fibre optics. These links and advanced receivers will allow astronomers to see in a single day what would have previously taken them more than a year of observations.

Prof. John Womersley of the STFC said: “e-Merlin is a flagship project for the UK in radio astronomy, a scientific field where the UK has a rich legacy, a strong future, and is proud to be the home of some of the very best researchers in the world.”

The new e-Merlin radio image of the jet has been superimposed on a Hubble Space Telescope optical image showing the quasar core, 9 billion light-years away. Light from the object – one of the Double Quasar pair, each with their radio jets – has been lensed by a foreground galaxy, visible on the left of the image. The quasar is a galaxy powered by a super-massive black hole, leading to the ejection of jets of matter moving at almost the speed of light. http://www.jb.man.ac.uk/news/2010/ emerlin1

The new e-MERLIN radio image of the Double Quasar combined with an earlier Hubble Space Telescope (HST) optical image. The radio emission generated by the black hole is visible as the compact bright region superimposed on the (yellow-green) optical emission seen by HST. The radio jet, moving at speeds approaching that of light, is seen in the e-MERLIN image arcing away from the black hole towards the upper left. The jet shows several regions of enhanced brightness before it ends in a hotspot where it is ploughing through the tenuous matter filling the space around the quasar.

The e-MERLIN data are shown in false-colour with blue, through red, to white, showing increasing brightness of the radio emission. The HST image is made from WFPC2 images through two filters: green for the F555W filter (V-band) and red for the F814W filter (I-band). (Jodrell Bank Centre for Astrophysics, University of Manchester) A&G • February 2011 • Vol. 52

News

News in Brief Brazil to join ESO On 29 December 2010, Brazilian Minister of Science and Technology Sergio Machado Rezende and ESO Director General Tim de Zeeuw signed the formal accession agreement aiming to make Brazil a Member State of the European Southern Observatory, the 15th and the first from outside Europe.

The agreement must now be submitted to the Brazilian Parliament for ratification, but the accession plan received unanimous approval by the ESO Council during an extraordinary meeting on 21 December 2010. http://www.eso.org.

Statistical seismology

First data released for SDSS-III …

The third major project of the Sloan Digital Sky Survey – SDSS-III – released its first tranche of data at the meeting of the American Astronomical Association in Seattle in January, in the form of the most detailed digital image of the sky, with associated astrometry and photometry. These new data build on all the SDSS work so far, and will contribute to the forthcoming SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), expected to be complete in 2014. This data release also sees the completion of the Sloan Extension for Galactic Understanding and Exploration (SEGUE), a project to map stellar motion in the outer regions of the Milky Way. This image of the northern galactic cap shows the coverage and detail of the SDSS-III release, with the large-scale structure of space visible in the form of clots and walls of matter. (M Blanton, SDSS-III)

… and first science results from Planck published The first release of data from Planck, Europe’s cosmic microwave background observatory, shows details not only of relict radiation from the Big Bang, but also a range of astronomical finds, including massive galaxy clusters, the origin of the microwave fog, and unexplained new objects.

Planck is a European Space Agency mission with significant UK involvement. The science results released in January 2011 come in the main from the Early Release Compact Source Catalogue, Planck’s millimetre and submillimetre survey of the coldest objects in the sky. Planck will eventually examine the CMB, but much of its usefulness lies in characterizing the universe through which we examine it. Planck’s whole-sky survey has detected emission from our galaxy and other galaxies, revealing processes in cold dust clouds, starforming regions and galaxy clusters, for example. There is evidence for an otherwise invisible population of A&G • February 2011 • Vol. 52

This composite image of the Rho Ophiuchus molecular cloud highlights the correlation between the anomalous microwave fog, probably arising from miniature spinning dust grains (observed at 30 GHz, shown here in red), and the thermal dust emission (observed at 857 GHz, shown in green). The complex structure of knots and filaments, visible in this cloud of gas and dust, represents striking evidence for the ongoing processes of star formation. The image covers about 5° on each side, ten times the apparent diameter of the full Moon.

(ESA/Planck Collaboration)

galaxies shrouded in dust billions of years in the past, which formed stars at rates some 10–1000 times higher than in our own galaxy today.

Eventually, Planck will show us the best views yet of the formation of the first large-scale structures in the universe, precursors to the first galaxies. These structures are traced by the CMB but, in order to see it properly, contaminating emission from a whole host of foreground sources must first be removed. These include the individual objects contained in the Early Release Compact Source Catalogue, as well as diffuse emission.

One source of such diffuse emission is the “anomalous microwave fog”, a diffuse glow most strongly associated with dense, dusty regions of our galaxy. Planck data confirm that it comes from dust grains set spinning at tens of billion times a second by collisions with either fast-moving atoms or ultraviolet photons. The fog can then be removed from data, without affecting the CMB signal.

A new online resource aims to support statistical seismology by providing knowledge and networking for the research community. CORSSA is the Community Online Resource for Statistical Seismicity Analysis.

It is an open online resource in which authors with known expertise will address complex problems, with authority enhanced by peer review and the attentions of an editorial board.

It is not a place for new scientific results, but rather an online forum that invites discussion and spreads ideas. It is intended to be educational, for active researchers and students alike, but also to cover areas such as data standards. As a forum, it stands and falls by the active involvement of the community, so all interested parties are encouraged to visit the website or contact the organizer, Mark Naylor, at the School of Geosciences at the University of Edinburgh. http://www.corssa.org

ESO’s hidden gems A competition run by the European Southern Observatory has shaken out some hidden treasures, thanks to enthusiastic amateur image processors.

ESO’s Hidden Treasures 2010 astrophotography competition attracted nearly 100 entries. Igor Chekalin from Russia won first prize: a trip to ESO’s Very Large Telescope at Paranal, Chile.

Entrants had to combine greyscale images in ESO’s archives to produce attractive images. The results and many of the entries can be seen at ESO’s website. http://www.eso.org

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News

Clear blue waters for VISTA in the Lagoon Nebula

The new VISTA camera – the world’s largest infrared survey camera – at the European Southern Observatory is continuing its survey of the central regions of the Milky Way with a closer look at the Lagoon Nebula. In visible light little can be seen of the structure of this dusty cloud, but VISTA, the Visible and Infrared Survey Telescope for Astronomy, shows that its reputation as a stellar nursery is well founded. There are many denser Bok globules within the dust and gas clouds, and young stars still surrounded by dusty discs. This image was taken as part of a larger survey of the central regions of the galaxy, called VISTA Variables in the Via Lactea (VVV). VISTA’s new infrared image (top) of the Lagoon Nebula (Messier 8) shows what is going on behind the swirls of dust and gas that make this such an attractive visible light object. The visible light image (below) was taken with the Wide Field Imager on the MPG/ESO 2.2 m telescope at La Silla in Chile. The infrared image reveals many cool red stars that are otherwise hidden in the dust, as well as the hot young stars that light up the clouds in visible light. (ESO/VVV/Cambridge Astronomical Survey Unit) http://www.eso.org/public/images/eso1101b

Hot Jupiter gives something back Hot Jupiters, gas planets orbiting close to their host stars, raise questions about the evolution of planetary systems. Their very existence suggests extraordinary dynamics in the evolution of planetary systems, with giant planets spiralling in towards their stars.

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Now researchers who have dated a hot Jupiter planetary system have concluded that the giant planet is losing orbital angular momentum to the central star. The HD 189733 system also includes a red dwarf companion, about 220 AU away, which allowed the team to date the planetary system

at more than 5 billion years old.

The star HD 189733 in the constellation Vulpecula has a hot Jupiter orbiting at around 0.03 AU with a period of 2.2 days. The central star rotates about twice as fast as our Sun and appears to be gaining angular momentum from magnetic and tidal interactions with the giant planet.

This may arise as the planet spirals

into its close orbit; they may become tidally locked, like the Earth and Moon. Edward Guinan, a professor of astronomy at Villanova University, Pennsylvania, and leader of a team including undergraduate students, presented the research at the American Astronomical Society meeting in Seattle in January 2011. http://www.villanova.edu A&G • February 2011 • Vol. 52

News

Fermi finds antimatter from thunderstorms The orbiting Fermi Gamma-ray Space Telescope has detected not only gamma-ray bursts and energetic electrons coincident with thunderstorms on Earth, but also positrons: terrestrial thunderstorms produce antimatter.

Terrestrial Gamma-ray Flashes (TFGs) were discovered in the 1990s with the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory. They showed a strong association with thunderstorms and lightning, a link confirmed by later observations. It is estimated that about 500 such flashes occur daily worldwide, but most go undetected. TGFs are thought to arise via bremsstralung radiation from electrons accelerated to high energies in strong electric fields. In addition, a TGF would be expected to produce secondary electrons by interaction with the atmosphere.

These electrons would follow magnetic field lines and could be detected directly, albeit only in a narrow beam aligned with the field lines.

Fermi’s Gamma-ray Burst Monitor uses scintillation detectors to pick up high-energy particles coming from space; they also detect TFGs coming from Earth, despite their orientation,

Lightning can generate gamma rays and antimatter. (NOAA)

because the radiation is so penetrating. So far, Fermi has detected 130 TGFs. Fermi’s improved timing accuracy compared to earlier instruments has strengthened the link with thunder­storms; use of the World Wide Lightning Location Network to map storms showed a pattern of lightning occurring along magnetic field lines linked to Fermi, rather than close to the position of the satellite at the time. One TGF occurred at the same time as an active storm

in Zambia, while Fermi was orbiting 4500 km to the north, over Egypt.

The geographical separation is important, because it means that the gamma rays Fermi detected were not part of the primary flash; they came from the particles travelling along the Earth’s magnetic field lines.

It is Fermi’s data on these particles that establishes the antimatter production associated with thunderstorms. As well as electrons, there is evidence of a substantial positron component; the annihilation of an electron–positron pair at Fermi produces gamma rays of a characteristic energy (511 keV). The researchers suggest that relativistic phenomena in conjunction with terrestrial lightning result in electron–positron pair production. The fate of the positrons is not yet known. Like electrons, they follow the lines of the Earth’s magnetic field, but whether they escape into space or add to the radiation belts remains to be seen.

Michael Briggs of the University of Alabama at Huntsville presented the results at the American Astronomical Society meeting in Seattle in January. Briggs et al. publish this work in Geophysical Research Letters. http://www.agu.org

Quasars help judge Earth’s internal magnetic field An estimate of the strength of the Earth’s internal magnetic field can be made thanks to the radio signals from distant quasars. This in turn provides information about the power of the geodynamo.

Earth’s magnetic field can be measured at and above its surface, but its value within the Earth, for example in the core, is rather more elusive.

Now Bruce A Buffet of the University of California at Berkeley has calculated the internal field strength

using variations in the direction of the Earth’s rotation – nutation – and the effect of differing angular velocities in the liquid and solid core.

Quasars play a mundane, albeit significant, role in this calculation; they form a frame of reference in which to measure the nutation.

Very Long Baseline Interferometry relates geodetic measurements of tidal oscillations to a reference frame based on radio signals from distant quasars, allowing the identification

of subtle patterns. Modelling of the geodynamo and internal field can reproduce the distortions within the Earth’s liquid core that produce the nutation. Buffet’s work suggests that an internal field strength of 2.5 mT on average, in the core, can match the observed nutation anomalies, without needing to suggest high fluid viscosity or an unusually high field at the inner core boundary. This research was published in Nature in December 2010.

A little bit of minor planet Vesta goes a long way A near-Earth asteroid identified as coming from the minor planet Vesta holds a key to the structure of this body, to be visited in August by the NASA space mission Dawn.

Vesta is a research target because it is unusual, if not unique, among minor planets in having a differentiated inner structure, with an iron and nickel core and a rocky mantle below its crust. In other words it may be a relict planetesimal, left over from the era of planet formation when such bodies collided and accumulated to form the planets. Asteroid 1999TA10 is identified as coming from Vesta by comparing its composition, measA&G • February 2011 • Vol. 52

ured with NASA’s Infrared Telescope Facility on Mauna Kea, Hawaii, with that of Vesta itself, and with the composition of the Vestoids, a family of asteroids in the Asteroid Belt with a similar composition to the larger minor planet. 1999TA10 shares a characteristic mineralogy (calciumrich wollastonite and iron-rich ferro­ silate) with these other bodies, but has far less iron than the surface layer of Vesta, pointing to an origin deeper within the body.

Vesta has a huge crater on its southern hemisphere, a relic of a major impact that could have produced the Vestoids. But if this body came from

Vesta’s mantle, it limits the crustal thickness to, at most, the depth excavated by the impact crater. Hubble Space Telescope data suggest a depth of 25 km (on a body 525 km across).

This is useful information for understanding the original composition and evolution of planetesimals, and ultimately for understanding the origin of the planets in the solar system.

More information will come from next summer, when the spacecraft Dawn will approach Vesta and orbit it for a year. Vishnu Reddy and colleagues publish this work in Icarus. http://www.mpg.de/english/portal/index. html

News in Brief Kepler’s rocky Earth A rocky planet 1.4 times the diameter of the Earth has been discovered by the photometric satellite Kepler. Follow-up Doppler shift observations using the 10 m W M Keck Observatory telescope in Hawaii confirmed the find. The planet, Kepler 10b, has a mass 4.6 times that of the Earth and an average density of 8.8 g cm–3, around that of iron.

It orbits its star every 0.84 days, 20 times closer than Mercury is to the Sun – by no means in the habitable zone. Kepler detects exoplanets from periodic dips in brightness of their stars. For Kepler 10b, the Keck telescope was used to pick out the Doppler shift in the starlight arising from the slight movements of the star as the planet orbits. The planet’s properties are well-defined because the star itself, Kepler 10, is bright and well-studied. http://www.nasa.gov/kepler

Mars before Space Age An excellent summary of the May 2009 RAS Specialist Discussion Meeting “Mars Before the Space Age”, organized by Barrie Jones and Peter Hingley, has appeared in Society for the History of Astronomy Bulletin 2010 issue 20 p29–40. http://sochistastro.webs.com

NASA balloons again NASA has restarted operations with high-altitude balloons, following a failed launch that threatened spectators in April last year. The NASA Mishap Investigation Board (MIB) has recommended more realistic risk assessment and safety management for future launches. During the launch at Alice Springs International Airport, Australia, the payload failed to release from the launch vehicle then broke free, hitting the airport fence and then a spectator’s car. No-one was hurt. The MIB was scathing: “In summary, the causes for this mishap evolved from: a flawed underlying assumption; a problematic historical mindset; and an ineffective organizational structure.” While the Board recognized significant financial and scientific loss as well, it points out that more realistic risk assessment and systems safety analysis would have avoided this. http://www.nasa.gov/centers/goddard/ business/foia/balloon_mishap.html

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News • Mission Update

Space Shorts 2000 comets from SOHO Citizen science reached another milestone with the discovery of the 2000th comet in images made available

by SOHO, the Solar and Heliospheric Observatory, designed to observe the Sun but coincidentally imaging many new comets. Michal Kusiak, an astronomy student at Jagiellonian University in Krakow, Poland, found the 1999th and 2000th comet on 26 December 2010. The SOHO comet-sighting website uses images from the Large Angle and Spectrometric Coronagraph (LASCO) cameras and, while the first 1000 comets took ten years to find, it took only five years to reach 2000. This is partly explained by better image processing and more volunteers looking. There also seems to be an unexplained increase in comets near the Sun.

Mission update Cassini maps icy Rhea

http://mars.jpl.nasa.gov/odyssey

A close approach to Saturn’s moon Rhea by the Cassini spacecraft has revealed details of its icy surface that suggest major fault scarps cutting across the trailing hemisphere of the moon (figure 1). These scarps gave bright radar reflections at a distance that mission scientists had thought might indicate cryovolcanism. The flyby and 3D images obtained show instead that significant tectonic activity has produced lines of cliffs and uplifted blocks cutting through the densely cratered plains that make up most of Rhea’s surface. This information strengthens the link between Rhea and Dione, suggesting that they are very similar in origin as well as being neighbours around Saturn.

Cassini data also show a difference between the leading and trailing hemispheres as the moon orbits Saturn. These are highlighted with false colour in the image, which combines ultraviolet, green and infrared information. The poles appear reddish in this image, while the trailing hemisphere appears bluer than the leading hemisphere. The variations may indicate compositional changes, or a change in the size distribution of grains at the surface. The poles may receive a different meteorite flux of different patterns of embedded ions, for example. And an orbiting moon is likely to have different patterns of meteoritic debris. The differences can also arise from “magnetic sweeping”, when ions that are trapped in Saturn’s magnetic field drag over and implant themselves in Rhea’s icy surface.

IBEX’s plasma pinch

http://www.nasa.gov/mission_pages/ cassini/multimedia/pia12808.html

http://sohowww.nascom.nasa.gov http://sungrazer.nrl.navy.mil

Martian record-breaker Odyssey, the NASA orbiter, is now the longest-serving of the many spacecraft that have observed Mars, surpassing the previous record of 3340 days on 15 December 2010, held by Mars Global Surveyor (MGS). Odyssey, launched in 2001, has mapped the martian surface and served as a communication relay, for rovers on the surface and to link NASA’s other martian satellites, MGS and Mars Reconnaissance Orbiter, for continuous weather observation. Odyssey will also support the 2012 Mars Science Laboratory mission, designed to look for traces of life.

NASA’s Interstellar Boundary Explorer has produced the first images of the plasma sheet within Earth’s magnetotail – as well as images that probably show a reconnection event. IBEX data appear to show a portion of the plasma sheet in the process of being pinched off from the rest, to produce a plasmoid. This process, involving disconnection and reconnection of field lines in the magnetotail, is an important part of the dynamics of the magnetosphere, but the data may arise from other processes. http://www.swri.org/press/2010/ ibeximage.htm

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Voyager 1: not quite gone The veteran spacecraft Voyager 1, launched 33 years ago, has now reached a region where the solar wind no longer flows outwards from the Sun, a sign that it is close to leaving the solar system. Voyager is in the heliosheath, the layer of the heliosphere beyond the termination shock, where the solar wind slows, heats up and curves sideways under pressure from the interstellar wind. Voyager scientists have found that the average speed of charged particles hitting the outermost face of the spacecraft over

1: The Saturn-facing side of Rhea (1528 km across; north is up) from the Cassini spacecraft wide-angle camera, approximately 35 000 km away.

Rhea is moving to the left; in this image the left side leads and the right trails. Image scale is 2 km per pixel. (NASA/JPL/Space Science Institute)

four months now matches the speed of the spacecraft itself, so there is no average outward flow. Voyager 1 is getting close to interstellar space, but it is not there yet; leaving the heliosheath by crossing the heliopause would entail a sudden drop in the density of hot particles, plus an increase in the density of cold ones.

This is expected to happen in about four years. http://www.nasa.gov/voyager

Space weather forecasts Near-simultaneous explosions in different regions of the solar atmosphere have every appearance of being connected, but hard evidence has been lacking. Now simultaneous observations by three spacecraft of a burst of solar activity on 1 August 2010 have pinned down the connection.

NASA’s Solar Dynamics Observatory and the twin spacecraft of STEREO were positioned to be able to observe all but a 30° wedge of the Sun’s far side when the burst of activity took place – including multiple magnetic filaments, radio bursts and several coronal mass ejections (CMEs).

The data suggest that the link between distant events on the solar

surface – 160° apart in this case – lies in their effects on the large-scale coronal field topology. Flux emergence on parts of the Sun’s surface not visible from Earth can influence the development of CMEs that reach our planet. It also explains some of the problems experienced in predicting solar flares and CMEs that would affect Earth. For effective space weather forecasting, researchers will need to analyse most, if not all, of the evolving global solar field. http://www.nasa.gov/stereo

APEX maps prestellar cores Sequences of chemical reactions in cold, dense molecular clouds are difficult to detect, yet important for understanding the processes that form stars and planetary systems.

Rare molecular species containing deuterium, such as H 2D+ and D2H+, are effective probes of these obscure regions, because they form only in these dense and extremely cold regions, at about 10 K, through a mechanism involving gas freezing onto dust grains. Researchers at the Max Planck Institute for Radio Astronomy in Bonn, led by Berengere Parise, used the CHAMP+ sub­milli­ A&G • February 2011 • Vol. 52

News • Mission Update

VIMS spots Saturn’s aurora Compilation of data collected by Cassini’s visual and infrared mapping spectrometer (VIMS) has revealed images of the infrared aurora that stretches 1000 km above the cloud tops. Here a composite false-colour VIMS image uses blue for reflected sunlight (2–3 µm), green for infrared light from hydrogen ions (3–4 µm) and red indicates thermal emission at 5 µm. The dark cloud bands across the face of Saturn are silhouetted in the thermal glow from the interior of the planet, highlighting the importance of cloud circulation, while the high-altitude haze and rings reflect sunlight. This image was made from 65 individual images, each taken over 6 minutes. (NASA/JPL/University of Arizona/University of Leicester) http://www.nasa.gov/mission_pages/cassini/multimedia/pia13402.html

metre receiver on APEX, the Atacama Pathfinder Experiment, to map the distribution of these species for the first time. This instrument can measure at seven positions simultaneously, a great advance on previous instruments that had needed a whole night’s integration to take comparable data at one position.

The team examined a prestellar core in the Rho Ophiuchi cloud, a star-forming region about 400 lightyears away. The surprise in the D2H+ data was that it was found not only in the central part of the core, where gas was expected to be densest, but also at the sides. This suggests that the key step in the formation process, gas freezing onto the dust grains, is efficient and takes place across these dense molecular clouds, rather than just in the cores. These results are published by Parise et al. in Astronomy & Astrophysics 2010 arXiv:1009.2682v1. http://www.mpifr-bonn.mpg.de/public/ pr/pr-d2h-en.html

Dragon into orbit and back SpaceX’s Dragon has become the first commercial spacecraft to orbit the Earth and return to splashdown safely. The spacecraft, developed with support from NASA, was launched from a Falcon 9 rocket at Cape Canaveral in Florida and A&G • February 2011 • Vol. 52

Blue sunset on Mars The Panoramic Camera on NASA’s Opportunity Mars Exploration Rover has been used to create movies showing a martian sunset, and a transit of the moon Phobos across the Sun as seen from the surface of Mars. While the movies use enhanced images and extrapolation, they are closely based on data from Opportunity and show the Sun shining in a blue haze in the otherwise red atmosphere, thanks to the dust normally present around Mars. (NASA/JPL-Caltech) http://marsrovers.jpl.nasa.gov/gallery/video/opportunity01.html

splashed down in the Pacific after two Earth orbits, in a flight lasting almost 3 hours and 20 minutes. This was the first flight under NASA’s Commercial Orbital Transportation Services (COTS) programme, intended to develop commercial resupply services to the International Space Station. Although not required for transport to the ISS, this demonstration flight also included firing the second stage engine, potentially important for future Geosynchronous Transfer Orbit missions where customer payloads need to be positioned at a high altitude. In addition, the descent was slowed using parachutes at around 3 km altitude, limiting the speed to around 5 m s –1, which is safe for humans. Both the

rocket and spacecraft were designed to be suitable for astronauts.

The COTS programme gave SpaceX freedom to select and develop products such as the PICA-X heat shield, a SpaceX variant of NASA’s Phenolic Impregnated Carbon Ablator heat shield. The shield was developed in collaboration with NASA, but much more quickly and cheaply than NASA had estimated. http://www.spacex.com

Hot spots heat solar corona?

How heat is transferred from the inner parts of the Sun to its tenuous corona is a long-standing puzzle in

soalr physics. Now combined observations from the spacecraft Hinode (an international collaboration led by Japan) and NASA’s Solar Dynamics Orbiter have highlighted one potential mechanism: the spicules in the Sun’s lower atmosphere.

The upper atmosphere of the Sun, at around a million degrees, is much hotter than the photosphere below.

The unknown heating mechanism is one stage in the process by which the Sun affects life on Earth, through the solar wind and mass ejections collectively termed space weather.

Spicules can be seen in visible light as fountain-like jets of plasma erupting through the photosphere. They involve a lot of plasma, perhaps a hundred times as much as escapes from the Sun in the solar wind.

The data from Hinode and SDO show that, while most of the plasma is heated to a hundred thousand degrees, some of it reaches millions of degrees.

And this hotter plasma tends not to fall back to the photosphere, as the cooler material does. Spicules are common on the Sun and this, together with their size means that even this small fraction of hot plasma could be a significant mechanism for coronal heating.

Bart De Pontieu of Lockheed Martin’s Solar and Astrophysics Laboratory, Palo Alto, California, is the lead author on the paper in Science. http://www.nasa.gov/sdo

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Bersier, Bode: Meeting report

Explosive transients: Meeting report Astronomers are finding common ground between gamma-ray bursters, supernovae and novae, thanks to more diverse and better data, report David Bersier and Michael Bode.

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xplosive transients have long been thought of as the one-hit wonders of the universe, spectacular and unpredictable, yet apart from the run-of-the-mill activity of the stars. But modern instrumental techniques and the sheer wealth of data mean that such time-dependent behaviour can be better examined and understood; novae, supernovae and gamma-ray bursters now represent a significant research field. This one-day conference in Liverpool, organized by David Bersier, Mike Bode, Shiho Kobayashi, Carole Mundell and Iain Steele of Liverpool John Moores University (LJMU), and sponsored by the RAS and LJMU, brought together those interested in all aspects of the field to discuss how transient behaviour illuminates astro­physical processes.

Paul Murdin (Institute of Astronomy, Cambridge, and Visiting Professor at LJMU), introduced the meeting, recalling the announcement of the discovery of gamma-ray bursters – some time after the actual discovery, which was made serendipitously with defence technology.

Once the details were out in the open, further discoveries followed, thanks to dedicated and not-at-all secret instruments. Time-dependent behaviour of all types has now become a significant element of astrophysical enquiry.

Peter Garnavich (Notre Dame University) spoke about finding and characterizing super­ novae and other transients with the Sloan Digital Sky Survey – and the potential to continue to do so, on a larger scale, with the Large Synoptic Sky Survey Telescope. Supernovae have become central to the understanding of the universe over the past 10 years, thanks largely to the use of Type Ia supernovae as distance indicators. Better sky coverage gave more data on nearby supernovae, and the cosmological focus has greatly increased the numbers at high redshifts. The result is a gap in the middle, the “redshift desert”, where very few supernovae are found (0.1 < z < 0.3). The SDSS-II Supernova Survey was designed to fill that gap.

The survey used an efficient technique covering 300 square degrees of sky every two nights.

Simultaneous detection of the u, g, r, i and z wavebands made measurement of supernova light curves very efficient; the short exposures allowed dense sampling. In the first year, software detected around 100 000 possible transients, which were scanned by eye to reduce to 11 000 possibilities and resulted in the discovery 1.10

of 130 Type Ia supernovae. The next year of operations produced 193 Type Ia supernovae and the third year, 175. The survey also found three cataclysmic variables, a gravitational lens, and a handful of unusual transients including a rare helium dwarf nova.

This great dataset allowed Garnavich and colleagues to fill out the Hubble diagram and fit a lot more light curves for cosmology. They also correlated the width of the light curve with peak luminosity and examined the colour variation, comparing low- to high-redshift supernovae.

Assuming that all colour variation was based on dust extinction, they found a statistically significant difference between low- and highredshift supernovae. The big uncertainty was the colour pattern of supernovae – data from the LSST should reduce this. The survey also revealed a large range of rise times. The physics of the rise and fall may help with fitting light curves (Hayden et al. 2010).

The LSST offers far more scope for supernova observations. It will observe 20 000 square degrees down to a magnitude of 24.5, with all data in the public domain. It will have a 3.5° × 3.5° field of view, with a 2 second readout at 0.2 arcseconds per pixel. It is expected to find 10 000 supernovae per year with good light curves, and 300 000 per year in the general survey. Just as in SDSS-II, we can expect to get our share of oddballs, too – supernovae impostors, rare novae and stars that just disappear.

Re-reading the records There are records of novae stretching back over 2000 years, and a vast database of nova observations from the 19th century onwards. Brian Warner (University of Capetown) described the recent burst of interest in optical observations of novae over the past three years as producing “a tsunami of papers”. The origin of this enthusiasm is in part due to the opportunity to track pre-nova light curves, but the key to success has been returning to the original photographic plates to measure the pre-eruption magnitudes.

Some published papers included errors and mis­ identifications; removing spurious results has produced a much more valuable dataset – and much that remains to be explained.

Detail in the data Collazzi et al. (2010) found that only two novae out of 22 studied showed significant

pre-eruption rises in brightness (V533 Her and V1500 Cyg). By contrast, the recurrent nova T CrB showed a pre-eruption dip. These events challenge theorists. For 30 classical novae plus 19 eruptions from 6 recurrent novae, they found that the average change in magnitude from before the eruption to long after the eruption is zero. However, they also found five novae (V723 Cas, V1500 Cyg, V1974 Cyg, V4633 Sgr, and RW UMi) with significantly large changes, where the post-eruption quiescent brightness is more than 10 times the pre-eruption level. These large post-eruption brightenings pose another challenge to theorists.

Schaefer and Collazzi (2009) showed that eight novae (V723 Cas, V1500 Cyg, V1974 Cyg, GQ Mus, CP Pup, T Pyx, V4633 Sgr and RW UMi) are significantly distinct from other novae. This group shares a suite of uncommon properties, characterized by post-eruption magnitudes much brighter than before eruption, short orbital periods, long-lasting super-soft emission following the eruption, a highly magnetized white dwarf (WD), and secular declines during the post-eruption quiescence. This may be explained if most novae do not accrete enough of their companion stars for continuous hydrogen burning, but some achieve this if the companion star is nearby (with short orbital period) and a magnetic field channels the matter onto a small area on the WD so as to produce a locally high accretion rate.

The past few years have also seen the emergence of a class of “luminous red novae” lying somewhere between novae and supernovae in absolute magnitude. Such objects have included nova M85 OT2006-1, which has a luminosity of five million times that of the Sun. More recently, Shri Kulkarni and collaborators (Kasliwal et al.

2010a) discovered the third example of these enigmatic objects: PTF10fqs, a luminous red nova in the spiral galaxy M99. These luminous red novae are of unknown origin, but have been speculated to arise from mergers or nova outbursts on very low mass white dwarfs.

At the other end of the luminosity scale, M M Kasliwal et al. (2010b) have recently announced the discovery with the Palomar 60 inch telescope of several sub-luminous, fast novae. These novae may arise on hot, massive white dwarfs, but again their origins and explosions are enigmatic.

Brad Schaefer and collaborators have now produced detailed light curves of 93 novae (Strope et A&G • February 2011 • Vol. 52

Bersier, Bode: Meeting report

a time-variable sky

1: Inside the laboratory at TRIUMF in Vancouver, Canada, where nuclear reactions are studied to improve our understanding of the processes powering stellar explosions.

al. 2010), resulting in a classification system with seven main types. Schaefer et al. (2010) have also conducted a great deal of work on recurrent novae of recent times, including the prediction for the first time of a nova outburst – that of the recurrent nova U Sco in January 2010.

One other startling finding of recent times is that by Bob Williams and collaborators of transient heavy element absorption systems around maximum light in a majority of novae studied (Williams et al. 2008). Most of these systems are accelerated outward, and they all progressively weaken and disappear over timescales of weeks.

The gas causing the absorption systems must be circumbinary and its origin is most likely to be mass ejection from the secondary star. Calculations of the amount of gas involved suggest around 10 –5 solar masses at least. The absorbing gas exists before the outburst and Bob Williams and Elena Mason (Williams and Mason 2010) suggest it originates in the L3 point of the binary system and that the outburst ejecta then run into this material. In a separate paper, they suggest that the broadening of spectral lines suggests the quadratic Zeeman effect is in operation, which implies the presence of extremely strong magnetic fields, at the mega-Gauss level. These must originate in the white dwarf.

Which stars explode?

Sumner Starrfield (Arizona State University) spoke about the types of stars that give rise to these remarkable explosions. He noted that what types of stars erupt to form Type Ia supernovae remains uncertain, speculating that they A&G • February 2011 • Vol. 52

may arise from close binary systems. A very interesting object in this context is the nova V445 Pup, which has no detectable hydrogen in its ejecta (described in detail in the poster by Patrick Woudt at this meeting).

Classical and recurrent novae arise from the white dwarf in a close binary pair. In the classical nova explosion the ejecta comprise about 10 –4 of a solar mass enriched in elements such as helium, lithium, carbon, oxygen, nitrogen, neon and aluminium. Starrfield highlighted problems with the theory, notably the origin of the tremendous amount of material ejected in these novae that is not hydrogen or helium. It must have come from the white dwarf, but how does that mixing happen? Chemical diffusion would be too slow, so is it shear mixing in the accretion process, or convective “undershooting” during the thermonuclear runaway itself?

The compositions of the binary components are important in calculations of where and how the explosion takes place. The time taken to get to a runaway reaction does not depend just on the white dwarf – there is a range of parameters involved, including the accretion rate. There are also still uncertainties in the fundamental nuclear reaction rates. Developments so far show ever shorter times to explosion as knowledge of the nuclear physics improves.

What drives the explosion?

Work on some of those nuclear reactions was described by Alison Laird (University of York), who focused on the nuclear reactions powering stellar explosions. She showed that the rates for

key reactions make a difference to the outcome of models, for example, in fitting light curves to X-ray bursts, where the rate of two key reactions involving argon and sulphur influence both the timing of the outburst and its peak luminosity.

Nuclear physics has an impact on what is seen, as well as on the energy and decay products.

Part of the problem is to find the most important reactions among the many possibilities.

Some 300 stable isotopes are known, and around 3000 unstable ones, but there may be around 7000 in total. It is impossible to study all of them, so the first stage is to determine which species and reactions are important, in terms of the conditions under which they take place, the reaction rates and their impacts on stellar evolution. Explosive nucleo­synthesis involves high-pressure and high-temperature reactions, taking place over short timescales, with radioactive species playing a role. Most experiments on quiescent burning are undertaken at very low energies and can involve measurements stretching over a year. Explosive nucleosynth­ esis is more accessible: a study can be complete in days or weeks.

Observations typically give the final state of the system, for example the final abundances of key isotopes such as 18F. The processes that produce and destroy this isotope are well understood, but the destruction processes have large uncertainties. Measurement in the lab is necessary in order to get the reaction rates and thence the final fluor­ine abundance. Laird and colleagues used the Canadian TRIUMF Isotope Separator and Accelerator (ISAC) (figure 1) beams to send an 18F beam to an H target, measuring the probability of the reaction at different energies.

Their results have shown that X-ray bursts are associated with breakouts from the hot CNO cycle, which is slowed by bottlenecks at β-decay points. The rp-process (rapid proton capture process) produces much more energy and there are a series of breakouts as temperature increases, which change the reaction pathway and increase the energy output. However, none of these breakouts have been measured at close to realistic temperatures and they involve hard-to-make chemicals. As a result, the rates of the reactions, and whether or not they provide enough energy to power the X-ray bursts we see, remain unknown.

For supernovae, the key information is whether the r-process (rapid neutron capture process) takes place and what the nuclear input is, in terms of masses, isotopes and Q-values.

The r-process is responsible for the production 1.11

Bersier, Bode: Meeting report

Observing transients – now and in the future Ian Steele (LJMU) highlighted the means for observing transients and following up new time-variable objects – in effect, providing the glue that joins together disparate observations.

In terms of providing targets, currently, there are the Palomar Transient Factory (PTF), PanSTARRS and, shortly, the LSST on the ground, together with Swift and Fermi in space. PTF and PanSTARRS are looking for transients, while LSST will open up a whole new era with its vast data collection rate. Swift has a trigger system to catch gamma-ray bursters, with gamma-ray, X-ray and UV coverage. Fermi is also a burst monitor, and generates a lot of data with an error box of around 1°.

For the future, Gaia will generate a lot of transient observations and will send out science alerts. It will return to fields about 80 times over the five-year mission and will generate alerts over 3–24 hours based on photo­ metric variation. Gaia has a large focal plane and so will pick up supernovae, gamma-ray bursters and microlensing events as well as all types of variable stars.

At high energies, VERITAS, HESS and the future CTA will all find flares. Gravitational

of 50–70% of iron and heavier elements, but there has to be seed material for the r-process to build on. The triple-a process could do it, although it is slow, but a reaction route via 8Li may have feasible reaction rates.

Laird stressed that there is a significant amount of work in this field now being undertaken in the UK. Nuclear physicists are investigating approachable aspects of explosive transients (Murphy et al. 2009, Beer et al. 2008). For novae, they are unravelling the hot CNO cycle, linking the production of 18F and 26Al with observations. For X-ray bursts, they study breakouts from the CNO cycle with waiting points where the rp-process is important. For supernovae, a target is the origin of seed material for the production of the highmass elements.

Massimo della Valle (INAF Napoli) returned to direct astronomical data with a review of the three ages of supernovae observations: heroic times from 1930–1970, the golden age from 1970–1997, and modern times after 1998.

Heroic observers such as Baade, Zwicky and Minkowski made their observations on photo­ graphic plates and used an empirical classification into Types I and II, working from limited photometry and light curves. Type I had no hydrogen and very deep Si ii features; they all had essentially the same light curve shape. Type II, on the other hand, were more individually varied and their spectra were dominated by hydrogen lines. In 1964 the story moved on 1.12

wave events will be picked up by enhanced ground-based GW detectors LIGO, GEO and VIRGO, which when working simultaneously will offer a very wide baseline and could locate events to 6°. If these instruments are able to locate sources of gravitational waves, there’s an opportunity to locate the optical counterparts and maybe determine, for example, the burst signature of merging neutron stars.

Alerts should give between half and one hour to find the optical counterpart.

The ICECUBE neutrino detectors in Antarctica pick up transient astronomical events from neutrino secondary radiation, with an error box of 1–2°. They look back through the Earth so the Antarctica instrument detects northern hemisphere events. There will also be the potential to discover transient events at other wavelengths, through LOFAR for example.

Follow-up requiring telescopes can be done robotically, but despite there being many such instruments, few of them are large. The Liverpool Telescope (figure 2) is particularly suitable. Transients such as gamma-ray bursts, gravitational microlensing events and events linked to exoplanets demand a wide

range of instruments.

The speed of follow-up also requires thought. Traditionally, International Astronomical Union circulars have been used to announce discoveries, but it is a slow and bureaucratic process. The Astronomer’s Telegram website (http://www. astronomerstelegram.org) offers circulars and reports by email, and requests a description and coordinates, etc. The limiting factor for speedy follow-up is that the reports are read by a human being. We need to get computer reading established if we are to scale up to the levels of discovery indicated for new instruments.

There are also different systems in place for different objects, such as Supernova News, an early warning system involving automated email, the GCN for gamma-ray burst notices, which are computer-readable and include follow-up observers reports, some computer-readable and some not. The standard adopted by many observers is the VO Event, a reporting system that is readable by both computers and people, and details who, what, when, where and why, giving a reference and citations, with embedded pictures and other

when Bertola observed a supernova with a light curve in the typical shape of a Type I but a spectrum similar to Type II.

ated with a GRB, with incredible energy output and a lack of H, He and Si ii. It has a broad-lined Type Ic supernova spectrum with a very large luminosity and emits about ten times the energy of a typical supernova. Around nine of these supernovae/GRBs have been found, six of which are well studied because they are close by, and three at high redshift. GRBs are rare compared to supernova explosions, so very few of them – only something less than between 0.4 and 3% of Type Ibc supernovae – can become GRBs. Why do these very few Type Ic super­novae become GRBs, and most don’t? The answer seems to lie in the special conditions required: a really massive star (>30–40 solar masses) that has lost most of its hydrogen and helium before collapse, and must be in a low-metallicity environment.

Della Valle went on to discuss the questions posed by unusual transients such as sub­luminous core-collapse supernovae, the so-called dark supernovae, and objects that fit between novae and supernovae, such as red faint supernovae, and SN2008ha. There are also ultra-bright supernovae, such as the hydrogen-rich 2006gy, which reached an absolute magnitude of –22 and showed an unusually broad light curve.

Are such objects supernovae? And what about a supernova whose predecessor star looked like a luminous blue variable? Was it a pulsing LBV that exploded, or an outburst of a Wolf–Rayet star? It was certainly something that had never been observed before.

Della Valle set out a continuum from Type II

The golden age In the golden age, an incredible number of bright supernovae were detected, with 4 m class telescopes and CCDs bringing higher quality spectra and follow-up observations to fainter magnitudes. The better spectra led to new classes of supernovae: Ib, Ic, IIb and IIc. This implies a certain amount of non-homogeneity, but the details also allowed observers to understand more about the process. For example, Bertola’s peculiar object was a Type Ib supernova and in the mid-1980s it was identified as a star that, by the time it came to collapse, had lost its hydrogen envelope. Some of these unusual types of supernovae, such as 1987k with its dis­ appearing hydrogen lines, are transition types that start as Type II but morph into Ib. Modern times brought systematic searches with 4–10 m optical telescopes and the Hubble Space Telescope. Together with the use of small and robotic telescopes working in synergy, this has brought out the supernova–GRB connection. Large-area supernova surveys have produced lots more examples each year, revealing new types of explosive transients such as luminous blue variables, rare types of SNe and corecollapse explosions in unusual environments.

The GRB–supernova connection is highlighted by supernova 1998bw which seems to be associ-

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Bersier, Bode: Meeting report

Jim Hinton (University of Leicester) spoke about the potential of the Cerenkov Telescope Array to pick out transient events. The CTA will pick out flares in blazars – transients that go through several orders of magnitude variability in minutes – although they are slow compared to GRBs. But it is their high energy and transient nature that make them good

targets for the CTA. At greater than X-ray energies, existing ground-based instruments such as HESS, Veritas and Magic have the sensitivity to detect astronomical phenomena – and there are things to see. One can pick out cosmic rays from supernovae, pulsars, starbursts and even carry out galactic surveys. Information is also obtained on the IR background, gamma-rays, dark matter and annihilation signatures, for example. GRBs were a big part of the case made for building CTA. High-energy astrophysics also has applications in fundamental physics and has been a very successful field over the past five years. CTA as the nextgeneration instrument will be a global facility.

It will be an array of Cerenkov telescopes and, while current facilities have two, three or four telescopes, CTA will have 100. It will be an order of magnitude more sensitive than current ground-based instruments, with a wider energy range (overlapping that of Fermi, but far more sensitive at equivalent energies), better angular resolution, and will be a major astronomical observatory for Europe in the next few decades.

The data coming in from Fermi suggest that

supernovae, through Types IIb, Ib and Ic, to GRB supernovae, with increasing mass on the main sequence. The mass of hydrogen involved decreases, as does their frequency. Type II supernovae, at more than 8 solar masses, are most frequent, while the GRB types originate in stars with around 40 solar masses but are much rarer.

The new subtypes coming out of this research raise questions about our understanding of the processes involved, for example the alternative sources of energy to power ultra-bright supernovae, which demonstrate 10 times the energy in standard core-collapse supernovae.

Stacey Habergham (LJMU) then presented results of an optical study looking at corecollapse supernovae in a range of nearby galaxies and different environments in order to constrain their progenitors. 140 local spiral galaxies produced 178 core-collapse super­novae, 110 Type II supernovae and 68 Type Ibc supernovae. The distribution of these super­novae within galaxies showed a deficit of Type II supernovae, and an excess of Type Ibc in the central regions, interpreted as an increase in metallicity from Type II to Type Ib to Ic. The survey also showed that many of the super­novae came from disturbed galaxies with signs of interactions such as tidal tails, double nuclei or strong asymmetries. In the central regions of these disturbed galaxies, supernovae are preferentially Type Ibc rather than Type II, to the extent that within the central 10% of these galaxies, only Type Ibc occur and no Type II has been found. Overall these

supernova distributions suggest a “top-heavy” initial mass function with a slope of close to 1 (Habergham et al. 2010). The strong central concentration may be a result of a nuclear starburst, although it could be that this central excess of Type Ibc supernovae is an artefact of survey incompleteness in the central regions, from the greater extinction among central region stars as a whole.

Zach Cano (LJMU) continued to explore the GRB–supernovae connection, looking at multi­ wavelength observations. Long-duration GRBs appear to have a link to the core collapse of massive stars; their light curves and spectra include broadened lines suggesting speeds of around 10% the speed of light, together with colour changes and a bump in the optical/infrared region. Three case studies were described to illustrate these unusual objects. GRB060729, studied using Swift data, showed a clear bump 26 days after the burst, but the U band showed no or very little light from an associated supernova. The (longer wavelength) R band did appear to show supernova light. Working with the Swift assumption that the rate of decay is constant across all wavelengths, they were able to remove the GRB flux from the data. What is left looks like a supernova. The same process applied to the data from GRB090618, the second case study, also produces the characteristic supernova curve, complete with bump, but in this case there is also a distinct colour change, from an early blue GRB to a later red supernova.

2: The 2 m robotic Liverpool Telescope in its fully opening enclosure, with other telescopes of the Observatorio del Roque de los Muchachos in the background. (R J Smith 2005)

detail that make it a good way to pick out interesting events. In the UK we have eSTAR, based at Exeter (http://www.estar.co.uk).

Even higher energies?

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there are photons at energies greater than 30 GeV, with some very large redshifts. There could certainly be 100 GeV photons. And, if the synchrotron radiation is interpreted correctly, the second component has the potential to be a powerful diagnostic of the conditions inside bursts. Fermi is already seeing a systematically delayed high-energy component, therefore late afterglows will be detectable. The design of the telescope is also flexible enough to be able to pick out optical afterglows. The fast cameras used to detect Cerenkov radiation flashes could also be used to find optical transients on a very short timescale – and it can be done effectively for free alongside the gamma-ray work.

In short, the CTA will be a powerful tool for detecting transient non-thermal phenomena. It won’t be cheap, but the scale of the consortium makes it manageable. EU funding has been awarded for the engineering preparatory stage – and some of that €5.2m has come to the UK. The first telescope should be available in 2013, and CTA will take five years to build. But the science starts immediately; data will be taken from the start and building up to the full working capacity.

In short, Cano presented clear photometric evidence of supernovae associated with GRBs.

GRBs Jonathan Granot (University of Hertfordshire) opened the GRB session with a whirlwind tour of what we know, do not know and would like to know about GRBs. These brief, intense and totally unpredictable flashes of high-energy gamma rays are the instantaneously most luminous objects in the universe. They are thought to be produced during the core collapse of a massive star or the merger of two compact objects (neutron stars or black holes) and provide direct access to regions of extreme physics (Lorentz factors Γ > 100), strong gravity and potentially large magnetic fields, as well as acting as important probes of the high-redshift universe. The technical challenge of observing completely unpredictable, time-variable and rapidly fading electro­magnetic signals from space has driven development of advanced technological capabilities in satellite- and ground-based observatories over the last decade.

Recent progress in the study of GRBs comes mainly from the discovery of long-wavelength counterparts (afterglows), and their rapid, multi­ wavelength follow-up. Afterglow observations have established that ultra-relativistic flows are associated with GRBs (Granot 2008). A blast wave caused by the interaction of the ejecta with the ambient medium provides a natural explanation for afterglow observations. Several well1.13

Bersier, Bode: Meeting report monitored afterglow light curves clearly indicate supernovae association with long GRBs (long defined as gamma-ray duration > 2 s). Their host galaxy type, star formation rates and location within the hosts also suggest massive stars as long-GRB progenitors. Some short GRBs arise in host galaxies with very low star-formation rates. This is consistent with the leading theor­ etical candidates for short-burst progenitors – compact stellar mergers – but direct observational confirmation is still required. Future detection of gravitational waves and neutrinos provide definitive tests of progenitor models.

Open questions The origin of the high-energy gamma rays of GRBs is also an open question (Granot et al.

2009). Although a widely accepted mechanism for producing the prompt gamma rays is an internal dissipation in a relativistic flow, the dissipation and emission mechanisms are still highly uncertain. More fundamentally, a longstanding problem is how to accelerate a GRB outflow to ultra-relativistic velocities. Recent Fermi satellite observations show that some events require a Lorenz factor Γ ~ 1000 to avoid a high-energy cut-off from intrinsic pair production. The jet acceleration mechanism could be magnetic or thermal.

The Swift satellite has provided a range of puzzling observations in recent years: the plateau phase and chromatic breaks in early afterglow light curves cannot be explained in the standard model, and calls for further observational and theoretical investigations. Particle acceleration in collisionless shocks is believed to be responsible for the production of the non-thermal GRB emission. However, a theory of collisionless shocks based on first principles does not exist yet. More theoretical investigation is needed, and GRB observations could provide an ideal opportunity for diagnosing the physics of relativistic collisionless shocks in general.

Peter Curran (CEA, Saclay) discussed the physics of Fermi acceleration of electrons in astrophysical plasmas. This mechanism is thought to be important for accelerating electrons to relativistic speeds in shock fronts in AGN and GRB jets, X-ray binaries, supernovae and solar flares, and the resulting synchrotron radiation provides an important diagnostic tool to probe the energy distribution, p, of the population of radiating electrons (Starling et al. 2008). Curran examined whether there is a single, universal value of p, a distribution and/ or variation across sources and source types. In the standard fireball model, a GRB afterglow is produced when the expanding relativistic blastwave collides with and is decelerated by the circumburst medium; forward and reverse shocks are produced, with the forward shock emitting broad-band synchrotron radiation that fades with time. The spectral energy distribution at 1.14

a given time has a characteristic shape that is determined by the location of the synchrotron injection and cooling frequencies and p. By selfconsistently modelling the afterglow behaviour, the value of p can be derived and is found to lie in the range 2–2.4 for most bursts, consistent with theoretical predictions, although larger values (~3–4) are inferred in some cases. Curran showed that modelling of X-ray data from Swift for approximately 300 bursts suggests a Gaussian distribution of p values with a mean p ~ 2.4 and a standard deviation σ = 0.6, with the cooling frequency already lying below the X-ray band at the time of observation in 94% of GRBs (Curran et al. 2010).

Baryonic or magnetic jets?

Shiho Koyabashi (LJMU) discussed magnetization of GRB outflows. GRBs are produced by shocks in ultra-fast outflows of material moving at speeds close to that of light when a new black hole is formed. The mechanism for accelerating the ejected material to these extreme speeds in GRBs and other cosmological jets, however, is a long-standing and important unsolved problem in modern astrophysics. There are two competing models for relativistic GRB jets: baryonic jets and Poynting-flux – magnetically dominated – jets. The magnetic jet models are now attracting more attention from researchers (Zhang and Yan 2010). An attractive aspect of the magnetic models is that intrinsic magnetic fields may provide a powerful mechanism for collimating and accelerating the relativistic jet. When the outflow impacts on the ambient medium to produce shocks, a transient shock called the reverse shock is generated inside the outflow itself. The short-lived optical flash radiated from reverse shocks is a key element when magnetic properties are discussed (Mundell et al. 2007, Gomboc et al. 2008). Koyabashi discussed two methods based on relativistic hydrodynamics and optical flash observations.

Because the jet is believed to radiate photons via the synchrotron process – where electrons radiate energy as they spiral around magnetic field lines in the expanding flow – the emitted radiation is predicted to be highly polarized.

The first detection of 10% polarization of an optical afterglow just 160 s after the explosion of GRB 090102 by the Liverpool GRB team (Steele et al. 2009) opens the exciting possibility of directly measuring the magnetic properties of GRB flows. A new polarimeter, RINGO2 on the Liverpool Telescope, was recently commissioned to allow detection of a larger number of fainter bursts and, for the first time, measure the temporal evolution of the polarization degree and position angle of early optical afterglows.

RINGO2 measurements will open a completely new observational parameter space.

Maurice van Putten (Université d’Orléans) drew the day to a close by unifying the physics

of some of the most energetic processes in the universe within a coherent theoretical framework centred on extraction of energy from spinning Kerr black holes via frame dragging and viscous spin down (van Putten 2009). The origin of high-energy (gamma-ray) photons from GRBs (in contrast to current accretion-driven models) and the generation of ultra-high-energy cosmic rays by low-luminosity AGN, such as Seyferts and LINERs (in contrast to powerful BL Lacs) would be explained within this single framework for stellar and super­massive black holes respectively (van Putten and Gupta 2009).

In particular, highly collimated relativistic jets, if common to GRBs and AGN, could produce high-energy emission by gravitational spinorbit coupling along the axis of rotation and low-energy emission from surrounding matter via a torus magnetosphere. Analysis of 600 GRB gamma-ray light curves suggests that viscous spin-down against matter at the innermost stable orbit is occurring and predicts future detections of radio and gravitational wave bursts with a bi-modal distribution of durations similar to that observed for GRB durations. Current and future astronomical facilities optimized for timedomain astronomy will provide exciting tests of these theoretical predictions and, together, will advance our understanding of gravito-magnetic processes in black hole-driven central engines and their role in the transient sky. ● David Bersier, Mike Bode (mfb@astro.ljmu. ac.uk), Shiho Kobayashi, Carole Mundell and Iain Steele, Astrophysics Research Institute, Liverpool John Moores University, UK. The meeting “Explosive Transients” was sponsored by the Royal Astronomical Society and LJMU. Further information The talks can be found at http://www.astro.ljmu. ac.uk/ras2010/schedule.shtml References Beer C E et al. 2008 Proceedings of the 10th Symposium on Nuclei in the Cosmos PoS (NICX) 058.

Collazzi A C et al. 2010 Astron. J. 138 1873.

Curran P et al. 2010 Ap. J. 716 L135.

Gomboc A et al. 2008 Ap. J. 687 443.

Granot J 2008 arXiv:0811.1657.

Granot J et al. 2009 arXiv:0905.2206.

Habergham S et al. 2010 Ap. J. 717 342.

Hayden B T et al. 2010 Ap. J. 712 350.

Hounsell R et al. 2010 Ap. J. 274 480.

Kasliwal M M et al. 2010a arXiv:1003.1720.

Kasliwal M M et al. 2010b arXiv:1005.1455.

Mundell C G et al. 2007 Science 315 1822.

Murphy A St J et al. 2009 Phys. Rev. C79 058801.

Schaefer B and Collazzi A C 2009 Astron. J. 139 1831.

Schaefer B et al. 2010 Ast. J. 140 925.

Starling S et al. 2008 Ap. J. 672 433.

Steele I A et al. 2009 Nature 462 767.

Strope R J et al. 2010 Astron. J 140 34. van Putten M and Gupta A C 2009 MNRAS 394 2238. van Putten M 2009 arXiv:0905.3367.

Williams R et al. 2008 Ap. J. 685 451.

Williams R and Mason E 2010 Astrophy. Space Sci. 327 207.

Zhang B and Yan H 2010 arXiv:1011.1197.

A&G • February 2011 • Vol. 52

Astrobiology • Horner: Introduction

Astrobiology:

young science, old questions W

hat was it that first got you interested in science? For many, the answer to that question probably goes back to the origin of life (“Where did we come from?”), or the question of life elsewhere (“Are we alone?”). Although people have asked these questions for as long as we have been aware of the world around us, it is only in recent years that there has been any chance for them to move from the realm of science fiction to become the subject of serious scientific scrutiny. Over the past 20 years or so, our knowledge of the universe around us has advanced in leaps and bounds – more than 500 planets have been discovered around other stars, and the nature of objects in our own solar system has been revealed in unprecedented, and often surprising, detail. At the same time, our understanding of life on Earth has advanced incredibly – from sequencing the human genome to understanding the fundamental microbiologial processes of ribosome formation in extremophiles (and what happens when it goes wrong). Researchers from across science have come together to attempt to uncover the origins of life, and to search for its signature beyond the Earth. The science of astrobiology has come into its prime.

One of the beauties of astrobiology is that it is, at heart, truly multidisciplinary. It is hard to imagine any other field of study where it is usual for geologists to collaborate with theoretical physicists, biologists to talk to observational astronomers, and chemists to examine data from satellites around other planets. Indeed, far from the scientific model in which people work and talk within fields that continually narrow and specialize, almost excluding all others, astrobiology encourages – and thrives on – its multidisciplinary nature. This is reflected well in the rapid growth of national and international societies, such as the Astrobiology Society of Britain (ASB – see the article by Terence Kee, Mark Burchell and David Waltham on page 1.29) and the Astrobiology Society, which aim to bring together interested scientists from all fields to form a cohesive and exciting community, which is nowhere better illustrated than at the ASB bi-annual conferences (the most recent of which, held last year, was the direct inspiration for this set of papers).

In this issue we present several invited articles that we hope will give the reader a glimpse into A&G • February 2011 • Vol. 52

Jonti Horner introduces a series of papers on astrobiology: the broad but focused discipline examining the possibilities for life off Earth.

Astrobiology • Horner, Jones: Exo-Earths

Astrobiology • Horner, Jones: Exo-Earths

Which exo-Earths should we search for life?

I

n the 15 years since the discovery of the first Jonti Horner and Barrie W Jones worlds, they will have such short lives that life planet orbiting a Sun-like star, 51 Pegasi on those planets can never truly get started.

(Mayor and Queloz 1995), the detection of work out how to select the most Even for those stars of sufficiently low mass to habitable planets, and even life, has become a likely targets among planets like allow the development of a significant biosphere real possibility. The new field of astrobiology Earth, to search for signs of life. in their main sequence lifetimes, if they are has gone from strength to strength, as scienyounger than a few hundred million years then tists from a range of disciplines come together to quickly or efficiently.

It is therefore vitally it is likely that life has not yet emerged. Low work on the question of how and where to find important to ensure that we choose the most priority must therefore be assigned to young life beyond the Earth. Unless we are fortunate promising candidates for initial observations, stars in the search for life. enough to detect a signal broadcast by another to maximize our chances of finding life.

It should also be noted that the luminosity of civilization, or detect life in situ elsewhere in a star increases throughout its main sequence Pick your planet the solar system, our main route to search for lifetime. For example, the Sun, now 4.6 Gyr life will be the study of exo-Earths: Earth-type How do we differentiate between one exo-Earth into its 11 Gyr main sequence lifetime, was only planets orbiting distant stars. Although no such and the next? What factors help determine about 70% of its present luminosity when it planets have yet been discovered, the first will which planets are most suited to the develop- was very young. Consequently, the HZ around be found within the next couple of years, and so ment of life? In this article we provide a brief a given star will gradually drift outwards. Just the time is right to discuss exactly what factors overview of the key factors that are currently because a planet is within a star’s HZ now does might come together to make such planets more, thought to influence the habitability of exo- not, therefore, mean that it will have been there or less, suitable for life to develop and thrive.

Earths. Although it would be foolish to entirely for long enough to have developed a detectable Throughout the history of astronomy it seems prejudge where we are likely to find life, it makes biosphere. Preference must therefore be given that, once the first member of a population of sense to concentrate our initial efforts on those to planets that have spent at least hundreds of objects has been discovered, many more follow that seem most likely to provide a positive detec- millions of years in the HZ. soon after. The faster technology improves, the tion. We therefore highlight criteria for comIf we adopt 1 Gyr as the minimum age for a more rapid the turnover between no objects parison, in order to determine the best target. planet to have a detectable biosphere, we rule being known, and many having been detected.

The classical habitable zone (HZ) extends over out stars whose main sequence lifetimes are Exoplanets are no exception. Since 51 Pegasi was the range of distances from a star at which an shorter than this – the massive stars that condiscovered, an increasing number of exoplan- Earth-like planet can have water as a stable liq- stitute the O, B and A dwarfs. However, the very ets have been found, first at a trickle, and now uid over at least part of its surface. The extent of great majority of stars are less massive, includalmost at a flood. At the time of writing, 494 the HZ for a star depends on mass and age. ing the Sun. Such a cut, therefore, only rules out planets are known within 416 planetary sysStars form with a wide variety of masses. The a tiny fraction of potential planets. If we extend tems (data from the Extrasolar Planets Encyclo- smallest are just 8% of the Sun’s mass, below our 1 Gyr minimum age to all young stars, over paedia, http://exoplanet.eu/catalog-all.php, 15 which objects can not sustain hydrogen fusion in 10% of all stellar systems are excluded.

October 2010). On average, 10 new planets are their cores. The most massive are over 100 times All stars are variable. Our Sun, for example, being announced per month, a rate that is des- the Sun’s mass. All stars spend the majority of varies in luminosity by ~0.1% through the 22tined to climb further as projects such as Kepler their lives on the “main sequence” – a narrow year solar cycle. Some stars, though, are far (http://kepler.nasa.gov) begin to yield results. strip across a plot of luminosity versus sur- more variable than others: young stars and To date, the least massive exoplanet discov- face temperature. A star moves on to the main old stars are particularly prone to significant ered around a Sun-like star is Gliese 581e, with a sequence after its formation (which is a relatively variability. The variable star Mira (ο Ceti), for minimum mass around twice that of the Earth. quick process) and remains there for most of its example, varies in luminosity over a period As new techniques come online, and missions lifetime. Stars leave the main sequence when of ~332 days, by a factor of ~4000, and such such as Kepler begin to bear fruit, the first true all the hydrogen in their cores has been used. behaviour is far from unusual. Other stars, exo-Earths will be detected. Once one is found, They then enter stellar “old age”, another phase such as our nearest stellar neighbour, Proxima many more will quickly follow, and the search that passes in an astronomical blink of an eye, Centauri, are prone to enormous stellar flares. for life beyond our solar system will begin. How compared to their main sequence lives.

It is clear that such stars would be bad hosts that search will be carried out is still under some The luminosity of a star during its main for the development of life.

At what point does debate – a wide variety of biomarkers have been sequence life is roughly proportional to the variability become too great a problem for life suggested that might help life be detected – but fourth power of its mass.

However, the amount to overcome? Whatever the answer, it is surely the observations necessary to give a conclusive of “fuel” the stars have is simply proportional prudent to focus our initial hunt for life on planresult for a given planet will certainly be long to their mass. The lifetime of a star is therefore ets around stars that are comparatively stable and arduous. Although new technologies and roughly inversely proportional to the cube of its telescopes will no doubt be developed to help mass. As a result, the most massive stars in the 1: An artist’s impression of a watery, ringed exo-Earth, speed along such work, it is unlikely that we universe live fast and die young. Even if the most viewed from its largest moon. (David A Aguilar CfA) will be able to survey all of the new exo-Earths, massive stars can form potentially “habitable” 1.16

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1.17

Astrobiology • ApAgyi, burchell: AstronAutics

Astrobiology • ApAgyi, burchell: AstronAutics

Does astrobiology inclu de human space flight?

is given to the relevance of human activity in space to astrobiology. It is thus clear that from a mainstream astrobiology viewpoint, there is a disconnection from astronautics. To understand if this is sensible we need to ask a further ques­ tion about the nature of astronautics.

Katinka Apagyi and Mark J Burchell argue that aspects of astronautics overlap with astrobiology, in the same way that aspects of geophysics and planetary science do. The gap between these disciplines is an artificial separation that should be overcome.

What is astronautics?

A

studying how life adapts to extreme environ­ ments on Earth are potentially contributing to the field, as are chemists who try to understand the pathways needed for the development of the complex organics needed for life, and so on.

Equally, scientists and engineers who design space vehicles or apparatus to be deployed on other solar system bodies, for example, are also potential astrobiologists depending on the type of research enabled by their efforts. Moving outside science, academics who consider the impact on mankind of the possible discovery of alien life are also working in an area related to astrobiology. This goes well beyond the obvious concerns of planetary protection – where we worry about impact on biospheres of transfer­ ring life to new potential habitats – and intro­ duces social aspects to the field. In general we need to remember that science does not exist in a vacuum apart from society, and those who consider the potential impact of astrobiology on society are clearly participating in the develop­ ment of the field of astrobiology.

One can get a feel for how academics in general see astrobiology, by looking at their manifestos for the discipline, i.e. the various roadmaps that occasionally appear. The most famous roadmap is probably that of NASA. Several versions have appeared – Des Marais et al. 2003 is an early version and Des Marais et al. 2008 is current.

In the later version, seven goals are set for astro­ biology. We can immediately note that in them­ selves the seven headline goals (table 1), which involve the use of space for collecting samples What is astrobiology? from other planets, delivering instruments to making astronomical As evident from the other papers in this issue, other solar system bodies, telescopes, etc, do not astrobiology is a broad discipline. It addresses observations from space that are “astro­ the origin or origins, distribution and develop­ specifically describe activities they neither require nor ment of life from a broader perspective than nautical” in nature – i.e. activity in space. that of a single planet, i.e. it considers life as a necessarily involve human One possible area where humans are directly cosmic phenomenon, but often does so with an version of the NASA acknowledgement that Earth is our sole example mentioned in the 2008 with regard to the is Roadmap Astrobiology thus is of a body containing life. Astrobiology of mankind beyond concerned with many different aspects of study. potential future expansion considers that micro­ As well as those people searching for evidence the Earth. The roadmap role in life­ of life itself, other researchers are also poten­ organisms will play a significant acquisition, and tially astrobiologists. For example, astronomers support systems or resource micro­organisms to who search for extrasolar planets are potential therefore the ability of should be explored as astrobiologists. Cometary scientists observing adapt and survive in space But beyond the presence of complex organic materials on part of an astrobiology programme. wider consideration comets are potential astrobiologists. Biologists this narrow approach, no

strobiology is a growing field of schol­ arly activity worldwide. It is multi­ disciplinary in nature, and encompasses a wide range of topics drawn from other well­ established academic disciplines (for a recent discussion see Burchell and Dartnell 2009).

However, while the relation of biology, chem­ istry, planetary science, etc, to astrobiology is readily apparent and often realized in collabora­ tive work across these disciplines, the relation of the field to astronautics is not so clear. To take the UK as an example, a recent survey of univer­ sity­based academics possibly active in the field of astrobiology found no respondents describing themselves as active in astronautics (Dartnell and Burchell 2009). It could be argued that such a survey might reveal as much about the biases of the authors as it does about the topic being surveyed, but this lack is also apparent at many conferences in the field. This split between astrobiology and astronautics is reflected in a wider split in the science community between those who favour robotic exploration of space and those who promote the use of astronauts.

The issue that arises is thus: is there any over­ lap between the two fields of astrobiology and astronautics? Since one of the authors has a space science background (MJB) and the other in astronautics (KA), we partially answer our own question, and it is the degree of overlap that is discussed below. Included in this discussion is also a consideration of the benefits of collabora­ tion between the two communities.

Astronautics, similarly to astrobiology, is a highly multidisciplinary subject, encompassing both the science and the technology of manned and unmanned space flight. It is essentially a very large engineering job, requiring input from both biological and physical research, and in return providing new and improved platforms for further scientific experiments, as well as flight equipment to be used by the public and the private sector.

While astrobiology is the legacy of the late 20th century, the roots of astronautics go back to the 17th century when Newton first outlined some of the mathematical basis of space travel.

In 1903, Tsiolkovskii, who became known as the father of astronautics, published his famous rocket equation, which predicts the final veloc­ ity of a rocket, calculated from the known vari­ ables of the mass of the rocket, and the mass and the exhaust velocity of the propellant.

In the early 20th century, this so far civilian and purely theoretical enterprise was largely taken over by the military in the USA, Germany and the USSR. Robert Goddard, a pioneer in developing rockets propelled by liquid fuel, was in charge of the US War Department’s rocket division during the first world war. He was both an engineer and a scientist, and since little was known about the biological effects of the heights his rockets later reached for the first time (~2.6 km), he supported using this new technol­ ogy for scientific experiments as well as recon­ naissance and other military applications.

Despite his best efforts, however, astronau­ tics did not include any biological research till the 1950s. Manned space flight imposed a new challenge for life scientists and thus the science of space biomedicine, as a new branch of astronautics, was born. The first “space doc­ tors” had to combine medical knowledge and the little that was known about the space envi­ ronment to form reasonable predictions about the health effects of space flight. In the 1960s almost all US astronaut candidates were test pilots, coming from aviation and engineering backgrounds. They were physically and men­ tally fit and their military training ensured a disciplined and organized attitude towards their missions. All of these were essential attributes

Astronauts at work on the Hubble Space Telescope.

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1.31

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Astrobiology • MArtins: Biomarkers

In situ biomarkers and the Life Marker Chip Zita Martins examines some of the challenges involved in the identification

Table 1: Components of a unicellular organism molecule proteins DNA and RNA lipids other total

Table 2: Abundances of soluble organic matter in Murchison meteorite compounds

more stable >2 billion years

carboxylic acids (monocarboxylic)

concentration (ppm) 332

sulphonic acids

16 × 10–14 6.9 × 10–14 2.6 × 10–14

dicarboximides

>50

dicarboxylic acids

>30

3.2 × 10–14 28.7 × 10–14

he existence of life on Earth now and in the past provides the basis for questioning whether life may be ubiquitous in the universe. In order to be able to detect life elsewhere in our solar system, it is crucial to know what to look for, i.e. molecules that are diagnostic of past or present life: biomarkers. Future life-detection missions, such as the European Space Agency’s ExoMars mission to the Red Planet, need to be able to detect these biomarkers, but also to determine the origin of any molecule that they detect. Therefore, it is necessary to distinguish between present life biomarkers, dead organisms/fossil biomarkers and abiotic molecules.

Following in the footsteps of the Viking landers, the future ESA ExoMars mission will look directly for signs of life on Mars. If life ever existed on Mars it would have left organic remains in the martian environment. It is crucial to address the questions of how the biomarkers of martian life were stored and in what form they remained after different periods of time; our understanding of these processes for extraterrestrial life is influenced by our knowledge of terrestrial biology. A summary of possible biomarkers present on Mars is given by Parnell et al. (2007). Biopolymers, such as DNA, RNA and proteins, would be the ultimate proof of present life on Mars (table 1). However, these molecules rapidly degrade under the strong UV radiation and oxidizing conditions of the martian surface (figure 1; e.g. Wayne et al. 1999 and references therein). Amino acids (the building blocks of proteins) are also degraded within a few hours when exposed to Mars-like UV radiation (Ten Kate et al. 2005, 2006). However, when buried at a depth of more than 2 m, amino 1.34

less stable

Astrobiology • MArtins: Biomarkers

dry weight (g/cell)

Summary of the main components and the dry weight composition of a unicellular organism, such as E. coli (g/cell). (Adapted from Brock et al. 1984)

T

and detection of biomarkers.

amino acids proteins

DNA

amino acids

hydrocarbons

polyols

1: Degradation rates of different biomarkers under martian conditions.

Less stable biomarkers include DNA, RNA and proteins, while the most stable biomarker includes isoprenoids (e.g. phytane).

ketones hydrocarbons (aromatic)

acids can persist for up to 3.5 billion years 1971, Yuen et al. 1984, Cooper et al. 2001, (Kanavarioti and Mancinelli 1990, Aubrey et Sephton 2002, Martins et al. 2008, Martins al. 2006, Kminek and Bada 2006). and Sephton 2009).

Typical biomarkers considered to indicate Carboxylic acids are the most abundant present life are L-amino acids, because on Earth molecules present in meteorites.

In addition, all living organisms use L-amino acids only. more than 80 different amino acids have been Over long periods of time biological amino acids detected in the Murchison meteorite, most in geological Earth samples are converted into of which are rare in terrestrial proteins (such equal amounts of L- and D- forms (i.e. a racemic as isovaline and α-aminoisobutyric acid). In mixture). However, on Mars racemization has addition, most non-protein chiral amino acids been estimated to be extremely slow because of present in meteorites are racemic (for a review environmental conditions (Aubrey et al. 2006), see Martins and Sephton 2009). However, so mixtures in which the L form dominates L-enantiomeric excess (up to 18.5%) has been may be fossil forms. Hydrocarbons, which are reported for a few non-protein amino acids often found as the molecular fossils of biological (including isovaline) (Pizzarello et al. 2003, lipids, are stable over long periods of time and Glavin and Dworkin 2009). on Earth have been found in rocks more than 2 billion years old (figure 1; Brocks et al. 1999). Mars and the Life Marker Chip It is important to remember that lipids can be Viking was the first life-detection mission to used as biomarkers for both past and present life Mars and did not find any molecules on the (table 1). For example, phytane is a membrane surface of the Red Planet (Biemann et al. 1976, component of methanogens (Woese et al. 1990), 1977). Intense UV radiation and highly oxiwhich are one of the possible sources for the dizing conditions on the surface of Mars may methane observed in the atmosphere of Mars have contributed to the destruction of organic (Mumma et al. 2009). compounds (Klein 1978, Benner et al. 2000, Large amounts of carbonaceous material are Squyres et al. 2004). In addition, the Viking gas thought to be delivered to the surface of Mars chromatograph–mass spectrometer (GC-MS) by interplanetary dust particles (IDPs) and may not have been sensitive enough to detect meteorites every year (Chyba and Sagan 1992, the degradation products generated by several Zent and McKay 1994, Flynn 1996, Bland million bacterial cells per gram of martian and Smith 2000), meaning that abiotic extra- soil (Glavin et al. 2001).

As preparation for terrestrial organic compounds may be expected future life-detection missions, in particular on Mars. Abiotic molecules found in meteor- for ExoMars, it is crucial to optimize detection ites are distinct from biomarkers; they exhibit methods of biomarkers and abiotic molecules, complete structural diversity with branched using terrestrial soils that resemble Mars (for a chains dominating, and present a decrease in review see Marlow et al.

2010). The Life Marker abundance with increase in carbon number Chip (LMC) is part of the ExoMars payload (fig(Sephton 2002). Several abiotic compounds ure 2) and is being designed to detect biomarkhave been detected in meteorites. In particular, ers in the martian soil (Sims et al. 2005). It is the Murchison meteorite includes among other an antibody-based instrument, aiming to detect molecules amino acids, nucleobases, carboxylic polar (e.g. amino acids) and non-polar molacids, polyols and hydrocarbons (table 2; Kven- ecules (e.g. isoprenoids) at the part-per-billion volden et al. 1970, Pering and Ponnaperuma (ppb) level. Court et al.

(2010) have optimized A&G • February 2011 • Vol. 52

hydroxycarboxylic acids

15

hydrocarbons (aliphatic)

12–35

alcohols aldehydes amines

2: Artist’s impression of the ExoMars rover, which will carry the Life

Marker Chip. (ESA)

Marker Chip (LMC) is one of the instruments onboard ExoMars that aim to detect a large set of biomarkers using an antibody array. Preliminary work currently being performed to optimize detection methods of biomarkers and abiotic molecules is crucial for successful future life-detection missions to Mars. ●

Conclusion

Zita Martins, Dept of Earth Science and Engineering, Imperial College London, UK. z.martins@imperial.ac.uk. Zita Martins is supported by the Royal Society.

Future life-detection missions, in particular the ExoMars mission to Mars, will search for biomarkers, i.e. organic molecules indicative of past and/or current life present in the regolith of the Red Planet. As the intense UV radiation and oxidizing conditions on the surface of Mars lead to the destruction of organic molecules, it will be necessary to search for those biomarkers at a depth of at least 2 m. The ultimate proof of life (as we know it) on Mars would be the detection of DNA, RNA or proteins. However, these biopolymers rapidly degrade under martian conditions. Amino acids, the building blocks of proteins, may survive up to 3.5 billion years when shielded from Mars-like UV radiation. Yet the most stable of all biomarkers are hydrocarbons, which can be used as a signature for both past and present life. Abiotic molecules delivered by IDPs and meteorites are also expected in the martian regolith, and future life-detection missions should be able to distinguish these from biomarkers. The Life A&G • February 2011 • Vol. 52

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the breadth of astrobiological study. Over the coming decade it is certain that the first Earthlike planets will be discovered around Sun-like stars, while our exploration of both our own planet and the rest of our solar system will continue apace. But where should we look? Unless we are fortunate enough to detect a definitive signal from another civilization, the search for life elsewhere will be long and arduous. Within our own solar system, the rapid growth in the

number of locations deemed potentially habitable means that the local search for life is no longer limited to up-close and personal studies of Mars, and speculative missions to drill through Europa’s thick ice crust to the oceans thought to lie below. Lewis Dartnell reviews the ever-widening boundaries within which life as we know it on Earth can survive, on pages 1.25–1.28, while Claire Cousins (pages 1.36–1.38) identifies close analogues on Earth of potential sites for life on Mars. On pages 1.34–1.35 Zita Martins targets the biosignatures that will identify life, while Lucy Norman and Dominic Fortes (pages 1.39–1.42) wonder what life might survive or thrive on Titan.

Beyond our solar system, any detected exoEarths are so distant and faint that any programme aiming to detect the signature of life will, by necessity, be both financially and temporally expensive, and will likely be limited to just the most promising targets. How we choose those targets is still under some debate, as is the best way in which to carry out the search – both in terms of what we look for, and how we look.

Jonti Horner and Barrie Jones (pages 1.16–1.20) try to narrow down the types of exo-Earth that we should examine in detail.

Various technologies will be employed in order to search for evidence of life, both within our own solar system and around distant planets – perhaps including astronautics, as Katya Apagyi and Mark Burchell argue, on pages 1.30–1.33.

Some (such as SETI, discussed here by Alan Penny, 1.21–1.24) will focus on the search for intelligent life, while others will concentrate on the smallest and simplest organisms.

At the same time as the search goes on, life will be carried into Earth orbit, to study its behaviour and survivability in drastically different conditions to those on the surface of our planet, while engineers elsewhere take great pains to ensure that other life does not accidentally get exported to the other potentially habitable parts of our solar system.

Despite the difficulties (and, in part, because of them), the next few years will prove to be an enormously exciting and rewarding time to be involved in astrobiology – possibly at once the oldest and the newest of the sciences. Who knows – in 10 years time, we might finally hold the answers to those two oldest questions: “Are we alone?” and “Where did we come from?” ● 1.15

Astrobiology • Horner, Jones: Exo-Earths

Which exo-Earths should we search for life?

I

n the 15 years since the discovery of the first planet orbiting a Sun-like star, 51 Pegasi (Mayor and Queloz 1995), the detection of habitable planets, and even life, has become a real possibility. The new field of astrobiology has gone from strength to strength, as scientists from a range of disciplines come together to work on the question of how and where to find life beyond the Earth. Unless we are fortunate enough to detect a signal broadcast by another civilization, or detect life in situ elsewhere in the solar system, our main route to search for life will be the study of exo-Earths: Earth-type planets orbiting distant stars. Although no such planets have yet been discovered, the first will be found within the next couple of years, and so the time is right to discuss exactly what factors might come together to make such planets more, or less, suitable for life to develop and thrive.

Throughout the history of astronomy it seems that, once the first member of a population of objects has been discovered, many more follow soon after. The faster technology improves, the more rapid the turnover between no objects being known, and many having been detected.

Exoplanets are no exception. Since 51 Pegasi was discovered, an increasing number of exo­ planets have been found, first at a trickle, and now almost at a flood. At the time of writing, 494 planets are known within 416 planetary systems (data from the Extrasolar Planets Encyclopaedia, http://exoplanet.eu/catalog-all.php, 15 October 2010). On average, 10 new planets are being announced per month, a rate that is destined to climb further as projects such as Kepler (http://kepler.nasa.gov) begin to yield results.

To date, the least massive exoplanet discovered around a Sun-like star is Gliese 581e, with a minimum mass around twice that of the Earth.

As new techniques come online, and missions such as Kepler begin to bear fruit, the first true exo-Earths will be detected. Once one is found, many more will quickly follow, and the search for life beyond our solar system will begin. How that search will be carried out is still under some debate – a wide variety of biomarkers have been suggested that might help life be detected – but the observations necessary to give a conclusive result for a given planet will certainly be long and arduous. Although new technologies and telescopes will no doubt be developed to help speed along such work, it is unlikely that we will be able to survey all of the new exo-Earths, 1.16

Jonti Horner and Barrie W Jones work out how to select the most likely targets among planets like Earth, to search for signs of life. quickly or efficiently. It is therefore vitally important to ensure that we choose the most promising candidates for initial observations, to maximize our chances of finding life.

Pick your planet How do we differentiate between one exo-Earth and the next? What factors help determine which planets are most suited to the development of life? In this article we provide a brief overview of the key factors that are currently thought to influence the habitability of exoEarths. Although it would be foolish to entirely prejudge where we are likely to find life, it makes sense to concentrate our initial efforts on those that seem most likely to provide a positive detection. We therefore highlight criteria for comparison, in order to determine the best target.

Stars form with a wide variety of masses. The smallest are just 8% of the Sun’s mass, below which objects can not sustain hydrogen fusion in their cores. The most massive are over 100 times the Sun’s mass. All stars spend the majority of their lives on the “main sequence” – a narrow strip across a plot of luminosity versus surface temperature. A star moves on to the main sequence after its formation (which is a relatively quick process) and remains there for most of its lifetime. Stars leave the main sequence when all the hydrogen in their cores has been used.

They then enter stellar “old age”, another phase that passes in an astronomical blink of an eye, compared to their main sequence lives.

The luminosity of a star during its main sequence life is roughly proportional to the fourth power of its mass. However, the amount of “fuel” the stars have is simply proportional to their mass. The lifetime of a star is therefore roughly inversely proportional to the cube of its mass. As a result, the most massive stars in the universe live fast and die young. Even if the most massive stars can form potentially “habitable” worlds, they will have such short lives that life on those planets can never truly get started.

Even for those stars of sufficiently low mass to allow the development of a significant biosphere in their main sequence lifetimes, if they are

younger than a few hundred million years then it is likely that life has not yet emerged. Low priority must therefore be assigned to young stars in the search for life.

A key concept is the classical habitable zone (HZ). This extends over the range of distances from a star at which an Earth-like planet can have water as a stable liquid over at least part of its surface. The extent of the HZ for a star depends on mass and age.

The luminosity of a star increases throughout its main sequence lifetime. For example, the Sun, now 4.6 Gyr into its 11 Gyr main sequence lifetime, was only about 70% of its present luminosity when it was very young. Consequently, the HZ around a given star will gradually drift outwards. Just because a planet is within a star’s HZ now does not, therefore, mean that it will have been there for long enough to have developed a detectable biosphere. Preference must therefore be given to planets that have spent at least hundreds of millions of years in the HZ. If we adopt 1 Gyr as the minimum age for a planet to have a detectable biosphere, we rule out stars whose main sequence lifetimes are shorter than this – the massive stars that constitute the O, B and A dwarfs. However, the very great majority of stars are less massive, including the Sun. Such a cut, therefore, only rules out a tiny fraction of potential planets. If we extend our 1 Gyr minimum age to all young stars, over 10% of all stellar systems are excluded.

All stars are variable. Our Sun, for example, varies in luminosity by ~0.1% through the 22-year solar cycle. Some stars, though, are far more variable than others: young stars and old stars are particularly prone to significant variability. The variable star Mira (ο Ceti), for example, varies in luminosity over a period of ~332 days, by a factor of ~4000, and such behaviour is far from unusual. Other stars, such as our nearest stellar neighbour, Proxima Centauri, are prone to enormous stellar flares.

It is clear that such stars would be bad hosts for the development of life. At what point does variability become too great a problem for life to overcome? Whatever the answer, it is surely prudent to focus our initial hunt for life on planets around stars that are comparatively stable 1: An artist’s impression of a watery, ringed exo-Earth, viewed from its largest moon. (David A Aguilar CfA)

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Astrobiology • Horner, Jones: Exo-Earths in their output. Fortunately, the detection of planets is significantly easier for quiescent stars, since significant stellar variability can mask any evidence of accompanying planets.

By far the most common stars in the universe are the M dwarfs. These stars have the lowest masses among main sequence stars, and therefore the lowest luminosities and the longest main sequence lifetimes, at many hundreds of gigayears. However, there are a few problems that may limit the habitability of planets in orbit around them. First, the low luminosity of these stars means that their HZs are very close to the star. Even for the most luminous M dwarfs, the HZ only stretches from around 0.2 to 0.4 AU. Exo-Earths located so close to their parent star will rapidly become tidally locked, just as the Moon has about the Earth. It has been suggested that such an exo-Earth, keeping one face pointed permanently toward its host star, would be inimical to life. Indeed, some authors have suggested that the entire planet­ary atmosphere would freeze on the dark side of the planet, leaving it airless and uninhabitable. However, such a scenario is not as likely as once thought. Heath et al. (1999) showed that an atmosphere just a tenth of the density of Earth’s would be sufficient to prevent this, were it mainly composed of CO2 . With denser atmo­spheres, the conditions could even support liquid water somewhere on the planet’s surface.

Secondly, M dwarfs exhibit variations in luminosity that are far greater, relatively speaking, than those experienced by more massive stars, ranging from the giant flares of stars such as Proxima Centauri, which can increase their luminosity by a factor of 100 over minutes, to the large, cool star-spots which can reduce their luminosity by several tens of percent for months at a time. However, even if the X-ray and UV flux from an M dwarf were to increase by a factor of 100, the total flux would still be relatively feeble – and should certainly pose no great problem to life on the surface. Similarly, short-term variations of stellar luminosity of a few tens of percent should pose no problem to any planetary biosphere, so long as the planet has even a moderate atmosphere. All in all, it seems that M dwarfs could be a promising places to look for life.

A significant fraction of stars form in multiple star systems. The exact fraction is unknown – widely separated stars are hard to associate with one another. When we look at our nearest stellar neighbours, roughly half are in multiple star systems, though it is likely that some stellar companions, even among those nearest stars, remain undetected. Whatever the true frequency of multiple stars, they are sufficiently common that any discussion of habitability beyond the solar system must consider such systems. Indeed, roughly a quarter of all known exoplanets are in multiple star systems (Horner and Jones 2010a). Typically, the separation of 1.18

the stars in those systems is significantly greater than the orbital radius of the planets that orbit about one or other of them.

Several studies have considered the orbital stability of such planets. David et al. (2003) consider the stability of an exo-Earth in the HZ of a Sun-like star. They found that a planet in such an orbit could remain within the HZ for at least the age of our solar system for a surprisingly wide range of binary scenarios, provided that the orbital eccentricity of the binary companion was low. The greater the eccentricity, the wider the mean separation needed to keep the system stable. Similarly, it appears that planets orbiting in the HZ beyond a close binary system can be stable, if the stars are sufficiently close to one another (Holman and Weigert 1999). It seems, that habitable exo-Earths could exist in a binary star system. If dynamical studies indicate orbital stability of an exo-Earth in the HZ, then such systems would be worthy of scrutiny.

The likelihood of stars hosting planets will vary as a function of the position of the star in our galaxy. However, given that we are likely to find many exo-Earths in our galactic back yard, it is certain that the first planets we survey for life will be in our local neighbourhood, and so a discussion of the galactic habitable zone is beyond the scope of this short review. For more information, see our recent lengthier review, Horner and Jones (2010a).

Dynamical effects and debris Many of the exoplanetary systems discovered to date are vastly different to our own. Systems have been found where the planets move on tightly packed, or highly eccentric, orbits.

Many giant planets have been found orbiting far closer to their host star than Mercury orbits our Sun, while other systems feature planets on mutually resonant orbits. With such a wide variety of systems, it is vital that the orbital stability and evolution of an exo-Earth be examined in some depth before conclusions on its habitability are drawn. The key point here is that, just because a planet’s orbit makes it currently appear habitable, it does not necessarily follow that that planet’s orbit will have been the same for a protracted period of time. It is easy to imagine, for example, planetary systems in which an exo-Earth’s orbit is periodically driven from being sufficiently circular for it to be habitable, to being eccentric enough to be a hostile environment, and back again, on geological timescales. The only way to check for such behaviour is to run suites of dynamical simulations of the planetary system in question, and follow its evolution on gigayear timescales. Any decision on which exo-Earth to study must take into account the long-term dynamical variation of the orbit of the planet in question.

Even if the long-term stability of a planet’s orbit appears to ensure its habitability, distant

perturbations applied by the other planets in the system might play a significant role in ensuring that an exo-Earth is less hospitable than would otherwise be expected. Subtle variations in the inclination and eccentricity of an exo-Earth’s orbit, coupled with variations in the tilt of its rotation axis, can combine to modify the climate of the planet. On Earth, these Milankovic´ cycles are linked to recent glaciations and interglacial periods. Were our solar system laid out differently, it is quite possible that these variations would be significantly larger, less regular, or happen over a shorter timescale, all of which could lessen the habitability of our planet.

Fortunately, Earth experiences only fairly small variations, over relatively long timescales.

Waltham (2010) has used Monte Carlo simulations to show that approximately 98.5% of randomly generated versions of our solar system would result in the Earth experiencing significantly more rapid and extreme Milankovic´ cycles. This suggests that Earth might be unusually favourable for the development of life!

The calculations needed to determine the frequency and size of the Milankovic´ cycles in a given system are not particularly computationally intensive, and therefore, so long as we have a reasonable degree of knowledge about the makeup of an exo-Earth’s planetary system, it should be relatively straightforward to draw quick conclusions about the degree to which the planet is habitable. While these cycles are not a key determinant of habitability, it is certainly well worth considering them – at least as a tie breaker between otherwise “optimal” planets.

For many years, it was thought that a key ingredient of planetary habitability was the presence of a large, Jupiter-like planet, orbiting beyond the HZ. Such a planet, it was argued, would shield an exo-Earth, protecting it from an overly punishing flux of hazardous objects from the outer reaches of the system. In a recent series of papers, we showed conclusively that this idea is, simply, wrong. In fact, giant planets in a planetary system are more of a doubleedged sword: what they give with one hand, in terms of protection from impacts, they can easily take away with the other, by drawing small bodies closer. We described our results in detail in a recent issue of A&G (Horner and Jones 2010b), where the interested reader will find a detailed discussion of this issue. It is certainly the case that the impact regime of any exo-Earths should be considered – dynamical studies of the regimes are relatively straightforward to carry out (although computationally intensive), and would definitely help the selection of the best target for the search for life.

All other things being equal, a system with more debris would deliver a greater impact flux to an exo-Earth. However, not all such systems are equal. Surveys of the sky using infrared telescopes (such as IRAS and Herschel) have A&G • February 2011 • Vol. 52

Astrobiology • Horner, Jones: Exo-Earths revealed debris discs around a wide variety of tem, the Moon is unusually large and massive stars. The discs span a wide range of masses, and relative to its host planet. It is thought to have occur at a large variety of distances from their formed during the latter stages of the Earth’s hosts. Note, however, that the dust we observe accretion, when a Mars-sized object collided, is strongly influenced by a variety of non-gravi- at relatively low velocity, with the proto-Earth. tational forces which result in it being removed Such events are, however, stochastic, and there on astronomically short timescales. Systems that is no guarantee that an exo-Earth would play contain large amounts of dust must therefore host to a similarly large satellite. Does having a have some mechanism by which that dust is large satellite affect the potential habitability? replenished, which suggests that kilometre-sized The supposed beneficial effects of the Moon objects are continually grinding one another can be broken down into two roles – the creadown. This infers either a very large population tion of significant tides (which might have faciliof such objects or some significant instability in a tated the transfer of life from the oceans to the reservoir containing them, resulting in increased land), and the stabilization of the Earth’s axial collision velocities and frequencies. tilt. The first does not, necessarily, appear to Alternatively, one can imagine systems with be a prerequisite – it is easy enough to imagmassive reservoirs of kilometre-sized objects ine a planet with oceans teeming with life, and that remain unperturbed from the dynamically un­inhabited continents, providing sufficient cold orbits (with low mean eccentricities and evidence for a firm detection of life. Furtherinclinations, as in protoplanetary discs) on more, even if the Moon was not present, the which they formed. Such systems would not, Earth would still experience tides from the Sun’s necessarily, show any significant infrared excess, gravitational pull. because their low rates of collisional grinding Secondly, the tilt of the Earth’s axis rocks back would result in little dust, and would experi- and forth by a degree or two – enough variation ence little collisional threat. Systems that show to contribute to the Milankovic´ cycles, but not a huge infrared excess would, at least initially, sufficient to cause catastrophic climate changes. seem to pose more of a collision hazard for exo- The axial tilt of Mars, however, experiences far Earths within them. However, if a debris disc greater excursions, sometimes even reaching (or that is somewhat stirred, and therefore exceeding) 60°. If this happened on Earth, very dusty, is a long way from an the Arctic circle would pass through exo-Earth, and there are no masCairo and south of Shanghai, and Weathering sive planets between the two the Antarctic circle to the north to source material from the of Perth and Santiago, with of material from reservoir to the exo-Earth, significant consequences continents might be then the abundant debris for the climate! It has been important for providing suggested that the Moon will pose little threat. key chemicals for Infrared data on some sysprovides the main reason life in the tems indicate large amounts for the Earth’s axial stability. of hot dust (which therefore Waltham (2006) found that the oceans is close to the host star), and mass of the Moon is remarkably could represent systems in which close to the maximum for which the the impact regime would be inimical to host planet’s axial tilt would be stable. the development of life. It might be that these Above that mass, a large “Moon” would destasystems simply have so much debris left over bilize the spin axis of the planet, potentially that planetary accretion is essentially still in making it less, rather than more, habitable. progress. Alternatively, such systems might While the precise role of giant satellites in be in the process of undergoing a Late Heavy determining planetary habitability is still under Bombardment episode, in which the impact flux debate, it would be premature to write off the through the inner planetary system has under- potential benefits of such a satellite to the develgone a recent significant increase. Such episodes opment of life. That said, we consider that the could be the result of the long-term migration of role of such satellites may well prove less signifithe giant planets in the system destabilizing any cant than many of the other features discussed reservoirs of kilometre-sized objects. in this review.

Information from infrared observations of exo-Earth host systems will no doubt prove Planetary features invaluable in assessing the local impact regime. The presence of liquid water is often considered However, such observations should be used the key ingredient for the development of life with dynamical simulations, to check whether on an exo-Earth, as evidenced by the definition the debris associated with an observed infrared of the HZ. Some models suggest that Earth’s excess truly poses a risk to the planet. water was accreted from local hydrated minerIt has been suggested that the Moon has played als, while others suggest that the source was mat­ a pivotal role in the development of life on Earth. erial that came from beyond the “ice line”, the Compared to the other satellites in our solar sys- boundary in the protoplanetary nebula beyond

‘‘

’’

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which water ice was able to condense. The latter models come in two versions: early and late arrival of ice – see Horner and Jones (2010a) for more information.

The amount of water possessed by exo-Earths will depend on the dynamical processes involved in the formation of systems significantly different to our own. For example, it is feasible that exo-Earths will have formed in systems that also feature a “hot Jupiter” – a Jovian planet orbiting much closer to its parent star than the exo-Earth. Models examining the formation of such planets suggest that the Jovian planet must have formed much further from the Sun, and then migrated inwards, dragging icy materials with it. This could result in exo-Earths that are covered with a planet-wide ocean tens, or even hundreds, of kilometres deep. Would such planets be habitable? It is often suggested that, in addition to planetary oceans, the presence of continental regions also plays an important role in the development of life. As the continents weather, they provide a constant source of minerals and metals that would otherwise rapidly be lost from the ocean. Without these materials, it is suggested, the development of life would be significantly stymied (e.g. Ward and Brownlee 2000). Equally, it is just as feasible that the late giant impacts at the end of planetary formation could strip an exo-Earth of its watery veneer, leaving it as an uninhabitable desert planet.

So, we come to our “ideal” exo-Earth. Clearly, we want to search for a planet that has liquid water on its surface, so orbits within the HZ. However, we probably want to avoid any planet that is too dry, or too wet, and focus on those that are just right.

As noted above, the weathering of material from the continents might play an important role in providing key chemicals for life in the oceans that would otherwise be depleted over time. A good example of this is calcium – a vital ingredient in shells, bones and teeth. Over time, the calcium in the oceans is sequestered as calcium carbonate, and were it not replaced, would eventually be exhausted. The removal of calcium from the oceans also removes carbon dioxide from the atmosphere. If there were no way to return that sequestered gas to the atmosphere, it is probable that continued removal would gradually cool the planet, as the greenhouse effect lessened, until the planet became uninhabitable. It is fortunate, then, that our planet is tectonically active, with plate tectonics acting to return sequestered carbon to the atmosphere (and calcium to its surface).

These examples highlight the importance of tectonic activity in ensuring the ongoing habitability of the Earth. Were plate tectonics absent (as is believed to be the case for Venus and Mars), then the Earth would certainly be a different place. There is evidence that the on­going tectonic activity of our planet is due 1.19

Astrobiology • Horner, Jones: Exo-Earths to the lubricative effects of the Earth’s water on plate motion. Recent research suggests that dry planets would have to be significantly more massive than the Earth in order to support long-term tectonic activity. Without our water, these studies suggest, the Earth would instead exhibit the “stagnant lid” behaviour of Venus and Mars, a significantly less efficient method of cooling the planet’s interior. As we discuss below, the ramifications of such a tectonic scen­ario might stretch beyond the maintenance of a clement climate.

The combined action of plate tectonics (which helps maintain the continental crust) and weathering (which attempts to rub it away) acts to both mediate our climate, and keep the oceans sufficiently nutriment-rich that life can thrive.

Therefore, any exo-Earth should be expected to be tectonically active. This could be determined by long-term observations following the brightness and colour of the planet as a function of time. If the planet has large continents, and broad oceans, it seems likely that it would also be tectonically active (or the continents would weather to nothing).

Tectonic activity also plays a role in the generation of the magnetic field, through the flow of liquid iron in the core. As discussed earlier, all stars expel prodigious amounts of material in the form of their stellar wind and more violent coronal mass ejections. If that material was unimpeded, it would directly interact with the atmospheres of any orbiting exo-Earths, resulting in gradual but unceasing erosion. In addition, the continual rain of fast moving particles from the star would penetrate the atmosphere, doing further damage to the habitability of the planet. Fortunately, the Earth has a relatively strong magnetic field, which acts to protect it from all but the worst vagaries of the solar wind. Without that magnetic field, the flux would be orders of magnitude higher, stripping our atmosphere, irradiating the planet’s surface, and likely destroying the ozone layer which protects us from excessive solar UV radiation.

The role of the magnetic field in preserving a planet’s atmosphere is particularly important when the system is young. Young stars are particularly active, and have powerful stellar winds.

As time goes by, the strength of the stellar wind decreases, and the efficiency with which it could remove a planet’s atmosphere falls away. In the case of Mars, observations suggest that the planet’s dynamo ceased to operate about 4 Gyr ago, after which the atmosphere has slowly been stripped away, leaving today’s tenuous atmo­ sphere, over a thousand times less dense than the Earth’s. By contrast, it is thought that the young Venus had plate tectonics and a magnetic field for long enough to retain its atmosphere when the Sun was young and especially active.

As the planet warmed, due to the Sun gradually becoming more luminous, the oceans would eventually have boiled, leading to the loss of the 1.20

planet’s water, and the shutdown of tectonics.

Outgassing of carbon dioxide from volcanoes would then have increased the atmosphere and warmed the planet further, with strong positive feedbacks. Lundin et al. (2007) suggest these two planets as extreme examples of the effects of loss of magnetic shielding.

It is very likely to be the case that plate tectonics greatly increases the probability that a habitable exo-Earth is, in fact, inhabited. Given that water might be essential for plate tectonics, this strengthens the argument that the first exo-Earths we search for life should be those that have a significant water budget. Without sufficient atmospheric pressure, it is impossible for water to exist as a liquid on the surface of a planet – below a surface pressure of 6.1 milli­ bars, any ice will pass directly to the vapour phase, without ever being stable as a liquid. As the atmospheric pressure increases above this value, the range of temperatures over which water can be liquid increases, such that on Earth, with a mean pressure of 1013 mb, water is liquid between 0 and 100 °C. The atmosphere also plays a central role in determining the temperature required for water to be liquid. Too cold, and the water will freeze out, too warm and it will boil away (and potentially even be lost via photodissociation). Maintaining this balance is not as straightforward as it seems. When life first appeared on the Earth, the Sun was shining with just ~70% its current luminosity. Had the Earth’s atmosphere at that point been the same as that today, the planet would have been frozen solid. Similarly, if the modern Earth had the same atmosphere as it had in the early days of life, then the greenhouse effect would be so severe that our water would have boiled away long ago. As the Sun has brightened, the Earth’s atmosphere has evolved (in part, due to the biosphere) in such a way that the mean surface temperature has enabled most of our planet’s water to be liquid.

Uniquely among the terrestrial planets, Earth’s atmosphere has a major temperature inversion at the top of the troposphere. This inversion helps “cap” the water content of the atmo­ sphere, keeping the great bulk of water vapour below that level. Over the aeons, this effect has prevented the otherwise crippling loss of water due to the vapour reaching sufficiently high altitude that solar UV can photodissociate it. At the Earth’s mass, hydrogen easily escapes from our atmosphere, so once water is dissociated in this manner, it is essentially lost.

What about other planets? Imagine the scen­ ario where Mars, instead of being a fraction of Earth’s mass, was instead somewhat more massive than our planet. As such, the young, wet Mars would have been able to maintain its tectonic activity, keeping up a dynamo that would prevent the loss of its youthful atmosphere. Suppose also that it had a thicker atmosphere than

the Earth’s and a correspondingly greater greenhouse effect. With such an atmosphere to offset the lower insolation received, such a “Mars” would be habitable at the current epoch, despite its position on the very outer edge of the present day HZ; in this scenario our solar system could feature two planets with thriving biospheres, rather than just the one.

These considerations are clearly relevant to choosing exo-Earths that are the best candidates for being habitable, or even inhabited.

Conclusion In the coming years we will for the first time discover potentially habitable planets orbiting distant stars. Despite the fact that it has taken so long for us to find the first, we will rapidly move to a situation where tens, hundreds, or even thousands of such planets are known. Once these planets are found, the scramble to detect the first evidence of life upon them will begin.

Unfortunately, the observations needed to provide a conclusive proof of life beyond the solar system will be lengthy, expensive and arduous; it is almost certain that we will not be able to study more than a small fraction of the planets we discover in sufficient depth to search for life.

As such, it is vital that we direct our efforts with care, and select those planets which represent the most promising targets for a positive detection.

In this review, we present a number of the major factors which are thought to play a role in determining the habitability of exo-Earths. In the coming years, it is imperative that scientists from the varied fields that make up astrobiology come together to prepare a template for “optimal habitability”, to help determine which of the exo-Earths we discover should be the first target for an intensive search for life. We have just a few short years to prepare ourselves for that search, which promises to yield the most exciting result science has ever witnessed – the detection of life beyond the Earth. ● Jonti Horner, Dept of Astrophysics, School of Physics, University of New South Wales, Sydney, Australia. Barrie W Jones, Dept of Physics and Astronomy, Open University, Milton Keynes, UK. References David E-M et al. 2003 Pub. Astron. Soc. Pacific 115 825–836.

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Horner J and Jones B W 2010a Int. J. Astrobiology 9 273 and references therein.

Horner J and Jones B W 2010b A&G 51 6.16–6.20.

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Ward P D and Brownlee D 2000 Rare Earth: Why Complex Life is Uncommon in the Universe (Springer New York).

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Astrobiology • Penny: SETI

SETI: peering into the future 1: Part of the SETI Institute’s 42-telescope Allen Telescope Array (ATA) in California.

(SETI Institute)

T

he first Search for Extra-Terrestrial Intelligence (SETI) project of the modern era was done by Frank Drake in the spring of 1960, using the Green Bank 26 m telescope.

He was looking for narrow-band radio emission from two nearby stars, t Ceti and ε Eri, over a frequency range of 400 kHz near the H i line. Since then there have been six major and many minor searches, made both on specific targets and also over the entire sky. The searches have extended to the optical and infrared, and to search for artefacts in the solar system and beyond. There have also been more than a thousand papers in the scientific press.

The searches have all come up negative. What does this mean? Can future searches extend in a significant way the present area of the “ETI phase space” that has been searched for the existence of extraterrestrial intelligences (ETIs)? This article will briefly describe some of the component parts of SETI and put forward the case that SETI does indeed have an exciting future.

SETI has two main aims. There is the expanding exploration of that phase space, always with the possibility of “contact” and the leap forward in our understanding of life in the universe and in many other fields of science and culture that would result. But SETI also addresses the future of humankind, looking for other civilizations that have trodden this path before us. If we find them, then we will know there is a possible way forward. If a particular SETI search comes up with a negative result, then we know that our A&G • February 2011 • Vol. 52

Fifty years of scanning the skies for signs of ET has produced nothing. But we mustn’t give up, argues Alan Penny, and describes an exciting future for the search for extraterrestrial intelligence. future may not include the path that that search would have revealed.

SETI activity has other components. It involves studies of: how evolution leads from the origin of life to intelligence; the rise and nature of technological civilizations; the problems of communications with fundamentally different entities; the possibilities of interstellar travel. It provides a logical extension to the growing field of astrobiology. Like all high-tech work, it has spin-offs, such as the Berkeley BOINC system of grid computing, originally designed to deal with the flood of SETI data from the Arecibo telescope with the SETI@home project, and which is now used in many fields, including medicine, molecular biology and climatology. And SETI provides a powerful forum for engaging with the public on the nature of scientific studies, using a subject in which the public is already interested.

What do we know about ETI? We know very little for sure. We know from our own example that technological civilizations can arise and persist for thousands of years,

and send both “leakage” and deliberate radio and optical signals of their existence out to the galaxy. We know that our civilization arose in the last 10% of the age of the Earth before the increase in the Sun’s output will render the surface of the Earth uninhabitable. We know that no ETI has left evidence of its existence in any of the searches that have been made, on the Earth, in the solar system, or further afield.

Our existence means that other civilizations could exist, but gives no indication of their probability. Our recent knowledge of extra­terrestrial planets suggests that Earths hospitable to life are common. However, since we do not know how life started, we do not know if life is common, rare, or if the Earth is the single case. Within the next decades the study of the atmospheres of Earth-like planets may resolve this point.

However, we next do not know the probability that once life has started whether it then evolves to a technological civilization. We cannot say that evolution is bound to produce intelligence, and we are unable to predict the nature of other technological civilizations, or how long such civilizations exist. Civilizations thousands, millions, or billions of years older than ours could be of a very different nature to our own. However, the late arrival of intelligence on Earth is most compatible with an average time for the arrival of intelligence being much longer than the lifetime of stars, and thus with us being alone. But this is only a probabilistic pointer, not a proof.

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Astrobiology • Penny: SETI

The lack of evidence of ETI is known as the Fermi Paradox. Once a civilization gets to our stage, it would only be a short time before it could build Von Neuman probes – autonomous self-replicating space probes – whereby every planet in our galaxy could be visited within a few tens of millions of years. The simplest explanation for the fact that we do not see such probes here, and that none of our searches have found signs of ETI, is that we are alone. However, there is no lack of credible alternative explanations of how the existence and even widespread existence of ETI would be compatible with the negative search results. These searches have as yet only explored a very small fraction of ETI phase space. So although there is some indication that we are alone, all we can presently say is that it is possible that ETI is out there, but we cannot with any degree of certainty predict how often ETI arises, or what their natures or lifetimes would be.

The SETI searches The most common way of looking for ETI is to look for narrow-band radio emission. Our civi1.22

lization emits such radiation from the 1 Hz wide carrier beam of analogue TV stations through to the kilohertz wide emissions of such things as airport radars. ETI may also emit such leakage radiation, although present searches are only sensitive to much more powerful radiation than we presently emit. Narrow-band radio waves are also the cheapest and most efficient method of interstellar communication that we know of, and so may be ETIs’ way of communication, and even of signalling their existence to us (“beacons”). The narrow-band signature can also be distinguished from natural sources, even rare natural narrow-band ones such as masers.

The Harvard and Argentinian searches with 26 m telescopes covered the entire sky, and the Arecibo “piggy-back” survey covered some 25% of the sky. But these have integration times of only a minute or so. The SETI Institute among others has done many longer integrations on individual targets such as nearby stars.

Searches have become more powerful as receivers, electronics and data handling and analysis software improve, as for example in the billion 1 Hz spectral resolution channels of the 42-tel-

escope Allen Telescope Array (ATA). The most recent surveys are now a trillion times more capable than Drake’s 1960 observations.

Following the recent development of highpowered lasers, which in theory could be matched with telescopes to outshine the Sun in nanosecond pulses, searches have started to look for such ETI signals in the optical. Pointed observations at Berkeley and Lick and an allsky survey at Harvard are now looking for such nanosecond pulses. Again these are distinguishable from natural sources. If an ETI were using one of our most powerful lasers and a 10 m telescope, these searches would pick them up from hundreds of light-years away. More exotic radiation sources, such as the neutrinos from supernova SN1987A, are also investigated for signs of an artificial nature. The most famous such search was in 1967 when the Cambridge pulsar discovery team checked that the pulses had no sign of orbital motion.

Different searches have different aims, usually based on some sort of premise of the nature of ETIs. The most obvious choice is of nearby long-lived stars, where ETIs on planets have A&G • February 2011 • Vol. 52

Astrobiology • Penny: SETI

2: The view shows the first LOFAR station to be built in the UK at STFC’s Chilbolton Observatory, with the Low Band Array in the foreground. Unlike conventional radio telescopes, there are no moving parts, but steering of the telescope is done in software.

When completed, LOFAR will consist of more than 5000 separate antennas spread in “stations” all over Europe. The project is based in the Netherlands where the core of the array is located. (STFC/SEPnet)

had time to evolve. Such searches range from Drake’s observation of two such stars in 1960, to the million stars planned for ATA. Since stars can differ in ages by billions of years, and ETIs take an unknown time to emerge, a search of a million stars gives a chance of picking up an ETI radiating for a thousand years, which may be a reasonable estimate of the time until an ETI changes into a fundamentally different mode.

Then there are the all-sky surveys and surveys of areas of the sky, such as the galactic centre, where no presumption is made of where ETI is – on or off planets, near or far. These necessarily have shorter integrations per pointing, so are sensitive to rarer but brighter sources.

The extreme of this is surveys of other galaxies, looking for extremely bright sources, but sources so rare that there is not one in our own Milky Way. There are also specialized searches.

A recent proposal is for a search on the ecliptic plane, where an ETI would have been aware for a long time, using the radial velocity and transit planet detection methods, that there is an Earth in orbit around the Sun. Perhaps this would prompt them to signal to us.

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Searches have also been done looking for artefacts of an ETI civilization. The most famous of these are Dyson spheres, where an ETI surrounds a star with solar panels, probably on many discrete mounts, to tap a significant fraction of the star’s energy. The outsides of these panels will be cool, shining in the infrared.

Each new infrared catalogue that comes out is scanned for objects of strange non-natural looking colours. There have been searches for strange colours in the asteroid belt objects which might indicate an artificial nature, and for objects in the unstable Earth–Moon L4 and L5 Lagrangian points. There are notoriously many “sightings” of UFOs, which all have either been explained or have not contained enough information to determine their natures. The most interesting ongoing scientific investigation is the Norwegian Hessdalen Valley Project where there have been repeated sightings.

The main limit on these searches is funding.

There are almost no public funds. Very little sustained work is done outside the US, and within the US the main work is done through private funding and the efforts of determined individu-

als at Berkeley and Harvard. The SETI Institute, which grew out of the NASA work of the 1970s and 80s, is privately funded and the Berkeley and Harvard projects are done from within radio astronomy and electronics groups with university funding and private support. Outside radio and optical searches there is almost no concerted academic work on the other areas of ETI phase space such as solar system searches or catalogue analysis. Theoretical work depends on the intermittent interest of individuals. There is a lack of resources to fund fresh blood.

Theoretical considerations Over the past 50 years there have been hundreds of papers describing the capabilities of searches and suggesting new methods. There have been as many speculating about the existence, origins, lifetimes and natures of ETIs, about composing and decoding messages, the prospects for interstellar travel and many allied matters. There has been much cross-fertilization with other fields including biology, philosophy, spaceship propulsion, linguistics and planetary science.

Is intelligence a convergent property, etc? Some 1.23

Astrobiology • Penny: SETI pointers to this extensive body of literature are given in the “Further reading”.

An important field for SETI is the evolution of intelligence. Once life is started, does it then always evolve to intelligence? Intelligence seems such a useful attribute that evolution would home in on it, but for two billion years bacteria reigned alone. Since then there have been millions of species on Earth, out of which only one, us, has evolved advanced technology. Were we inevitable? Is evolution convergent?

And then there is the “Man from Mars” problem, as it is known in linguistic studies. Can there be ways of communication that are so fundamentally different from our own that the message may be incomprehensible? Concepts such as “signs” and “signifiers” may not be present.

How would a communication system based on smells be coded into a radio message?

A standing controversy is whether it is dangerous to send out signals. In fact any advanced ETI would probably know about us already from our various radio emissions of the past six decades, or from visible signs such as the existence of our cities over the past four thousand years. And because we do not know about the nature of any ETI, a signal might either provoke or forestall an attack by any ill-intentioned ETI. So there is no reason not to transmit. But in any case there is probably presently little point, as signalling for thousands of years would be needed to give the class of ETIs not much more advanced than us a reasonable chance to pick us up. (Only such ETIs would not necessarily know all about us already.)

Peering into the future In studying the future of humankind, we already know that certain classes of ETI, those that our searches would have picked up, are not common.

How much does that tell us about the long-term evolution of civilizations like our own? Will we become a civilization that SETI searches could detect? Will we survive the bottle­necks of the near future: global warming, nuclear war, biological terrorism, grey goo, a catastrophic meteorite strike, the rise of the machines? In the more distant future, will we establish selfsustaining colonies off the Earth that will lessen our vulnerability? In the very distant future, will we become a race that can persist for a million or a billion years? SETI provides an avenue, the only observational avenue presently available to us, for exploring these puzzling questions.

An example of how SETI thinks about our own future is the “Great Filter”. Taking from the Fermi Paradox that advanced ETIs are not common, Hanson (1998) pointed out that in the progress from star formation to such ETIs there must be a limiting pinch point. If this is behind us, then we are one of the extremely rare cases to have got this far, and our future prospects are not limited. But if it is in front of us, then 1.24

we will very probably be extinguished. Paradoxically, discovery of ETIs like us, but not too advanced, would be bad news, as then it must be easy to get as far as us, and the Great Filter must be in front, and quite close.

Contact If we detect an ETI, what would happen next?

First of all, there is the getting out of the news, and present SETI searchers subscribe to the International Academy of Astronautics SETI Permanent Study Group’s “Post-detection Protocol”, which basically says “be sure, have it confirmed, and then spread the news widely”.

No signal should be sent back until inter­national agreement has been reached. In practice, the experience of search groups is that, when investigating ambiguous signals, the news can leak out in an uncontrollable way.

What happens next would depend on the origin and nature of the signal. A solar system detection would have its own possibilities and problems. The result of a radio or optical detection of a distant source would depend on its nature. A continuous narrow-band signal which simply says “I am artificial” would revolutionize the scientific field and trigger funds for a great search for more details. Does it show signs of orbital motion? Is it associated with a star?

It would also trigger public and philosophical interest. It is generally thought that the public would be intensely interested, but would not overreact. However, if there were to be some sort of code seen in the signal, then as well as the scientifically fascinating cryptological and linguistic tasks of finding out what the message is, the public interest would be overwhelming.

Coming from an advanced civilization, does the message tell us how to behave, explain about religions, contain a cure for cancer? Is there some sort of danger in the message? If we respond, how do we have a conversation that may involve time lags of centuries? Astronomers would be interacting with the community in ways that are difficult to envision.

The future Given the negative results so far, is it worth going on? We do not know what ETIs are like, so we cannot say how large the phase space of possible ETIs is and thus we neither know if we are looking in the best way nor what our chances of success are. Many SETI searchers remain optimistic. The quotation from Cocconi and Morrison’s 1959 foundational paper that “The probability of success is difficult to estimate; but if we never search, the chance of success is zero” has many supporters. However, without knowing the nature of ETIs we cannot estimate by how much we improve our chances by any particular SETI search. Within the next decade we should be able to rule out (or discover) leakage radiation similar to our own

from nearby habitable Earths – but the chance of hitting the perhaps thousand-year window for such radiation for a planet millions or billions older or younger than us must be very small.

The author’s personal opinion is that although we cannot know what our chances are it would be a failure of nerve not to go on looking, as long as each new search does cover significant new phase space at a reasonably modest cost.

Planned radio searches will get more powerful, from the privately funded ATA array partly dedicated to SETI searches, to the use of new telescopes such as the European LOw Frequency ARray (LOFAR), for which the author is PI on a SETI Pilot Programme, and the South African 64-dish MeerKAT array, which has recently announced that it wishes to “explore further the potential for SETI”. And there is the giant Square Kilometre Array (SKA), on which funding decisions are imminent, to come. With advanced receivers, electronics and software we are poised for another giant step forward.

A major present limitation is in the electronics and computing and so Moore’s Law predicts an exponential growth in capabilities, especially with wide-band receivers and telescope aperture arrays that lead to simultaneous part- and all-sky coverage. Included with growth in the other types of searches, such as the nanosecondpulse optical surveys, new solar system surveys, searches of new catalogues, and investigations into unexplained phenomena, the next decade is poised with possibilities to greatly extend the ETI phase space exploration.

Presently we are limited by the almost total lack of public funding. When I speak to the public about SETI and tell them that almost none of their taxes supporting astronomy goes to SETI they are amazed that such an interesting field is being ignored. If the panels of the astronomy funding agencies were to decide to fund SETI at a level of just one half of one percent of their budgets, SETI would be transformed, and much more powerful and wide-ranging searches could be done. That would be an inspiring thought for us all – that we were taking the search seriously and in this journey into the unknown the human race is truly looking outward. ● Alan Penny, University of St Andrews, UK. References Cocconi G and Morrison P 1959 Nature 184 844.

Hanson R 1998 The Great Filter – Are We Almost Past it? http://hanson.gmu.edu/greatfilter.html Further reading Barrow J, Tipler F, Wheeler J 1988 The Anthropic Cosmological Principle (Oxford Paperbacks).

Davies P 2010 The Eerie Silence (Houghton Mifflin Harcourt).

Ekers R et al. 2002 SETI2020: A Roadmap for the Search for Extraterrestrial Intelligence (SETI Press).

Tarter J 2001 The search for extraterrestrial intelligence Ann. Rev. Astron. & Astrophys. 39 511.

Webb S 2002 Where is Everybody? (Springer).

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Astrobiology • Dartnell: Extremophiles

Biological constraints on habitability Lewis Dartnell discusses how extremophiles have pushed the survival envelope of terrestrial life – and what this means for the possibility of extraterrestrial life.

S

o far, the search for life beyond Earth has largely focused on the search for bodies of liquid water, ancient or current. While all life on Earth fundamentally relies on liq­ uid water as the biosolvent, finding wet extra­ terrestrial environments, or at least locales exhibiting signs of ancient water, is only the first step. Liquid water is a necessary but not suffi­ cient prerequisite for life. Multiple physical and chemical parameters further limit the suitability of a wet environment to support life. It is these factors that define the ultimate habitability of an environment, the volume of physicochemical parameter space that can be tolerated by living organisms. So the study of terrestrial organisms that can survive on the extreme boundary of these conditions, the so-called extremophiles, greatly informs astrobiology and the search for life beyond Earth.

Given there is no “normal” or average envi­ ronment on Earth, and we don’t yet know the physical and chemical conditions under which life first arose, it is difficult to set a baseline from which to define an “extreme” environmental condition. Out of necessity, extreme conditions are taken to be those that deviate from what our mesophile cells could tolerate, chauvinistic though this may seem! “Extreme” is in the eye of the beholder, and for bacterial cells thriv­ ing in the hot anaerobic deep environment of marine crust, it is our kinds of life, lumbering around in the cold, low-pressure, oxygen- and radiation-bathed environment of the planetary surface, that are the evolutionary freaks.

This article presents a short overview of the main physical and chemical parameters that affect an environment, and the extremophilic organisms found flourishing in the greatest extremes of each. Taken together, these maxi­ mum ranges for biology on our planet describe the survival envelope of terrestrial life, and they inform the science of astrobiology. What are the A&G • February 2011 • Vol. 52

limits of terrestrial habitability and what do they mean for the possibility of life beyond Earth?

Desiccation While liquid water is absolutely essential for the growth and reproduction of all terrestrial life, certain organisms can tolerate periods of extreme desiccation: the xerophiles. They sur­ vive by entering a state of anhydrobiosis, in which minimal water remains and cells’ meta­ bolic activity enters dormancy. Cases of anhydro­ biosis have been discovered within bacteria, yeast, fungi, plants and animals. When envi­ ronmental conditions deteriorate, tardigrades, or “water bears”, enter a desiccated, dormant tun stage, retracting their legs into their bodies.

In this anhydrobiotic state they can tolerate very high levels of radiation, vacuum exposure, and temperatures between –253 °C and 151 °C.

Temperature Heat-loving organisms are termed thermophiles, with the most extreme cases named hyper­ thermophiles (optimum growth at temperatures above 80 °C). High temperatures denature pro­ teins and nucleic acids. As the temperature rises, the thermal vibration and flexing of these poly­ mers increases until the non-covalent chemical bonds are overcome and the biomolecule begins to lose its precise functional structure. For soluble proteins, water molecules gain access to the hydrophobic core and the macromolecule precipitates out of solution. Membrane fluidity also increases with temperature, until it loses its barrier function and the cell can no longer regulate its own internal composition. Chloro­ phyll is only stable up to 75 °C, so photosynthesis does not take place in hyperthermophile envi­ ronments. The most hyperthermophilic organ­ isms are members of the archaea, with one strain growing at up to 121 °C. Bacteria manage up to around 90 °C, while eukaryotic cells cannot cope

much beyond 60 °C. Adaption to hot environ­ ments involves regulating membrane fluidity and incorporating more amino acids that are able to form non-covalent bonds in order to reinforce the conformational structure of the proteins.

Psychrophile organisms (optimum growth at less than 15 °C) in very cold environments face the opposite problem. At low temperatures, thermal activity is so diminished that proteins become too rigid and inflexible and they cannot function as effective catalysts. Such environ­ ments are encountered in the deep sea, Antarc­ tic and Arctic marine environments (including surface waters and sea ice), and glaciers world­ wide. Veins of water within marine ice remain liquid down to –35 °C because of the high con­ centration of dissolved salts as the surrounding sea­water freezes, and thin films of water remain fluid around mineral grain boundaries to even lower temperatures. Freezing water further endangers organisms through ice crystal growth and cellular rupture. Solubility of gases in water at low temperature can also be a limiting factor, particularly for organisms requiring oxygen or carbon dioxide for growth. Claims have been made for microorganism metabolism and growth at temperatures of –20 °C or lower, but such cold environments result in exceedingly slow reaction kinetics; even with cold-adapted enzymes, cellu­ lar metabolism proceeds at a glacial place. This has frustrated efforts to reliably detect evidence of growth at such low temperatures.

Neutral, acid or alkali The pH scale measures the concentration of hydrogen ions (H+, protons) in solution. A pH of 7 denotes neutrality, solutions with a pH value less than 7 are acidic and those greater than 7 are alkaline.

As with high temperatures, very acidic con­ ditions act to denature proteins and hydrolyse cellular components and so are extremely haz­ ardous to life. Acidic environments include geo­ thermal pools with a low pH from dissolved volcanic gases, such as Grand Prismatic Spring, USA (figure 1), and the run-off streams from mines such as Rio Tinto in Spain. Organisms able to tolerate such conditions are termed acido­philes, and members of all three domains of life, archaea, bacteria and eukarya, have been discovered to tolerate conditions down to pH 0. The acidophilic champions are currently archaea in the genus Picrophilus, which can tol­ erate a pH of –0.2. Certain acidic environments, such as acid mine drainages, are also rich in dis­ solved metal ions (due to the high solubility of metals in acidic water), and so cells must also contend with the toxicity of high concentrations of copper, arsenic, cadmium and zinc.

Organisms living in the opposite extreme, at very high pH, are termed alkaliphiles and are found in environments such as the soda lakes of East Africa or North America. Figure 2 shows 1.25

Astrobiology • Dartnell: Extremophiles Lake Magadi, a soda lake in Kenya. Extreme alkalinity presents a survival challenge because of the paucity of hydrogen ions in the solution.

Cellular metabolism transduces energy by trans­ porting hydrogen ions across the cell membrane to create an electrochemical gradient. These protons are then allowed to flow back in again through the enzyme ATP synthase, which gener­ ates molecules of ATP as a chemical energy store for the cell. In extremely alkaline environments, however, the low availability of hydrogen ions for this crucial bioenergetic process becomes limiting. The most alkaliphilic organisms are cyanobacteria able to grow at pH 12–13.

Both acidophiles and alkaliphiles survive by regulating their internal pH to that of meso­ philic relatives – near neutral – and so do not require molecular adaptations to pH extremes in their internal cellular components. This is achieved by both active pumping of protons across the cell membrane (outwards for acido­ philes, inwards for alkaliphiles), and passive mechanisms including negative surface charges on alkaliphiles (to enhance scavenging of H+) and positive charges on acidophiles.

Salinity High salinity reduces the water activity of a solution and disrupts the distribution of charges around macromolecules such as proteins and DNA, forcing them to denature and fall out of solution. Organisms able to flourish in highsalinity environments, such as evaporitic lakes, are termed halophiles and representatives are found in each of the three domains of life. Halo­ philes exhibit one of two survival strategies to cope with their environment. The first, the “saltin” approach, maintains intracellular salt con­ centrations comparable to the external solution (and thus controlling the loss of water from the cell by osmosis), but requires all cellular systems to be adapted to high salinity. The second strat­ egy, “compatible solute”, relies on excluding ions such as Na+ from the cytoplasm by active transport across the membrane, and intracellu­ lar accumulation of organic osmolytes such as glutamate, glycine or trehalose, which preserve the osmotic balance with the environment but are not themselves toxic. This strategy is more energetically expensive, requiring constant active maintenance of intracellular conditions, but does not necessitate extensive adaptation of all intracellular machinery to high salinity.

Environments exposing inhabitants to extremes of salinity are often desiccating (as evaporation and water loss is often the mechanism for con­ centrating dissolved salts) and so organisms must exhibit high tolerance to both stresses.

Pressure High pressure poses several challenges to sur­ vival for piezophilic (also called barophilic) organisms. High pressure compresses the 1.26

Acidic and alkaline environments 1

packing of lipids in cellular membranes, and so restricts membrane fluidity, giving a similar outcome to low temperatures. Many organisms respond to this by increasing the proportion of unsaturated fatty acids in the composition of their membranes. Secondly, by Le Châtelier’s principle, high pressure causes a shift in the equilibrium of chemical reactions that involve a change in volume; notably in the consump­ tion or production of gases. Thus, biochemical reactions that produce an increase in volume are inhibited by high-pressure environments and piezophilic organisms must adapt to this.

Natural high-pressure environments on the Earth include deep lakes and seas, or the subsurface. Pressure increases at a rate of 10.5 kPa per metre depth in water (hydrostatic pressure), while lithostatic pressure increases at over twice the rate, 22.6 kPa per metre beneath the Earth’s surface. The greatest pressure in the Earth’s oceans is at the bottom of the Mariana Trench, at just over 11 km depth, corresponding to ~110 MPa water pressure (over a thousand times that at sealevel). Organisms were found even at this crushing depth, many of which were obli­ gate piezophiles only able to grow at pressures above 50 MPa, but many isolates were found to grow at standard temperature and pressure.

Microbiological sampling has been performed from a borehole 5.3 km deep in granitic rock in Sweden, discovering a population of anaerobic thermophilic bacteria. Lithostatic pressure at this depth would be around 55 MPa, far less than the Mariana Trench. The ambient tem­ perature at 5.3 km depth was already around 70 °C (and the thermal gradient in oceanic crust is even greater than that in old continental crust), so the limit for life at depth in the Earth’s crust is largely defined not by pressure but by temperature – the hardiest piezophiles are likely to reside in the deep cold oceans rather than the planetary crust.

On the other hand, atmospheric pressure diminishes with increasing altitude above the Earth’s surface. The limiting factor for growth at high altitude, however, is not reduced pres­ sure but the freezing temperatures atop moun­ tains. As pressures drop very low, towards vacuum, water sublimes and organisms become desiccated. Different exposure experiments on space missions have found that organisms, particularly those in a dormant or spore state, are able to survive the vacuum and consequent desiccation of the space environment, provided they receive adequate shielding from solar ultra­ violet radiation. Indeed, freeze-drying or lyopi­ lization, is a standard laboratory procedure for preserving microbial samples for storage.

Radiation Radiation that affects the survival of organ­ isms comes in a variety of different forms, from short-wavelength electromagnetic waves, such as ultraviolet, X-ray and gamma-ray, to energetic subatomic particles such as the alpha and beta emissions of radioactive decay (helium nuclei and electrons, respectively) or the accelerated protons and heavier atomic nuclei that constitute cosmic rays. Ultraviolet radiation is absorbed by proteins and DNA to drive photochemistry, leading to photolysis or base dimerization and subsequent mutations.

The destructive effects of the other radiation types are primarily through ionization: liberat­ ing electrons and breaking covalent bonds, and so damaging biomolecules through both direct radiolysis and free radical chemistry.

Protection against ultraviolet radiation is afforded by UV-screening pigments such as carotenoids, or by living endolithically (within cracks or between mineral grains of surface rocks), as do microbial communities in the Antarctic Dry Valleys. Organisms cannot shield themselves against the flux of ionizing radia­ A&G • February 2011 • Vol. 52

Astrobiology • Dartnell: Extremophiles of the MgCl2 chemocline in the Discovery basin of the Mediterranean. It is the chaotropicity of these environments that limits their habitability, more so than their salinity.

2

Polyextremophiles

1: Grand Prismatic Spring, a geothermal pool in Yellowstone Park, USA, populated by acidophile thermophiles.

2: Lake Magadi, a highly saline soda lake in Kenya inhabited by halophilic alkaliphilic organisms. (Courtesy of Lottie Davis)

tion, they can only repair cellular damage once faster growth rates during irradiation and thus it has been inflicted. Deinococcus radiodurans, purportedly able to harvest energy delivered by shown in figure 3, is able to survive gamma-ray ionizing radiation (see the further reading sec­ doses of up to 5000 Grays without measurable tion below for references). loss of viability. Such doses are far higher than ever experienced in the natural environment, Chaotropicity and radiation resistance is in fact thought to be A further factor limiting environmental habit­ a spin-off of desiccation resistance: intra­cellular ability has come to light recently, in addition drying-out produces similar DNA fragmen­ to the physicochemical parameters listed tation and protein oxidation damage above that are most often discussed as does ionizing radiation. The in terms of the survival limits survival mechanism employed of terrestrial life. High salt

by D. radiodurans is only concentrations reduce the recently being elucidated water activity of a solution as the protection of cel­ and desiccate and stress a lular proteins from cell in the ways described radiation-induced oxi­ here under the section on dation; safeguarding the salinity. But the growthproteome so that repair limiting effect of certain enzymes can subsequently salts seems to extend reconstitute the fragmented beyond their role in reduc­ DNA (Daly 2009). ing the water activity. MgCl 2 The strict definition of an is additionally chaotropic 3: Scanning electron microscope image extremophile is an organ­ in nature, meaning that of tetrad clusters of the bacterium ism that requires a particu­ it exhibits a tendency to Deinococcus radiodurans, the most lar environmental extreme radiation-resistant organism on Earth. destabilize biological mac­ in order to grow; a ther­ romolecules (Hallsworth mophile with thermostable adapted enzymes et al. 2007). MgCl2 is even more destabilizing that can only grow above 70 °C, or a halo­ than ethanol and urea that are commonly used phile that needs 1.5 M NaCl to maintain their in laboratories to disrupt molecular structures structural integrity, for example. In this sense, or prevent biological activity. xerophiles are not true extremophiles in that Natural environments where this effect they do not require near-complete desiccation becomes significant include the briny water of for growth; they are merely able to tolerate it by the Dead Sea, which is dominated by MgCl 2 entering a metabolically dormant state. Simi­ rather than NaCl or KCl salts, and even more larly, there are no known examples of organisms extreme cases are provided by the deep anoxic that actually require high dose rates of radiation basins at the bottom of the Mediterranean Sea, in order to grow, just those, like D. radiodurans, where MgCl2 concentrations can approach sat­ that are able to repair the molecular devasta­ uration. While inhabited by microorganisms, tion wrought by irradiation. However, there actual microbial reproduction in Dead Sea brine have been some, as yet unconfirmed, reports has not been demonstrated, and no microbial of cyanobacteria, algae and fungi exhibiting activity at all could be detected at the bottom A&G • February 2011 • Vol. 52

While each of these environmental factors lim­ its biology in different ways, it is important to stress that rarely are physicochemical extremes encountered in isolation. In their natural habi­ tat, extremophiles must often tolerate several extremes in combination. For example, organ­ isms living in Grand Prismatic Spring in Yel­ lowstone Park, USA (figure 1), must tolerate both high water temperatures from geothermal heating and low pH from dissolved volcanic gases; inhabitants here are both thermophilic and acidophilic. Similarly, an environment such as the surface mineral soils of the Antarc­ tic Dry Valleys requires tolerance to a combi­ nation of low temperatures (and indeed often great swings in temperature due to variability in winds and insolation), desiccation, ultraviolet radiation, and often high salinity. Thus, many extremophiles are in fact better termed “poly­ extremophilic” to describe their resilience to several stressors simultaneously.

Survival envelope of terrestrial life A habitable environment is, by definition, one that can support biology. The limits of habit­ ability are thus delineated by the maximum ranges of conditions that life can tolerate: the domain of the extremophiles. In addition to the seven listed environmental parameters that life must contend with, there are many other physico­chemical factors that can be limiting such as concentrations of different chemicals including free oxygen or heavy metal ions.

The three most crucial environmental factors that limit biology are commonly taken to be temperature, pH and salinity. Figure 4 displays a plot of the ranges of conditions that known extremophiles can tolerate. The green bound­ ary is the survival envelope of life on Earth (in temperature–pH–salinity space), and at any point within the green volume an organism has been discovered flourishing in that particular combination of hostile conditions. The domain of the mesophiles – non-extremophilic organ­ isms such as H. sapiens – lies at the base of this survival envelope: conditions of low–moderate temperature and salinity, and pH neutrality.

Polyextremophiles are situated at the very corners of this survival envelope, such as the archaeon Sulfolobus acidocaldarius thriving at 80 °C, pH 3 and low salinity. In fact, this figure is slightly out of date and the currently known survival envelope has expanded a little beyond the limits shown here – for instance, an archaeal strain has since been reported to grow at up to 121 °C, slightly extending the “toe” of this boot­-shaped envelope.

1.27

Astrobiology • Dartnell: Extremophiles

Limits on habitability The furthest extent of environmental param­ eter space that is theoretically habitable is delin­ eated by physical conditions that permit flowing water and stability of organic macromolecules.

Pure water is liquid between 0 °C and 100 °C under standard conditions, but the freezing point can be depressed by dissolved salts and the boiling point delayed by higher pressures.

Hydro­thermal vents on the ocean floor pump out water at up to around 400 °C, prevented from boiling by the overbearing pressure of the water column, but above about 150 °C organic molecules start to fall apart. So the actual upper survival limit for temperature is defined by chemical constraints and the molecular stabil­ ity of the components of life.

A meaningful exercise is to consider diverse extremophiles on the outer fringes of the sur­ vival envelope of terrestrial life; to identify regions of this parameter space where life appears to be right up against the theoretical maximum limit, and other parameter combi­ nations where terrestrial life could perhaps be improved upon. For example, the hottest limit recorded for growth is 121 °C, with survival for short periods reported even at 130 °C, which isn’t far off the theoretical maximum prescribed by organic molecule stability. On the other hand, no examples of terrestrial acidophilic psychrophiolic organisms are known, yet there is no major physical or chemical constraint on why this may be. The reason may simply be a consequence of the lack of such environments on Earth, and so biology has had no opportunity to adapt to such a combination. Perhaps, provided with the appropriate environmental conditions, extraterrestrial life would encounter no bar­ rier to extending its habitability range into the region of low temperature and low pH. The most notable gap in the extent of ter­ restrial life shown in figure 4 is the wide space above the “toe” of the boot-shaped survival envelope. This region of the parameter space corresponds to conditions of high temperature and high salinity. Why does biology appear not to tolerate such a combination? Is it that very hot, salty water presents some form of insur­ 1.28

32 salinity (%w/v)

The extent of this survival envelope of ter­ restrial life is staggering, protruding into hos­ tile environments that had previously been thought to be completely sterile. These dis­ coveries in terrestrial extremophile research have really encouraged optimism in the possi­ bility of similarly hardy life in extraterrestrial environments.

But the question remains: why doesn’t life on Earth bulge even further within this parameter space? Why are certain regions of this physico­ chemical parameter space not populated by terrestrial organisms? Why has biology not colonized these apparent gaps?

24 16 8 0

0

12

40 80 120 temperature (°C)

8 4 pH

4: A slightly dated figure depicting the survival envelope of terrestrial life. Three environmental parameters are considered here – pH, temperature and salinity – and the tolerance ranges of known extremophiles are plotted to produce this boot-shaped volume. (Courtesy of Julian Wimpenny)

mountable challenge or stress to survival, per­ haps by compromising membrane integrity or destabilizing DNA? Or alternatively, does ter­ restrial life not occupy that region of parameter space simply because such a niche is not avail­ able in the natural environment? Has terrestrial life simply lacked the opportunity to adapt to such a combination? Cold salty environments exist, such as in Antarctic lakes or the briny veins within sea ice; indeed, it is the high salinity that allows the water to remain liquid, and thus able to sustain active life, at low temperatures.

But where in the natural environment would hypersaline solutions become superheated?

So a crucial topic in extremophile research is what defines the boundaries of the survival envelope of terrestrial life. Are the limits deter­ mined by genuine biological restrictions (such as the degradation of organic molecules), which may be shared with extraterrestrial organisms, or are they more a consequence of the available range of environments on Earth to which life has had the opportunity to adapt, and so per­ haps idiosyncratic to terrestrial life?

Origin of life There is a final key point on extremophiles and what they reveal about the ranges of hab­ itability and possibility of life beyond Earth.

Extremophiles demonstrate the incredible adaptability of life once it has arisen, but make no statement about the likelihood of life aris­ ing in the first place. It is almost certain that the physicochemical conditions able to nurture emerging self-organizing networks of prebiotic chemistry are much more tightly constrained than the conditions for survival of cells, pro­ tected within a membrane barrier and able to regulate internal conditions.

We can derive hints of the environmental con­ ditions in which the universal common ancestor

of terrestrial life lived (which presumably are similar to the conditions for prebiotic chemistry leading to the first cells). Organisms that lie at the root of the three-domained phylogenetic tree of life on Earth, the organisms alive today that appear most closely related to all others, are all thermophiles, suggesting that the habitat of the earliest life was hot and not very salty (which fits with expectations of the environment of the pri­ mordial Earth). Additionally, the fact that both acidophiles and alkaliphiles actively maintain their cytoplasm near neutrality suggests that the universal ancestor lived in waters neither very acidic nor alkaline. So it seems likely that the first life on Earth occupied an environmental niche somewhere within the toe of the bootshaped envelope in figure 4 – hot, neutral and not particularly salty – and has since diversified and expanded outwards from this point to fill the whole survival envelope of modern life.

But if the environment of another world, such as Mars or Europa that are championed as potential habitats for extraterrestrial life, can­ not provide the right combination of physico­ chemical conditions for the emergence of life in the first place, then the astounding extent of habitability revealed by the extremophiles is largely irrelevant.

There are, therefore, two major challenges that remain in fully defining the biological con­ straints on habitability, and thus which extra­ terrestrial locales offer the best hopes for life.

Firstly, it is key to characterize the theoretical limits for supporting biological processes as distinct from the limits exhibited by terrestrial extremophiles, which may be idiosyncratic to the Earth and its repertoire of habitats available for life to adapt to.

Secondly, researchers in the field of prebiotic chemistry can provide insights into the neces­ sary physical and chemical conditions for the original synthesis of life, from which starting point organisms can adapt to the full survival range of terrestrial extremo­philes. ● Dr Lewis Dartnell, The Centre for Planetary Sciences at UCL/Birkbeck, University College London, UK; l.dartnell@ucl.ac.uk; http://www.lewisdartnell.com Further reading Dadachova E et al. 2007 PLoS ONE 2 (5) 457.

Daly M J 2009 Nat. Rev. Microbiol. 7 (3) 237–45.

Deming J W 2002 Current Opinion in Microbiology 5 (3) 301–309.

Hallsworth J E et al. 2007 Environmental Microbiology 9 (3) 801–813.

Kminek G et al. 2010 Advances in Space Research 46 (6) 811–829.

Luckey T D 2008 21st Century Science & Technology Fall–Winter 4–6.

Pikuta E V et al. 2007 Critical Reviews in Microbiology 33 (3) 183–209.

Rothschild L J and Mancinelli R L 2001 Nature 409 1092–1101.

A&G • February 2011 • Vol. 52

Astrobiology • Kee et al.: Astrobiology Society of Britain

The Astrobiology Society of Britain Terence P Kee, Mark J Burchell and David A Waltham outline the network at the heart of this young but rapidly growing field in the UK.

T

he Astrobiology Society of Britain was founded in 2003 as an organization affiliated to the Royal Astronomical Society (RAS) and which promotes the study and wider dissemination of astrobiology to its membership (which currently exceeds 100) and the astrobiology community as a whole (Burchell and Edwards 2004). Essentially, the ASB acts to bring people with complementary interests and expertise together both for research and for educational purposes.

Who does astrobiology?

In 2009, one of us (MJB) along with ASB Committee Secretary Lewis Dartnell, published the results of a survey of astrobiological teaching and research activity in the UK (Dartnell and Burchell 2009). This survey, using data collected over one year, provides a snapshot of activities in the years 2007–8. More than 800 students were taking university-level courses with a significant proportion of astrobiological content, and more than 250 researchers reported themselves active in the field, across more than 30 research teams. Astrobiology now stands as a significant scientific discipline, perhaps more so at the upper, research-directed end than teaching at present, but that may logically reflect the cross-disciplinary nature of the field and lack of direct student exposure at pre-university level, as with nanotechnology and nanoscience.

Consequently, many of the current ASB activities focus on research, training and outreach.

Central to these activities are the biannual ASB conferences, summer schools in astrobiology and themed workshops. Following the first conference at Cambridge in 2003, whose proceedings were published in Burchell and Edwards 2004 and papers in the Int. J. of Astrobiology 3(2) 2004, further ASB conferences have been held at the universities of Kent in 2006 (Burchell 2006 with papers in the Int. J. Astrobiology 5(3–4) 2006), Cardiff in 2008 (Burchell 2009a with papers in the Int. J. Astrobiology 8(1) 2009), and Royal Holloway in 2010 (Burchell 2010 with papers in the Int. J. Astrobiology 9(4) 2010). In total, the proceedings of these four meetings now contain around 50 original, refereed papers. Each meeting was arranged A&G • February 2011 • Vol. 52

under the umbrella of an overarching theme, but also covered all key aspects of astrobiological research. The ASB has been extremely fortunate to enjoy the support of the UK government Science and Technology Facilities Council (STFC).

STFC has supported the society’s conferences as well as funding its own biennial summer school for training new astrobiology research students from across the UK. Events at the Open University (2007) and the University of Kent (2009) saw more than 40 postgraduate students engage in astro­biological topics from astro­physics, through prebiotic chemistry to extremo­philic microbiology. For 2011, we are considering introducing national ASB Workshops in Astrobiology, designed to be smaller, more focused meetings than our biannual conference. These workshops will allow groups of between 10 and 30 researchers to meet to discuss recent developments in key astrobiological topics, producing reports for the ASB website.

Connecting the community As well as these major organized activities, the society issues a quarterly newsletter to its members and runs a website which draws about 4000 hits per week. The website includes book reviews (of astrobiology-related titles) and recent news items. The volume of traffic through the site indicates an increasing level of interest in the subject. Committee members hear concerns finding places to undertake PhDs; in this respect the survey paper by Dartnell and Burchell 2009 is a useful tool, listing as it does many of the active UK groups. Committee members also distribute leaflets about astro­biology at meetings and outreach events (e.g. Café Scientifiques and local astronomical societies) and at major conferences distribute copies of the proceedings of past ASB conferences. To help popularize the subject, articles are written for science magazines (e.g. Burchell and Dartnell 2009 in A&G, Burchell 2009b).

Internationally, the ASB is a member of the wider, global community of astrobiological societies. We are a member of the European Astrobiological Network Association (EANA) and an affiliated partner of the NASA Astrobiological Institute (NAI). ASB members are

regular contributors to the annual EANA conferences and many ASB members attend the biannual NAI-supported AbSciCon meetings; this helps the profile of UK astrobiology work on the international stage.

Big questions There are few human questions as fundamental as “How did we come to exist?” and “Are we alone in the universe?” Thousands of years of philosophical enquiry and more than 200 years of scientific study demonstrate just how difficult such problems are to address. In a way we need to ask ourselves two further questions: “Do these problems actually have answers?” and, if so, “Are these answers scientifically tractable?”

We believe that the answer to both these questions is yes and that the key to getting closer to answers is by employing new ways of thinking.

This lies at the heart of UK, European and US space exploration efforts including programmes such as the joint ESA–NASA ExoMars mission to search for signs of extinct or extant life on our cousin planet. Irrespective of whether we find life (or evidence of past life) on Mars, the emerging subject of astrobiology does, we believe, provide a new framework and driver for developments in astronomy, microbiology and planetary geology, and may shed light on how life could emerge. Ultimately, astrobiology may indeed be seen to provide more than simply the sum of its parts. ● Terence P Kee, School of Chemistry, University of Leeds, UK; Mark J Burchell, School of Physical Sciences, The University of Kent, UK; David A Waltham, Royal Holloway College, University of London, UK. The authors are all committee members of the Astrobiology Society of Britain and wish to extend their thanks to the Science and Technologies Facilities Council for its gracious and continuing support of the society since its inception.

More information Astrobiology Society of Britain http://www.astrobiologysociety.org European Astrobiology Network (EANA) http://www.astrobiologia.pl/eana NASA Astrobiology Institute http://astrobiology.nasa.gov/nai Royal Astronomical Society http://www.ras.org.uk Science and Technologies Facilities Council (STFC) http://www.stfc.ac.uk References Burchell M J 2006 Int. J. Astrobiology 5(3) 181.

Burchell M J 2009a Int. J. Astrobiology 8(1) 1–2.

Burchell M J 2009b Significance (Royal Statistical Society) 6(3) 142–144.

Burchell M J 2010 Int. J. Astrobiology 9(4) 191–192.

Burchell M J and Dartnell L R 2009 A&G 50(4) 27–30.

Burchell M J and Edwards H G M 2004 Int. J. Astrobiology 3(2) 71–71.

Dartnell L R and Burchell M J 2009 Astrobiology 9(8) 717–730.

1.29

Astrobiology • Apagyi, Burchell: Astronautics

Does astrobiology inclu Katinka Apagyi and Mark J Burchell argue that aspects of astronautics overlap with astrobiology, in the same way that aspects of geophysics and planetary science do. The gap between these disciplines is an artificial separation that should be overcome.

A

strobiology is a growing field of schol­ arly activity worldwide. It is multi­ disciplinary in nature, and encompasses a wide range of topics drawn from other wellestablished academic disciplines (for a recent discussion see Burchell and Dartnell 2009).

However, while the relation of biology, chem­ istry, planetary science, etc, to astrobiology is readily apparent and often realized in collabora­ tive work across these disciplines, the relation of the field to astronautics is not so clear. To take the UK as an example, a recent survey of univer­ sity-based academics possibly active in the field of astrobiology found no respondents describing themselves as active in astronautics (Dartnell and Burchell 2009). It could be argued that such a survey might reveal as much about the biases of the authors as it does about the topic being surveyed, but this lack is also apparent at many conferences in the field. This split between astrobiology and astronautics is reflected in a wider split in the science community between those who favour robotic exploration of space and those who promote the use of astronauts.

The issue that arises is thus: is there any over­ lap between the two fields of astrobiology and astronautics? Since one of the authors has a space science background (MJB) and the other in astronautics (KA), we partially answer our own question, and it is the degree of overlap that is discussed below. Included in this discussion is also a consideration of the benefits of collabora­ tion between the two communities.

What is astrobiology?

As evident from the other papers in this issue, astrobiology is a broad discipline. It addresses the origin or origins, distribution and develop­ ment of life from a broader perspective than that of a single planet, i.e. it considers life as a cosmic phenomenon, but often does so with an acknowledgement that Earth is our sole example of a body containing life. Astrobiology is thus concerned with many different aspects of study.

As well as those people searching for evidence of life itself, other researchers are also poten­ tially astrobiologists. For example, astronomers who search for extrasolar planets are potential astrobiologists. Cometary scientists observing the presence of complex organic materials on comets are potential astro­biologists. Biologists 1.30

studying how life adapts to extreme environ­ ments on Earth are potentially contributing to the field, as are chemists who try to understand the pathways needed for the development of the complex organics needed for life, and so on.

Equally, scientists and engineers who design space vehicles or apparatus to be deployed on other solar system bodies, for example, are also potential astrobiologists depending on the type of research enabled by their efforts. Moving outside science, academics who consider the impact on mankind of the possible discovery of alien life are also working in an area related to astrobiology. This goes well beyond the obvious concerns of planetary protection – where we worry about impact on biospheres of transfer­ ring life to new potential habitats – and intro­ duces social aspects to the field. In general we need to remember that science does not exist in a vacuum apart from society, and those who consider the potential impact of astrobiology on society are clearly participating in the develop­ ment of the field of astrobiology.

One can get a feel for how academics in general see astrobiology, by looking at their manifestos for the discipline, i.e. the various roadmaps that occasionally appear. The most famous roadmap is probably that of NASA. Several versions have appeared – Des Marais et al. 2003 is an early version and Des Marais et al. 2008 is current.

In the later version, seven goals are set for astro­ biology. We can immediately note that in them­ selves the seven headline goals (table 1), which involve the use of space for collecting samples from other planets, delivering instruments to other solar system bodies, making astronomical observations from space telescopes, etc, do not specifically describe activities that are “astro­ nautical” in nature – i.e. they neither require nor necessarily involve human activity in space.

One possible area where humans are directly mentioned in the 2008 version of the NASA Astrobiology Roadmap is with regard to the potential future expansion of mankind beyond the Earth. The roadmap considers that micro­ organisms will play a significant role in lifesupport systems or resource acquisition, and therefore the ability of micro-organisms to adapt and survive in space should be explored as part of an astrobiology programme. But beyond this narrow approach, no wider consideration

Astronauts at work on the Hubble Space Telescope.

A&G • February 2011 • Vol. 52

Astrobiology • Apagyi, Burchell: Astronautics

de human space flight? is given to the relevance of human activity in space to astrobiology. It is thus clear that from a mainstream astrobiology viewpoint, there is a disconnection from astronautics. To understand if this is sensible we need to ask a further ques­ tion about the nature of astronautics.

What is astronautics?

Astronautics, similarly to astrobiology, is a highly multidisciplinary subject, encompassing both the science and the technology of manned and unmanned space flight. It is essentially a very large engineering job, requiring input from both biological and physical research, and in return providing new and improved platforms for further scientific experiments, as well as flight equipment to be used by the public and the private sector.

While astrobiology is the legacy of the late 20th century, the roots of astronautics go back to the 17th century when Newton first outlined some of the mathematical basis of space travel.

In 1903, Tsiolkovskii, who became known as the father of astronautics, published his famous rocket equation, which predicts the final veloc­ ity of a rocket, calculated from the known vari­ ables of the mass of the rocket, and the mass and the exhaust velocity of the propellant.

In the early 20th century, this so far civilian and purely theoretical enterprise was largely taken over by the military in the USA, Germany and the USSR. Robert Goddard, a pioneer in developing rockets propelled by liquid fuel, was in charge of the US War Department’s rocket division during the first world war. He was both an engineer and a scientist, and since little was known about the biological effects of the heights his rockets later reached for the first time (~2.6 km), he supported using this new technol­ ogy for scientific experiments as well as recon­ naissance and other military applications.

Despite his best efforts, however, astronau­ tics did not include any biological research till the 1950s. Manned space flight imposed a new challenge for life scientists and thus the science of space biomedicine, as a new branch of astronautics, was born. The first “space doc­ tors” had to combine medical knowledge and the little that was known about the space envi­ ronment to form reasonable predictions about the health effects of space flight. In the 1960s almost all US astronaut candidates were test pilots, coming from aviation and engineering backgrounds. They were physically and men­ tally fit and their military training ensured a disciplined and organized attitude towards their missions. All of these were essential attributes A&G • February 2011 • Vol. 52

1.31

Astrobiology • Apagyi, Burchell: Astronautics for the first space travellers, from both a profes­ sional and a political perspective.

After the 1960s’ “astronautics gold rush”, fuelled by the space race between the USA and the USSR, the costs of these activities began to seriously strain the economies of the two super­ powers and, as a consequence, funds as well as political expectations were lowered. The Apollo programme could no longer be sustained. With their decreased budgets, the American and Soviet space agencies decided to establish continued human presence on low Earth orbit (LEO), as opposed to focusing on returning to the Moon.

Both governments began to develop space sta­ tions (Skylab, Salyut, etc) as well as the Space Shuttle system. Most components of the latter could be reused in several missions, potentially cutting costs by a significant amount. The crew of these vehicles, once transferred to their space stations, could stay in LEO from two weeks to more than a year. They could conduct scientific experiments, as well as doing the essential main­ tenance work on the space stations themselves.

One obvious area of research was studying the effect of micro­gravity on humans. By the early 1990s, textbooks concerning humans in space had appeared (e.g. Nicogossian et al. 1993).

Other published work addressed the conditions needed to sustain human life in space (e.g. Eck­ art 1994). Indeed, there are now whole MSc courses devoted to space physiology and health, even in the UK (at King’s College London), a country not renowned for its astronauts.

During this period, NASA no longer needed all its astronauts to be test pilots. While flight com­ manders on the Shuttle, for example, were still recruited from a test pilot/high-performance jet aircraft background, space agencies increasingly started recruiting civilian astronaut candidates.

In parallel to this, the arrival of scientists and teachers to the American, Russian and Euro­ pean astronaut corps saw space biomedicine and other space-related sciences flourish.

Astronautics also includes unmanned space vehicles. The past 30 years saw a rapid increase in unmanned missions carrying biological experiments, remotely controlled from terres­ trial biolabs or from space stations. Space probes were sent to the Moon, Mars and Titan look­ ing for biosignatures or blueprints of life. The astronautical engineering background of those developing such probes should not be neglected; their work enables the work of astrobiologists working on the specific payloads.

Thus, as time spent in space has increasingly included life-science-related experiments, astronautics can no longer be considered an adventure focused purely on engineering and physical science. It is a discipline that researches the space environment, rocketry and human physiology and also develops equipment that is used to send manned and unmanned missions beyond the atmosphere of the Earth, in order, 1.32

among other things, to establish human habi­ tation outside our planet, and look for signs of extraterrestrial life.

As we will examine in the next section, astro­ nautics and astrobiology should be considered connected by their shared interest in the endur­ ance of living organisms when exposed to extreme conditions such as those in space.

them and pollute the environment they visit.

Astrobiologists want to understand this hazard before it can be allowed under planetary protec­ tion policies. And it is those who are active in human space flight who will best understand what sort of contamination will potentially occur, as they are the ones who design the lifesupport systems, etc.

Common ground

Steps forward

Despite this convergence of interests, the two communities remain somewhat separate. The gap between the two disciplines is illustrated by two recent UK meetings. The UK Space Biomedicine Association (UKSBA, see http:// uksba.org for details) conference in November 2009 (UKSBA 4th Annual Conference) included 17 talks, only one of which focused purely on astrobiology. Similarly, a meeting on lunar astrobiology held at the RAS in London in June 2010, included only one talk related to space biomedicine. Even within one country, there appears to be little connection between these disciplines and their research communities.

Internationally, it is little better. Some meetings on each side of this divide do include sessions on topics from each other’s field, but relatively few researchers attend both types of meeting.

It is useful to ask what areas of scientific study span this divide between disciplines. Many immediately spring to mind. For example, the study of dust is important. On the Moon, dust will contaminate the interior of space vehicles and habitable quarters. But what impact does it have on health (Khan-Mayberry 2008)? An area of recent interest is the study of increased bacterial virulence in microgravity: terrestrial bacteria change their behaviour after space adaptation (Wilson et al. 2007). Related to both of these topics is the study of how the human immune system responds to microgravity (and pressure differences, elevated levels of radiation, etc). It has been found that the immune system is dampened somehow – important to know, especially in the context of bacteria changing their effects (for better or worse) (Sonnenfeld and Shearer 2002). As hinted at in the NASA roadmap (Des Marais et al. 2008) there is need to be able to design closed-circuit recycling sys­ tems, such as MELISSA (Mergeay et al. 1988) to enable long-term habitation in space and on planetary surfaces. Knowing what environmen­ tal conditions bacteria and algae can tolerate and how such an environment changes their genetic and phenotypic profile are crucial in the design of such systems.

And finally in our examples, there is the topic of planetary protection. The need to preserve habitats and potential habitats from contamina­ tion has long been an issue considered by space agencies (Rummel et al. 2002). It is of great con­ cern for both astrobiology and astronautics. If astronauts go anywhere they will take life with

One reason why there is little apparent over­ lap between the two fields may lie in the back­ grounds of the workers in those fields. For example, astrobiology is a purely scientific field with little or no application domain, whereas astronautics maintains a more active interaction with industry and military projects. People in these two disciplines thus come from different domains. Further, human space flight is often seen as separate from the rest of space explora­ tion. It is restricted to LEO; it is seen (by those outside it) as expensive and relatively inflexible; and it is often seen as an end in itself. This has undoubtedly limited collaboration between researchers/workers in the space sector.

In order to overcome some of these barriers there are a range of steps that could be under­ taken. For example, joint conferences or work­ shops would encourage mutual recognition and offer opportunities for closer dialogue. Simi­ larly, bodies or societies active in these fields could recognize each other, offering cross membership (at reduced rates), provide joint platforms for disseminating literature, news, etc, in the overlapping areas. Cross-linking of relevant websites would be an obvious first step.

For more in-depth contacts, summer schools and training courses exist in both fields. Link­ ing these would offer obvious attractions as regards a mutual increased awareness

Summary Astrobiology and astronautics are both multi­ disciplinary subjects, stemming from a primarily theory-based past. However, while astronautics is becoming increasingly application-focused and pragmatic when it comes to human space exploration, astrobiology still largely belongs to the academic domain of its discipline and has little connection to politics and engineer­ ing projects. Successful access to space and robotic exploration of the solar system are needed for deployment of the instruments that astro­biologists wish to use to search for life (e.g. robotic spacecraft travelling to other planets, increasingly sophisticated space telescopes, etc).

This is part of the desire to understand the origin and natural distribution of life, often described as one of the major scientific goals of this cen­ tury, and is of enormous public interest.

One could ask why any government would fund human or robotic exploratory activity in space. Political capital is an odd concept for A&G • February 2011 • Vol. 52

Astrobiology • Apagyi, Burchell: Astronautics

1: The seven key goals for astrobiology in the 2008 NASA Astrobiology Roadmap (From Des Marais et al. 2008)

1

Understand the nature and distribution of habitable environments in the universe.

Determine the potential for habitable planets beyond the solar system, and characterize those that are observable.

2

Determine any past or present habitable environments, prebiotic chemistry, and signs of life elsewhere in our solar system. Determine the history of any environments having liquid water, chemical ingredients and energy sources that might have sustained living systems. Explore crustal materials and planetary atmospheres for any evidence of past and/or present life.

3

Understand how life emerges from cosmic and planetary precursors. Perform observational, experimental and theoretical investigations to understand the general physical and chemical principles underlying the origins of life.

4

Understand how life on Earth and its planetary environment have co-evolved through geological time. Investigate the evolving relationships between Earth and its biota by integrating evidence from the geosciences and biosciences that shows how life evolved, responded to environmental change and modified environmental conditions on a planetary scale.

5

Understand the evolutionary mechanisms and environmental limits of life.

Determine the molecular, genetic, and biochemical mechanisms that control and limit evolution, metabolic diversity and acclimatization of life.

6

Understand the principles that will shape the future of life, both on Earth and beyond. Elucidate the drivers and effects of microbial ecosystem change as a basis for forecasting future changes on timescales ranging from decades to millions of years, and explore the potential for microbial life to survive and evolve in environments beyond Earth, especially regarding aspects relevant to US Space Policy.

7

Determine how to recognize signatures of life on other worlds and on early Earth.

Identify biosignatures that can reveal and characterize past or present life in ancient samples from Earth, extraterrestrial samples measured in situ or returned to Earth, and remotely measured planetary atmospheres and surfaces. Identify biosignatures of distant technologies.

many scientists, but is essential when persuad­ ing governments into large-scale funding activi­ ties. In the US in the 1950s, the development of a space programme coincided with political ends regarding safeguarding access to space, the need for reconnaissance satellites, etc (e.g. see Laun­ ius 1994). A civilian agency (NASA) emerged, but again driven by political desire. And by the early 1960s the massive expansion of NASA in the race to the Moon was again fuelled by politi­ cal considerations (Launius 1994). Astronautics played a major role in achieving these goals and in so doing generated sufficient political capital to ensure a future (albeit at a reduced level) after the Apollo era.

Other aspects of space flight, such as the explo­ ration of the solar system, have rarely enjoyed such a fund of political capital on which to draw, but have nevertheless been very successful scien­ tifically. The resulting tension between the two parts of the NASA programme (human space flight and robotic exploration of deep space) often emerges in debate inside the space commu­ nity. Should we fund human space flight? Could exploration of deep space survive as a fund­ A&G • February 2011 • Vol. 52

ing line if human space flight was restricted?

The failure in the US to adequately plan for human space flight beyond the withdrawal of the Space Shuttle and the apparent termination of the Return to the Moon programme, may indeed illustrate that in the US at least, human space flight has spent its political capital. And despite occasional ministerial speeches, the UK has never properly funded human space flight, although it may be moving towards increased investment in space through the new UK space agency (albeit mostly from a desire to increase commercial activity in the sector).

At the European level, the development of the Aurora programme (with the goal of exploring the solar system via both robotic and human activity in space, with a secondary goal of searching for life beyond the Earth) may offer a way forward, and in the process is increasingly drawing together people in the fields of astro­ nautics and astrobiology.

People working in these two fields of astronau­ tics and astrobiology, albeit representing dif­ ferent organizations and research/engineering backgrounds and funding streams, etc, share a

great deal in terms of their training, the types of problems they face and, last but not least, their struggle to achieve appropriate recognition by the public and the private sectors – in other words, to gain more political capital. These similarities and the overlap between the activi­ ties described here call for the establishment of more connections between astrobiology and astronautics.

Any successful, sustained human presence in space requires a deeper understanding of how biological systems, such as single-celled organ­ isms or our own immune system, respond to the space environment. Thus the knowledge gained from research, conducted by both astrobiologists and those working in the space biomedicine sector of astronautics, needs to be integrated into common knowledge, recognized by the peer-reviewed platforms of scientific com­ munication channels. Such exchanges will serve both communities well, sharing knowledge and expertise that can then be fed back into indi­ vidual goals. For some these will lie in enabling and accelerating long-term human exploration of space. For others, it will be better and more sensitive tests for life to be deployed on robotic spacecraft. And for astronomers it will be better space telescopes.

A new challenge of the 21st century for all space scientists, from both astrobiology and astronautics, is thus to adapt and grow, to develop closer links with each other (and par­ ticularly with the biological sciences). We con­ clude that the twin goals of understanding how humanity can operate in the space environment and the search for life elsewhere are thus best served by an integrated community. ● Katinka Apagyi, Dept of Biochemistry, University of Cambridge, UK, and Mark J Burchell, Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, UK (M.J.Burchell@kent.ac.uk).

References Burchell M J and Dartnell L R 2009 A&G 50 4.27–4.30.

Dartnell L R and Burchell M J 2009 Astrobiology 9(8) 717–730.

Des Marais D J et al. 2003 Astrobiology 3(2) 219–235.

Des Marais D J et al. 2008 Astrobiology 8(4) 715–730.

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1.33

Astrobiology • Martins: Biomarkers

In situ biomarkers and the Zita Martins examines some of the challenges involved in the identification and detection of biomarkers.

Table 1: Components of a unicellular organism molecule

16 × 10–14

DNA and RNA

6.9 × 10–14

lipids

2.6 × 10–14

other

3.2 × 10–14 –14

28.7 × 10

Summary of the main components and the dry weight composition of a unicellular organism, such as E. coli (g/cell). (Adapted from Brock et al. 1984)

T

he existence of life on Earth now and in the past provides the basis for questioning whether life may be ubiquitous in the universe. In order to be able to detect life elsewhere in our solar system, it is crucial to know what to look for, i.e. molecules that are diagnostic of past or present life: biomarkers. Future life-detection missions, such as the European Space Agency’s ExoMars mission to the Red Planet, need to be able to detect these biomarkers, but also to determine the origin of any molecule that they detect. Therefore, it is necessary to distinguish between present life biomarkers, dead organisms/fossil biomarkers and abiotic molecules.

Following in the footsteps of the Viking landers, the future ESA ExoMars mission will look directly for signs of life on Mars. If life ever existed on Mars it would have left organic remains in the martian environment. It is crucial to address the questions of how the biomarkers of martian life were stored and in what form they remained after different periods of time; our understanding of these processes for extraterrestrial life is influenced by our knowledge of terrestrial biology. A summary of possible biomarkers present on Mars is given by Parnell et al. (2007). Biopolymers, such as DNA, RNA and proteins, would be the ultimate proof of present life on Mars (table 1). However, these molecules rapidly degrade under the strong UV radiation and oxidizing conditions of the martian surface (figure 1; e.g. Wayne et al. 1999 and references therein). Amino acids (the building blocks of proteins) are also degraded within a few hours when exposed to Mars-like UV radiation (Ten Kate et al. 2005, 2006). However, when buried at a depth of more than 2 m, amino 1.34

more stable >2 billion years

dry weight (g/cell)

proteins

total

less stable

proteins

DNA

amino acids

hydrocarbons

1: Degradation rates of different biomarkers under martian conditions. Less stable biomarkers include DNA, RNA and proteins, while the most stable biomarker includes isoprenoids (e.g. phytane).

acids can persist for up to 3.5 billion years (Kanavarioti and Mancinelli 1990, Aubrey et al. 2006, Kminek and Bada 2006).

Typical biomarkers considered to indicate present life are L-amino acids, because on Earth all living organisms use L-amino acids only.

Over long periods of time biological amino acids in geological Earth samples are converted into equal amounts of L- and D- forms (i.e. a racemic mixture). However, on Mars racemization has been estimated to be extremely slow because of environmental conditions (Aubrey et al. 2006), so mixtures in which the L form dominates may be fossil forms. Hydrocarbons, which are often found as the molecular fossils of biological lipids, are stable over long periods of time and on Earth have been found in rocks more than 2 billion years old (figure 1; Brocks et al. 1999).

It is important to remember that lipids can be used as biomarkers for both past and present life (table 1). For example, phytane is a membrane component of methanogens (Woese et al. 1990), which are one of the possible sources for the methane observed in the atmosphere of Mars (Mumma et al. 2009).

Large amounts of carbonaceous material are thought to be delivered to the surface of Mars by interplanetary dust particles (IDPs) and meteorites every year (Chyba and Sagan 1992, Zent and McKay 1994, Flynn 1996, Bland and Smith 2000), meaning that abiotic extra­ terrestrial organic compounds may be expected on Mars. Abiotic molecules found in meteorites are distinct from biomarkers; they exhibit complete structural diversity with branched chains dominating, and present a decrease in abundance with increase in carbon number (Sephton 2002). Several abiotic compounds have been detected in meteorites. In particular, the Murchison meteorite includes among other molecules amino acids, nucleobases, carboxylic acids, polyols and hydrocarbons (table 2; Kvenvolden et al. 1970, Pering and Ponnaperuma

1971, Yuen et al. 1984, Cooper et al. 2001, Sephton 2002, Martins et al. 2008, Martins and Sephton 2009).

Carboxylic acids are the most abundant molecules present in meteorites. In addition, more than 80 different amino acids have been detected in the Murchison meteorite, most of which are rare in terrestrial proteins (such as isovaline and α-amino­iso­butyric acid). In addition, most non-protein chiral amino acids present in meteorites are racemic (for a review see Martins and Sephton 2009). However, L-enantiomeric excess (up to 18.5%) has been reported for a few non-protein amino acids (including isovaline) (Pizzarello et al. 2003, Glavin and Dworkin 2009).

Mars and the Life Marker Chip Viking was the first life-detection mission to Mars and did not find any molecules on the surface of the Red Planet (Biemann et al. 1976, 1977). Intense UV radiation and highly oxidizing conditions on the surface of Mars may have contributed to the destruction of organic compounds (Klein 1978, Benner et al. 2000, Squyres et al. 2004). In addition, the Viking gas chromato­graph–mass spectrometer (GC‑MS) may not have been sensitive enough to detect the degradation products generated by several million bacterial cells per gram of martian soil (Glavin et al. 2001). As preparation for future life-detection missions, in particular for ExoMars, it is crucial to optimize detection methods of biomarkers and abiotic molecules, using terrestrial soils that resemble Mars (for a review see Marlow et al. 2010). The Life Marker Chip (LMC) is part of the ExoMars payload (figure 2) and is being designed to detect biomarkers in the martian soil (Sims et al. 2005). It is an antibody-based instrument, aiming to detect polar (e.g. amino acids) and non-polar mol­ ecules (e.g. isoprenoids) at the part-per-billion (ppb) level. Court et al. (2010) have optimized A&G • February 2011 • Vol. 52

Astrobiology • Martins: Biomarkers

Life Marker Chip

Table 2: Abundances of soluble organic matter in Murchison meteorite compounds carboxylic acids (monocarboxylic)

67

amino acids

60

dicarboximides

>50

dicarboxylic acids

>30

polyols

24

ketones

the solvent system used to transfer biomarkers from the martian soil into the LMC: organic solvents would denature the antibodies, and aqueous solvents would not extract non-polar biomarkers. Court et al. (2010) found that the addition of surfactant to an aqueous solution is an effective compromise between biomarker extraction and antibody compatibility.

Conclusion Future life-detection missions, in particular the ExoMars mission to Mars, will search for biomarkers, i.e. organic molecules indicative of past and/or current life present in the regolith of the Red Planet. As the intense UV radiation and oxidizing conditions on the surface of Mars lead to the destruction of organic molecules, it will be necessary to search for those biomarkers at a depth of at least 2 m. The ultimate proof of life (as we know it) on Mars would be the detection of DNA, RNA or proteins. However, these biopolymers rapidly degrade under martian conditions. Amino acids, the building blocks of proteins, may survive up to 3.5 billion years when shielded from Mars-like UV radiation. Yet the most stable of all biomarkers are hydro­carbons, which can be used as a signature for both past and present life. Abiotic molecules delivered by IDPs and meteorites are also expected in the martian regolith, and future life-detection missions should be able to distinguish these from biomarkers. The Life A&G • February 2011 • Vol. 52

Marker Chip (LMC) is one of the instruments onboard ExoMars that aim to detect a large set of biomarkers using an antibody array. Preliminary work currently being performed to optimize detection methods of biomarkers and abiotic molecules is crucial for successful future life-detection missions to Mars. ● Zita Martins, Dept of Earth Science and Engineering, Imperial College London, UK. z.martins@imperial.ac.uk. Zita Martins is supported by the Royal Society.

References Aubrey A D et al. 2006 Geology 34 357–360.

Benner S et al. 2000 Proc. Nat. Acad. Sci. 97 2425–2430.

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Biemann K et al. 1977 J. Geophys. Res. 82 4641–4658.

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Botta O and Bada J L 2002 Surv. Geophys. 23 411–467 Brock T D et al. 1984 Biology of Microorganisms (Prentice-Hall, Englewood Cliffs, NJ, USA) 14–93.

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Flynn G 1996 Earth, Moon and Planets 72 469–474.

Glavin D P and J P Dworkin 2009 Proc. Nat. Acad. Sci. USA 106 5487–5492.

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Kanavarioti A and R L Mancinelli 1990 Icarus 84 196–202.

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332

sulphonic acids

hydrocarbons (aromatic)

2: Artist’s impression of the ExoMars rover, which will carry the Life Marker Chip. (ESA)

concentration (ppm)

17 15–28

hydroxycarboxylic acids

15

hydrocarbons (aliphatic)

12–35

alcohols

11

aldehydes

11

amines

8

pyridine carboxylic acid

>7

phosphonic acid

1.5

purines

1.2

diamino acids

0.4

benzothiophenes

0.3

pyrimidines

0.06

basic N-heterocycles

0.05–0.5

Abundances (in ppm) of the soluble organic matter found in the Murchison meteorite.

(Adapted from Pizzarello et al. 2001, Sephton 2002, Botta and Bada 2002, Sephton and Botta 2005)

Marlow J J et al. 2010 Int. J. Astrobiology doi:10.1017/S1473550410000303 available online since 19 August 2010.

Martins Z and M A Sephton 2009 Extraterrestrial amino acids, in Amino Acids, Peptides and Proteins in Organic Chemistry ed. Hughes A B (Wiley-VCH) 1 3–43.

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1.35

Astrobiology • Cousins: Life on Mars

Volcano–ice interaction: The interaction between volcanism and ice on Earth provides numerous habitats for exploitation by microbial life – and may have created potential islandsof habitability on Mars, argues Claire Cousins.

T

he search for life on Mars is inherently rooted in our understanding of life and environments here on Earth. As the only currently known planet inhabited by what we identify as modern day biology, life on Earth provides us with the blueprint on which we can base the search for life elsewhere. A major avenue of this terrestrial-based research has been the investigation into environments believed to be analogous to either past or present environments on Mars. These environments are wideranging, both in scale and physicochemical conditions, and provide us with a natural laboratory within which to understand the biological processes that may have once operated on Mars. Crucially, we can find out how life within these environments not only survives, but also how it changes the environment itself – producing biosignatures within the rock record.

Mars, like the Earth, has a geologically diverse terrain, suggesting the planet was once much more dynamic and active than its current quiet state. The original view of Mars as a vast windswept and pock-marked terrain has been comprehensively swept aside as orbital and in situ data gathered during the past decade have revealed complex valley networks, past hydrothermal activity, delta plains, and evidence of glaciological processes. Likewise, orbital hyperspectral data from OMEGA and CRISM have shown Mars also to be mineralogically diverse, highlighting the potential for a wide range of palaeoenvironments – some of which may well have once been habitable.

Analogue environments There are many well-studied and valuable analogue terrains within the context of searching for life on Mars. Environments such as the Atacama Desert (Navarro-Gonzalez et al. 2003), Antarctic Dry Valleys (Wierzchos et al. 2005), acid mine drainage sites (Amils et al. 2007), and evaporites (Rothschild 1990) have all been found to host a resilient array of microbial life.

In particular, life in such extreme environments is closely associated with the local geology, an interaction that is not only directly related to metabolism and survival, but also fundamental to the production of biosignatures. For example, the UV-shielding properties of gypsum crystals at the Haughton impact crater are used by photosynthetic bacteria, which in turn alter the local geochemistry of the mineral deposits by their activity (Cockell et al. 2010). Similar 1.36

examples can be found in the Antarctic Dry Valleys, where the cold, desiccating winds are avoided by microorganisms who seek refuge within porous rock substrates, and at acid mine drainage sites where redox coupling between iron and sulphur drive bacterial metabolisms (Amils et al. 2007). While all these environments are relatively clement in comparison to present-day conditions on Mars, there is the distinct possibility that past climatic and geological conditions here allowed the formation of such environments, and it is therefore imperative to understand the geobiological processes that take place within these systems on Earth.

Mars is predominantly volcanic, and it is widely observed that a continuous global cryosphere has been present for much of its history, with the vast majority of water currently frozen at the poles (Clifford 1993, Hvidberg 2005). The past interaction between this cryosphere and volcanic activity may have produced a variety of habitable environments (Schulze-Makuch et al.

2007), the deposits of which could be suitable targets in the search for martian life (Boston et al. 1992, Hovius et al. 2008).

Volcano–ice interaction The interaction between volcanic activity and ice can manifest itself in many forms, and ranges from the flow of lava over ice-rich ground, to the production of whole volcanic edifices beneath a glacier. These volcanic edifices can be built entirely subglacially within a zone of meltwater above the eruption site (figure 1a). Alternatively, where the eruption is long-sustained or the ice thinner, the continual transference of geo­thermal heat will eventually melt the ice, resulting in a subaerial eruption. In these cases, basaltic volcanic edifices typically display a sequence of basal pillow lavas, overlain by volcaniclastic deposits (e.g. hyaloclastite, hyalotuff) as the eruption becomes more explosive due to reduced overlying pressure from the glacier (Jakobsson and Gudmundsson 2008).

Where the edifice has emerged completely through the ice, it will often be capped by horizontal subaerial lavas (see figure 1b). Examples of such past and present volcano–ice interaction can be found in places such as Iceland, British Columbia and Antarctica.

The melting of so much glacial ice inevitably leads to the generation of liquid water. This water can be stored within the confines of the glacier, forming a melt­water lens that cycles

through the lava edifice and mixes with hydrothermal fluids (Björnsson 2002). Eventually this meltwater zone becomes unstable, and is commonly released from the glacier as a catastrophic outflow flood – termed a jökulhlaup – depositing volcanic sediments and ice across an outwash plain. These “sandur” plains are characteristic features of volcano–ice terrain in Iceland, where they take their name (jökulhlaup means “glacier burst”); similar large-scale features have been identified on Mars.

Iceland and Mars The volcanic country of Iceland lies on a high point of the Mid-Atlantic Ridge (figure 2).

This unique geological setting, while being responsible for the production of Iceland in the first place, leads to ongoing and often intense volcanic activity, much of which is highly analogous to Mars surface processes. Due to the near-Arctic latitude at which Iceland lies, much of the volcanism here is in direct inter­ action with glacial activity. Indeed, the elevated topography of the currently dormant volcanoes leads to increased glaciation of volcanic centres, despite their relatively high heat flow. Vatnajökull – Europe’s largest glacier – overlies seven volcanic centres (figure 2), including those associated with the hot spot. Similarly, much of Iceland’s glaciers coincide with the active volcanic zones that cut through the centre of the island.

The relatively small eruption of Eyjafjallajökull in southern Iceland earlier this year is a recent example of this volcano–ice interaction. Here, an initially subglacial eruption quickly became subaerial as the overlying ice was melted away (Gudmundsson et al. 2010). Previous to this were the eruptions at Grimsvotn in 1998 and 2004 (Jakobsson and Gudmundsson 2008), and Gjalp in 1996 (Gudmundsson et al. 1997), all beneath Vatnajökull.

Iceland shares many similarities with martian volcanism, and volcanic systems here are often used as analogues for those on Mars (e.g.

Keszthelyi et al. 2004). As far as volcano–ice inter­action is concerned, numerous localities on Mars have been suggested to be the result of subglacial volcanism or magma–cryosphere coupling. These are found both at the poles (such as in the Dorsa Agentea Formation, see Head and Pratt 2001), as well as the equatorial regions (e.g. Kadish et al. 2008), and at major volcanic centres (such as Elysium, see Pedersen et al.

2010). As such, the environmental conditions A&G • February 2011 • Vol. 52

Astrobiology • Cousins: Life on Mars

a haven for life on Mars? (a)

(b)

(c) overlying ice fully melted

ice

4–50°C

meltwater

hyaloclastite foreset beds

circulation of hydrothermal fluids

subaerial capping lava

ice

meltwater pool

hydrothermal soils and sediments

hyaloclastite pillow geothermal mound heat flow

H 2O, H 2S, CO2, CH4, SO4 fumaroles lava edifice geothermal heat flow and release of volcanic gases

magma body (d)

(e)

(f)

1 (a–c): Examples of volcano–ice interaction processes and associated environments. (a) Edifice growth during a subglacial eruption within a zone of meltwater. (b) Continual edifice growth leading to the development of a subaerial tuya. (c) Manifestation of geothermal activity at the glacier surface. (d–f): Present-day volcano–ice interaction environments as observed in Iceland. (d) Ice cave and subglacial outflow stream. (e) Herðubreið basaltic tuya. (f) Hydrothermal activity at the glacial surface induced by underlying volcanic heat.

generated by this activity warrant investigation with regards to their potential for life.

Subaerial and subsurface habitats The key to volcano–ice interaction habitability lies principally in the generation of liquid water and geothermal heat. Taking Iceland as a model, both subaerial and subsurface environments exist that are exploited by microbial life. One of the most exciting of all these environments are the subglacial “caldera lakes”. These exist beneath the glacier surface, maintained by high geothermal heat between eruptions (Björnsson 2002), and are confined by the surface topography of the underlying volcanic caldera or edifice.

Despite the thermal input, these lakes are generally cold, and have been found to be inhabited by psycrotolerant and chemotrophic bacteria (Gaidos et al. 2008). Equivalent environments are thought to have existed on Mars, where the geometry of the volcano Ceraunius Tholus, for example, would favour similar meltwater accumulation from the geothermal melting of summit snowpack (Fassett and Head 2007).

While meltwater can be ponded to form subglacial lakes, in other cases it is released gradually via fissures and drainage tunnels within A&G • February 2011 • Vol. 52

the ice. Where the meltwater eventually is released at the edge of the glacier, it can form a substantial cave network within the ice. Ice caves have been observed in Iceland (figure 1d) and at Mount Rainier, USA, where fumaroles produced caves over 1.5 km long (Zimbelman et al. 2000). These caves provide a sheltered, water-rich environment, and are likely to have formed on Mars wherever geothermal heat flow coincided with overlying ice deposits. In addition, such subglacial drainage networks have the potential to link nearby caldera lake systems.

In contrast to the aqueous and hydro­thermal habitats is the lava edifice itself. Basaltic lava has proven to be an environment widely exploited by microbial life on Earth, both within a sub­aqueous setting and in cold volcanic deserts. Combined with hydrothermal activity, the basaltic edifice has the potential to be colonized by a variety of chemosynthetic life (Boston et al. 1992). Basaltic pillow lavas and hyalo­clastite erupted at the seafloor from mid-ocean ridge systems is rapidly colonized by bacterial and archaeal communities (Santelli et al. 2008). Likewise, terrestrial basaltic lava habitats include those within the deep subsurface (McKinley and Stevens 2000) and now-exposed subglacially erupted lavas in

Iceland (Herrera et al. 2009). Therefore, any microbiota with volcano–ice systems on Mars would probably exploit the basaltic lava edifice along with its surrounding aqueous and hydrothermal environments.

Finally, while most environments likely to have once been habitable lie in the subsurface, volcano–ice interaction also has surface manifestations (figure 1c), in the form of large glacial meltwater lakes, fumaroles and hot springs (figure 1f). These environments form isolated “islands” of habitability within an otherwise barren terrain, and can also be found in places such as the Atacama, Antarctica and Iceland.

Finding life In the same way that the variety of environments created by volcano–ice interaction provides a wide-range of putative microbial habitats, geological deposits are equally diverse, and provide several opportunities for the preservation of biosignatures. In particular, the possibility for the preservation of biomolecules within extensive jökulhlaup deposits would mean large, expansive flood plains accessible to rover exploration could be searched for evidence of life. Additionally, the clay-rich nature 1.37

Astrobiology • Cousins: Life on Mars

2: Google Earth satellite image of Iceland, showing the Icelandic Rift Zone (black dashed line), with associated fissure systems (green) and central volcanoes (yellow). (Adapted from http://gullhver.os.is/website/hpf/orkustofnun_english/viewer.htm)

of jökulhlaup deposits is conducive to the preservation of organic molecules (Ehlmann et al.

2008). Recently, Warner and Farmer (2010) demonstrated how visible to near-infrared and shortwave-infrared remote-sensing data can be used to identify hydrothermal minerals (such as clays) within Icelandic jökulhlaups.

In addition to jökulhlaups, the hydrothermal activity associated with volcano–ice interaction can lead to the deposition of concentrated mineral deposits. As with many hydrothermal and hot spring systems, these mineral deposits have the potential to produce biosignatures, through the fossilization and preservation of microbiota.

The subglacially erupted volcano Sigurdfjellet in Svalbard is rich in hydrothermally deposited carbonate, and is indeed a good analogue for the carbonate globules in martian meteorite ALH84001 (Treiman et al. 2002); it provides an example of the mineralogical deposition that could potentially preserve biosignatures. Alternatively, subsurface silica-charged hydrothermal fluids have been found to precipitate amorphous silica upon freezing when they are erupted into a sub-zero environment (Channing and Butler 2007) and it has been suggested such precipitation would simultaneously silicify any microbial life present. In particular, this mechanism of fossilization would provide a window into any subsurface microbial communities brought up by the erupting spring fluid.

Lastly, basaltic lavas themselves have the potential to yield signs of past microbial activity.

Basaltic lavas on the seafloor are often riddled 1.38

with so-called “bioalteration” textures, thought to be traces of microbial life burrowing into volcanic glass (Furnes et al. 2007). Such textures have been putatively identified in Archaean rocks on Earth, clearly lending themselves to long-term survival within the rock record. However, little is known regarding the generation of these features – biogenic or otherwise – and indeed those lavas from subglacial environments do not always display these textures in the abundance with which they are found in seafloor lavas (Cousins et al. 2009).

Summary Active volcano–ice systems and their associated environments can potentially provide all the ingredients for life, including protection from the harsh surface extremes present on Mars. In addition the geological processes in themselves produce a number of mineralogical and sedimentary deposits that are potentially conducive to the preservation of biosignatures. Much work needs to be done to uncover the true value of present-day volcano–ice systems as a martian analogue environment, including the microbial life that resides within them. ● C R Cousins, Centre for Planetary Sciences at UCL/Birkbeck, London, and Dept of Earth and Planetary Sciences, Birkbeck College, University of London.

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A&G • February 2011 • Vol. 52

Astrobiology • Norman, Fortes: Life on Titan

Is there life on …

Titan?

Lucy H Norman and A Dominic Fortes consider the possibilities for life on and in Saturn’s complex icy moon, and the nature of organisms that might live there.

S

aturn’s giant satellite Titan is the only moon in the solar system with a substantial atmosphere, and which as a consequence appears to have a remarkably Earth-like weather cycle: there is evidence for storm cloud activity and rainfall, extensive dendritic networks likely to be fluvial systems (Elachi et al.

2005, Soderblom et al. 2007), and many lakes and seas in the polar regions (Lopes et al. 2010, Stofan et al. 2007), shown as they might appear to an in situ observer in figure 1. However, at the surface temperature of 94 K, the liquid involved cannot be water; speculation has suggested it could be a mixture of methane and ethane with some heavier hydrocarbons, dissolved nitriles, and/or atmospheric gases. Evidence for the presence of liquid on Titan’s surface comes from the very low radar backscatter of the purported lakes and seas, the identification of a specular reflection in the near-infrared from the feature named Kraken Mare – Titan’s largest sea, near the moon’s north pole – (Stephan et al. 2010) and the identification of near-IR absorption features from ethane, propane and butane in Ontario Lacus – a large south polar lake (Brown et al. 2008). The observational evidence is supported by chemical potential models of Titan’s A&G • February 2011 • Vol. 52

1: An impression of Titan’s polar seas of liquid hydrocarbons, fed by extensive drainage networks, and illuminated by the first rays of the spring Sun under an orange fog of photochemical haze.

2: Synthetic Aperture Radar (SAR) image and passive radiometry data (inset) of Ontario Lacus, a large south polar lake on Titan. The line of small coloured dots crossing Ontario Lacus reports radar altimetric data (Wall et al. 2010). (© 2010 American Geophysical Union.

Reproduced by permission of the AGU)

170°E

175°E

180°E

70°S

72°S 72°S

74°S 74°S

lake chemistry (Cordier et al. 2009, Raulin et al. 1995, Dubouloz et al. 1989), and thermodynamic models of the lakes’ stability against evaporation (Mitri et al. 2007). These bodies of standing hydrocarbon liquid are estimated to cover approximately 15% of Titan’s surface above 65°N (Lunine 2009), in a hemisphere that was in its winter season until 2009. The tilt of Saturn’s spin axis (26.7°) with respect to its orbital plane provides Titan with strong seasonal modulations of solar insolation as a function of latitude; it is expected that the distribution of lakes in the polar regions will change

as northern summer/southern winter progresses (the solstice is in 2017). Indeed, repeated observations by the Cassini radar instrument have revealed evidence of an evaporation sequence at Ontario Lacus, indicating that the shoreline may have already receded inward by about 10 km (Wall et al. 2010), see figure 2.

Titan’s lakes and seas appear to be replenished by extensive fluvial networks, formed in all likelihood by precipitation of liquid methane and/ or ethane rain from storm clouds. The type of clouds found to date vary from the occasional high altitude, transient, tropospheric clouds 1.39

Astrobiology • Norman, Fortes: Life on Titan in tropical latitudes (Schaller et al. 2009, McDonald et al. 1991, Griffith et al. 2009), to relatively frequent polar tropospheric cloud systems (Griffith et al. 2000, Brown et al. 2002, 2010, Schaller et al. 2006, Wang et al. 2010), and “fogs” (Brown et al. 2009). The “fogs” are believed to form by evaporation of the lakes or a methane-soaked regolith, and have been observed within just a few kilometres of the surface. Further replenishment of the lakes may occur through methane-based aquifer systems; subterranean bodies of liquid hydrocarbons have been hypothesized (Hayes et al.

2008), and the Huygens probe, which landed on Titan’s surface in January 2005, found evidence for liquids in the near-surface regolith (Niemann et al. 2005).

As well as the liquid products of Titan’s atmo­spheric photochemistry, the surface is the ultimate sink of the solid photochemical products that snow out of the atmosphere. These solid hydrocarbons and nitriles accumulate into vast equatorial dune-fields (Lorenz et al.

2006b) and probably form thick sedimentary deposits on the polar sea- and lake-beds as well.

Quasi-circular structures and flow-like features thought to be cryovolcanic in origin have been observed (Lopes et al. 2007), as well as a few impact craters (Artemieva 2003, Lorenz et al.

2007) and several mountain ranges (Barnes et al. 2007). These features attest to a geologically active body, with a dynamic interior leading to partial melting and ascent of aqueous magmatic fluids to resurface Titan over geological time. A warm and watery interior is also one possible interpretation of Titan’s observed moment of inertia factor (Iess et al. 2010); both CastilloRogez and Lunine (2010) and Fortes (in press) have independently concluded that Titan is likely to have a fully differentiated interior comprising a warm hydrous silicate core overlain by a shell of high-pressure ice between 500 and 600 km deep (figure 3). This icy shell may also contain a global liquid layer some tens of kilometres beneath the surface (Beghin et al. 2010).

Although there are no observational constraints on the chemistry of this subsurface ocean, a mixture of ammonia and water has traditionally been preferred, and only more recently has an alternative model with a briny ocean (dominated by sulphate salts) been proposed (Fortes et al. 2007, Grindrod et al. 2008).

Astrobiological potential of Titan Long before Cassini arrived at Saturn in 2004, laboratory study of the likely organic chemical processes on Titan demonstrated the production of tholins (complex C- H- O-, and N-rich polymers) from high-energy electric discharges into a simulated Titan atmosphere (e.g. Sagan et al. 1992 and references therein, and Coll et al. 2001), or soft X-ray irradiation of methane-bearing ices (Pilling et al. 2009). Further 1.40

3: A possible structural model of Titan’s interior, showing the range of possible habitats, from the warm hydrous silicate core, through the high-pressure ice mantle and the putative subsurface ocean, to the organic-rich surface environment.

hydrolysis of these tholins would produce some amino acids (Khare et al. 1986).

This work suggests that the building blocks of proteins may be produced, even today, in the atmosphere and surface environment of Titan; we infer that these processes could have proceeded as far as the emergence of life under the warmer conditions that we expect to have prevailed at or below the surface of Titan shortly after its formation and differentiation. While it has long been recognized that the organic chemistry in Titan’s atmosphere may provide useful insights into the proto-biotic evolution of early Earth (Clarke and Ferris 1997, Raulin et al. 1994), astrobiologists have speculated only recently about the possibility that life might have arisen and could persist in these alien environments. There are a plethora of possible habitats for exotic biota on Titan, including life that may be indigenous to the surface and living in liquid hydrocarbons, or have originated in a sub­surface ocean and adapted to life in aqueous media such as an ammonia–water mixture. The range of environments that might be conducive to living organisms extends from the surface down to a few kilometres into the rocky core, giving a biosphere volume of ~4 × 1010 km3, at least double the volume of the terrestrial biosphere. ● The silicate core. The outermost few kilo­ metres of the rocky core at <400 K are probably cool enough for organisms to survive; the maximum growth temperature for known terrestrial hyperthermophiles is 394 K. This environment is almost certainly permeated by liquid water from the ice layer above, and this water can mediate a range of chemical reactions yielding substances of use to obligate anaerobes.

The hydrostatic pressure at the base of the ice mantle is likely to be ~1 GPa, which is substantially greater than most organisms on Earth are adapted to. However, microbial metabolism

has been reported at pressures of 1–1.2 GPa, while pressures >1.6 GPa were found to kill the microbes under investigation (Sharma et al.

2002). We can speculate that a carbon-based biochemistry would employ similar metabolic pathways to those found in deep terrestrial ecosystems (e.g. Pedersen 2000). Alternatively, the biochemistry may be completely alien; SchulzeMakuch and Irwin (2006), for example, pointed out that low-temperature serpentinization can lead to the formation of silanes, which might provide the basis for a polymerized silicon biochemistry quite unlike anything on Earth. ● The high-pressure ice mantle. The experiments by Sharma et al. (2002) observed microbial activity, in fluid inclusions in ice VI using a diamond anvil cell. This extends the range at which microorganisms are known to be able to survive in pore fluids in solid ice (Price 2000, 2004) over the entire pressure range of relevance to Titan’s ice shell. There is nothing to prevent interstitial liquids rich in likely nutrients from becoming trapped in the matrix of the ice VI shell as it forms. Also, if microorganisms were present in the early history of Titan they could have been incorporated into the icy matrix of the high-pressure mantle and have evolved into an ecosystem separate from other Titan biota. ● The subsurface ocean and crust. A sub­surface ocean of aqueous ammonia might have the requisite properties (in terms of temperature, pressure, pH, viscosity and nutrient availability) to support a modest biosphere, particularly if life was able to originate inside the warmer proto-Titan (Fortes 2000, Simakov 2000, 2001, 2008). However, a relatively warm aqueous ammonium sulphate subsurface ocean (Fortes et al. 2007, Grindrod et al. 2008) is a considerably more attractive environment for life. At 100–500 MPa, a eutectic solution of ammonium sulphate is likely to be ~60 K warmer, and A&G • February 2011 • Vol. 52

Astrobiology • Norman, Fortes: Life on Titan almost three orders of magnitude less viscous, than a eutectic solution of ammonia. The pH is comparable to rain water, under the expected conditions, and the salinity within the region of comfort for halophiles (Rodriguez-Valera 1991). Energy production by the dissimilative reduction of dissolved sulphate is a common metabolic pathway among terrestrial obligate anaerobes (examples include Archaeoglobus, and the well-known Desulfovibrio). There are numerous electron donors employed in sulphate metabolism (Brock et al. 1997), and those which might be available in the subsurface ocean as a result of inorganic synthesis (e.g. Shock and Mckinnon 1993) include H 2 and ethanol.

Interestingly, a methane/sulphate-bearing ocean is not very different from cold seeps on the Earth’s ocean floors. At cold seeps, sulphate reduction (SR) and the anaerobic oxidation of methane (AOM) are usually syntrophically linked (Joye et al. 2004). These environments, even on Earth, are poorly understood, and an area of active research (e.g. Knittel et al. 2005).

The metabolic products of SR and AOM are H 2S and dissolved CO32–; the identification of H 2Sclathrate and carbonates (probably ammonium carbonate monohydrate) in liquids erupted at Titan’s surface, particularly if these show signs of biological isotope fractionation, would be a strong indication of microbial activity in the subsurface ocean. Microorganisms could survive in pore fluids or grain-boundary fluids to within a few tens of kilometres of the surface globally (cf. Price 2000, 2004), and possibly for short periods locally in plutonic cryomagmas intruded into the crust, or in liquids erupted onto the surface. ● Surface liquids. Benner et al. (2004) first suggested that the liquid hydrocarbons on Titan could be the basis for life, playing the same role as water on Earth. This initiated theories about the metabolism of such hypothetical organisms; both McKay and Smith (2005) and Schulze-Makuch and Grinspoon (2006) computed the energy available from the reaction of H 2 with organic material, with the production of methane as a waste product. They showed that the metabolism of acetylene yielded the most energy, although the high abundance of ethane makes it a competitive source of energy for Titanian biota. These reactions will not proceed spontaneously; they require either metal or biological catalysts to promote the reaction. However, the Committee on the Limits of Organic Life in Planetary Systems (2007) noted that many enzymes function in organic solvents, and many organic reactions fundamental to biochemistry can occur in non-aqueous media, so there appears to be no barrier to the adoption of suitable catalytic enzymes by hypothetical methanogens on Titan’s surface.

McKay and Smith (2005) predicted that organisms living in liquid methane on Titan’s surface A&G • February 2011 • Vol. 52

would produce anomalous depletions of hydrogen, acetylene and ethane, as they consumed these substances: recent Cassini data appear to provide intriguing evidence for such depletions.

A deep global ocean consisting principally of ethane was predicted after the Voyager flybys, based on photochemical modelling; the limited liquid present in lakes and small seas reveals that there is an unexpected lack of ethane on the surface (Lorenz et al. 2008). Secondly, Strobel (2010) modelled the hydrogen concentration in Titan’s atmosphere and found that the observational data are best explained by a strong flux of hydrogen to the surface, for which the only current explanation is a gradient in the hydrogen concentration created by metabolism of H 2 by methanogenic organisms. Finally, Clark et al.

(2010) report an apparent depletion of acetylene at the surface compared to the expected rates of atmospheric production and subsequent deposition of acetylene onto the surface; in support of this there was no evidence of acetylene in the gases released from the surface after the Huygens Probe landed (Niemann et al. 2005, Lorenz et al. 2006a). Although these depletions are intriguing, they do not constitute unambiguous proof of life on Titan’s surface (there are abiological explanations in each case), but they certainly endorse the argument that Titan is a target of high astrobiological interest.

There have been discussions about the possible temporary presence of surface water, normally frozen at Titan’s ambient temperatures.

For example, localized “warm” spots are possible from geothermally heated liquid methane trapped deep in sedimentary basins rising to the surface at springs lines or in mud volcanoes (Fortes and Grindrod 2006) or, more vigorously, at geysers (Lorenz 2002). Bodies of liquid water (with or without ammonia?) produced by hypervelocity impacts into an icy substrate are predicted to last for periods of hundreds to thousands of years (Artemieva 2003). Similarly, aqueous cryovolcanic flows may remain partially molten for very long periods, particularly if they contain significant quantities of ammonia (Sarker et al. 2003), causing hydrolysis (and/or ammonolysis) of tholins to produce amino acids (e.g. Neish et al. 2007, 2008, 2010).

Possible biochemistries of life on Titan It is plausible that Titan’s subsurface ocean could deliver a cornucopia of chemicals to the surface environment dissolved in cryomagmatic liquids; these may also bear the signature of life in the subsurface ocean, or else – like black smokers on the Earth’s ocean floors – provide a concentrated source of nutrients for specialist organisms on Titan’s surface. Inputs of dissolved material from the subsurface ocean (sourced from Titan’s rocky core, or post-accretionary impactors) may have a significant impact on the habitability of Titan’s hydrocarbon lakes and have an important

selective effect on the potential biochemistries that lacustrine biota might use. For example, naturally occurring organo-silicon compounds, such as silanols, are likely to be soluble in liquid hydrocarbons, and could plausibly be used by an exotic Titanian biota (Bains 2004).

It is likely that the physiology of exotic life in hydrocarbon lakes will have many differences to life on Earth. Even if the biota is carbon-based it must adopt organic molecules with alternative functional groups, or alternative structural arrangements (bioisosteres) in order to carry out the functions of equivalent terrestrial macro­molecules employed for catalysis, storage of hereditary information, compartmentalization and structural integrity, to name a few. For hypothetical hydrocarbon-dwelling organisms a combination of experimental and computational work is required to establish the suitability of any potential bioisoteric adaptation.

Arguably, the most basic requirement for Titanian microbes is a mode of compartmentalization – the development of a capsule that allows the organism to concentrate nutrients, to conduct metabolism under controlled conditions, and to store genetic information. This may be a capsule of occluded hydrocarbon liquid in the case of organisms that originated – and are adapted to life – on the surface, or else may be a capsule of aqueous fluid separated from the surrounding liquid hydrocarbon environment in the case of organisms that evolved in Titan’s interior but subsequently adapted to life on the surface. Since the Committee on the Limits of Organic Life in Planetary Systems (2007) has observed that there is currently no information about the possible alternative membrane structures that would be stable in organic solvents, we have begun a programme of experimental study into plausible bioisosteres that may perform just such a function.

Ordinarily, organic solvents permeate cell membranes, resulting in leakage of the cell contents into the external environment and cell death. However, some terrestrial organisms are able to tolerate high concentrations of organic solvents (e.g. Heipieper et al. 1994, de Carvalho et al. 2007, de Carvalho et al. 2005) and can even live in such places as Pitch Lake, Trinidad (Ali et al. 2006), which is the largest natural deposit of asphalt in the world. These terrestrial organisms cope by incorporating surfactants with a greater hydrophilic nature into their cell membranes, which can then repel hydro­carbons more efficiently, and/or they have efflux mechanisms that remove solvents that have diffused into the cell. For those Titan biota that are hydrocarbon based, one possible solution to this problem would be to adopt a modification of the arrangement of surfactant molecules in the cell membrane, forming a socalled “reverse” vesicle structure (figure 4). Such structures are known to occur in the laboratory, 1.41

Astrobiology • Norman, Fortes: Life on Titan

4: “Reverse” vesicle structures (right) consist of spherical bilayers of close-packed surfactant molecules (such as phosphatidylcholine, shown on the left). The hydrophilic groups (orange spheres) are packed into the centre of the bilayer, with the hydrophobic chains of hydrocarbons (pale yellow sticks) extending inwards and outwards. The phosphatidylcholine molecule shown on the left is ~25 Å in length.

and have been extensively characterized in systems composed of phospholipid surfactants in high-molecular-weight hydrocarbons and nonpolar liquids, such as cyclohexane, hexane, toluene and chloroform, at room temperature (e.g. Tung et al. 2008, Kunieda et al. 1993a, 1993b, 1994, Li and Hao 2007, Nakamura et al. 1995). Reverse vesicles are excellent candidate analogues for cell membranes that could maintain a hydrocarbon-based cyctoplasm in a hydrocarbon liquid medium. For those biota that still retain an aqueous cytoplasm (after originating from the subsurface and migrating to the hydrocarbon lakes) a reverse micelle-like structure could have evolved; or else the “normal” terrestrial type vesicle is maintained with numerous coping mechanisms. We are carrying out experiments to determine whether certain surfactants will form reverse vesicles (or other similar structures) in low-molecular-weight liquid hydrocarbons under low-temperature conditions; reducing the molecular weight of the hydrocarbons and the temperature until attaining environmental conditions comparable to the surface of Titan.

Titan an extremely enticing object for astrobiological research.

Further work on exotic, hypothetical biota requires a combination of research pathways, including study of possible analogue habitats (such as terrestrial tar pits), computational modelling of plausible biochemical phase spaces (Bains 2004), and experimental investigation of potential bioisoteric substances that could be used in extreme environments. Such research forms the basis for future in situ astrobiological observations, including the design and delivery of instrumentation for the next planned robotic mission to Titan (e.g. Coustenis et al. 2009). ●

Summary

Lucy H Norman, Dept of Space and Climate Physics, Institute of Origins, University College London and Centre for Planetary Sciences at UCL/Birkbeck, UK (lucy.norman.09@ucl. ac.uk). A Dominic Fortes, Dept of Earth Sciences, University College London.

Acknowledgments. The authors wish to thank Dr Ian Crawford for commenting on the manuscript.

LHN is funded by a PhD studentship from the UCL Institute of Origins, and ADF is funded by an Advanced STFC Fellowship (grant number PP/E006515/1).

Titan meets the absolute requirements for the presence of life: it is not in thermodynamic equilibrium, it has abundant carbon-bearing molecules at the surface and there is a plausible liquid substance in which biological activity may be mediated. Moreover, there are a wide range of possible habitats for exotic biota extending to depths of several hundred kilo­metres into Titan’s interior; Titan could be home to numerous, separate ecosystems, with completely independent evolutionary histories (or else their only connection lies in the distant past when Titan formed). This combination of factors makes

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A&G • February 2011 • Vol. 52

Obituaries

Allan R Sandage 1926–2010 Honorary Fellow, Eddington and Gold Medalist of the RAS, stalwart of observational cosmology whose observations helped to determine the scale of the universe.

A

llan Sandage led observational cosmology through the dark nights till a new era dawned with the interpretation of the fluctuations seen across the sky in the cosmic microwave background. Unlike Hubble, who simply accepted that his estimate of the time since all the galaxies overlapped was even less than the age of the Earth, Sandage believed that he was discovering the age of creation. He studied under Walter Baade but became Hubble’s assistant and, when Hubble died suddenly in 1953, it was on Sandage’s broad shoulders that his cloak fell. Almost at once he realized that many of Hubble’s “brightest stars” in nearby galaxies were actually tight groups of bright stars and this led to a doubling of the distances estimated to them.

Milton Humason, as a mule driver, had helped to get telescope parts up the old Mount Wilson trail, but he had become Hubble’s chief observer of galaxy redshifts. In 1956 Humason, Mayall (of Lick Observatory) and Sandage gathered the 800 known galaxy redshifts and magnitudes in what was essentially a survey of observational cosmology. The largest redshift was then 61 046 km s –1 (z = 0.204) for a galaxy in the Hydra cluster. A preliminary rough estimate of Hubble’s constant was in the region of 200 km s –1 Mpc–1.

Sandage’s main work on refining distance measurements in astronomy had barely begun.

For his thesis he had studied the stars in the globular cluster Messier 3 and the RR Lyrae variables it contains. Refinements in the accuracy of the distances to variable stars became a recurring theme of his work, because these were the first step in determining the ladder of distances through which the true scale of the universe was measured. For a variable star of known period, both its colour and its chemical composition can affect its intrinsic brightness and the intervening interstellar dust can both dim and redden its light. Distance accuracy only comes after detailed studies of how A&G • February 2011 • Vol. 52

to compensate for such effects. No rung of the distance ladder escaped his attention and he found good secondary distance indicators in the brightest galaxies in clusters and in supernovae. Overcoming personal difficulties caused

by Zwicky’s persecution complex, he attributed the idea of using supernovae as distance indicators to Zwicky and indeed, through detailed refinements to which Sandage and Tammann contributed greatly, type IA supernovae have now become the standard candles for measuring the largest cosmological distances.

Sandage’s interests spread far and wide through astronomy, with quasars, active galaxies, X-ray sources, the Virgo cluster and the origin of our galaxy dominating his attention at different times. By nature Sandage was open to new ideas and both generous and helpful to those who worked with him. He was a true gentle­man and expected equally high standards of honour and acknowledgement from others.

Sandage’s fine Hubble Atlas of Galaxies was a basic guide to many young observers and theor­ists alike. Later he produced two further atlases with Bedke. His definitive Revised Shapley-Ames Catalog of Bright Galaxies was co-authored by his long-time collaborator Gustav Tammann of Basel.

In his early years Sandage had studied the theory of stellar evolution under Martin Schwarzschild at Princeton, work which gained them the 1963 Eddington Medal of the Royal Astronomical Society. The age of the globular clusters must be less than the age of creation but they seemed uncomfortably old (with age estimates that at times reached 18 billion years, whereas 12 billion is nearer the truth) compared with the universe’s expansion rate. He was convinced that nature must be consistent and worked unceasingly with Gustav Tammann to refine distances and redetermine the Hubble constant. For some years their best estimate was around 52±5, but their fine paper on the linearity of the Hubble flow based on more accurate

distance estimators – and only published this May – gives 62.3 km s –1 Mpc–1. Current consensus based on space telescope data gives values closer to 72, but astronomical consensus is not always truth. The paper referred to probably gives the most accurate relative distances of local structures in the universe, up to about five times the distance to the Virgo cluster of galaxies.

Sandage was elected to the National Academy of Sciences in 1963, but resigned when the Academy failed to elect his friend Olin Eggen, with whom he had collaborated on a famous paper on the formation of our galaxy. This paper, commonly called ELS after its authors, generated the subject of galactic archaeology.

Although Sandage was the first to isolate and to take the spectrum of a quasar, 3C48 (number 48 in the third Cambridge catalogue) he was unable to interpret its spectrum. For help he turned first to Bowen and then to Greenstein. Meanwhile he monitored the “radio-star” and showed that it varied over a few months. When, almost two years later, Schmidt showed that 3C273 had a large redshift and he with Greenstein finally understood and published the spectrum of 3C48, Sandage, who had taken the identification plates and the first spectrum, felt excluded.

Later he discovered that there are many more radio quiet quasi-stellar objects than there are quasars, a discovery that contributed to the current view that most large galaxies contain giant black holes (dead QSO) in their nuclei.

The deterioration of the Mount Wilson site as a result of increasing light from Los Angeles led to administrative changes that Sandage opposed. Caltech, who owned Palomar Observatory, wanted Carnegie to spend the money on new instruments there. However, Horace Babcock foresaw that the Californian sites would be overtaken by those in the Chilean Andes. So he concentrated effort and resources on developing Carnegie’s fine site at Las Campanas. This led to tension between the Caltech astronomers and Carnegie, which eventually led to a resentful severing of the strong links that had existed ever since Hale, Carnegie’s first director, had been one of the founding fathers of Caltech. When money got even tighter, Sandage, who acknowledged Babcock’s foresight over Las Campanas, was appalled by the decision of Carnegie to withdraw from Mount Wilson, its former home and still resonant with the history of astronomy. For the centennial history of the Carnegie Institution, Sandage wrote a definitive history of the astronomical discoveries made on Mount Wilson.

Sandage was awarded the Crafoord Prize in 1991, was the joint recipient of the first Gruber Prize for Cosmology in 2000, and was a Foreign Member of the Royal Society. He was also awarded all the major astronomical medals in both the USA and the UK. His wife Mary and their two sons survive him.

Donald Lynden-Bell

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Obituaries

Audouin Charles Dollfus 1924–2010 Honorary Fellow of the RAS, planetary astronomer, champion of historical telescopes and record-breaking balloonist.

W

ith the death of Audouin Dollfus, planetary astronomy has lost one of its greatest exponents. He had been an Associate Member (now styled Honorary Fellow) of the RAS since 14 February 1975.

He was born in Paris on 12 November1924, son of famous balloonist and airship pilot Charles Dollfus. He joined the Société Astronomique de France in 1941. Dollfus spent his professional career at Meudon, entering the laboratory of optical genius and RAS Gold Medallist Bernard Lyot in 1945, after graduating in Mathematical Sciences and Physics. He did his first planetary work at Pic du Midi in the same year, a time when everything had to be carried up on foot (or ski) from the HQ at Bagnères. At the 1948 opposition, Dollfus studied Mars with the Pic’s 60 cm folded refractor, leading to his first professional publication where he threw new light on the “canal illusion”.

A technique that Dollfus took up from the very start was visual polarimetry, building on the whole-disc measurements of Lyot on martian global dust storms. Dollfus went further and examined the polarization of individual regions of the planet’s disc, discriminating between bare ground and overlying dust or white cloud. For decades, Dollfus would collaborate on this with Shiro Ebisawa in Japan; they obtained likely values for the diameters of dust particles lofted during major storms.

Dollfus was also studying the polarization of the Moon’s surface in the 1950s. He concluded that fine-grained pulverized basalt covered the lunar surface, a valuable result for the future soft landings. NASA invited Dollfus to collaborate in the study of the Apollo 11 landing site and in the design of the astronauts’ Moon boots.

Dollfus contributed to the analysis of the lunar samples returned by the Apollo programme and to studies of martian soil before the Viking mission. He also collaborated with NASA on the Ranger and the Venus Mariner programmes, and with the Russians on Mars-5 in 1973.

In the 1950s Dollfus had the chance to combine profession with hobby in a series of balloon ascents. In 1956 a flight to an altitude of 6000 m established (via high-resolution photography of the solar granulation) that convection rather 1.44

than turbulence was the mechanism of energy escape from the Sun. In his most ambitious flight, in 1959, he reached 14 000 m in a pressurized gondola – to this day a French record – and detected water in the atmosphere of Mars.

These programmes produced a massive volume of data, and between 1961 and 1980 Dollfus directed the Centre for Planetary Documentation at Meudon, accumulating copies of planetary photographs from all over the world.

This international cooperation made possible, for example, the study of the rotation of the ultraviolet clouds of Venus. The data also enabled Dollfus, together with UK astronomer J B Murray, to make the best pre-Mariner telescopic albedo chart of Mercury, only recently surpassed by the Messenger mission.

In addition to his work at Meudon and the Pic, Dollfus was chairman of IAU Commission 16 for the Physical Study of Planets and Satellites.

He was instrumental in the compilation of a standard IAU map of Mars (drawn by G de Mottoni) and a new rational system of nomenclature (1957). He used Lyot’s coronograph at the Pic to conduct a photographic survey for new Saturnian satellites, discovering a faint outer ring of the planet as well as the moon Janus.

For much of Dollfus’s career it helped greatly if the planetary astronomer was also talented at sketching. Once, hoping for a tip, I asked him which pencil he preferred: “Oh, just the one on my desk” he answered modestly. In the 1980s the Pic du Midi observatory acquired a new 2 m telescope to continue the high-resolution tradition. Dollfus tested it out on Mars in 1982, and his fine drawings show that his observing eye (for Dollfus had sight in only one) had lost none of its celebrated powers.

Dollfus published some 330 scientific papers.

His impressive chapters in the classic Planets and Satellites, edited by Kuiper and Middlehurst (1961), stand as a testament to the skill of the French planetary observers at the Pic. Dollfus was enthusiastic about amateur astronomers. In 1988 it was decided to have a special observing campaign for the close opposition of Mars, and he invited me to make free use of the 83 cm Grande Lunette for several weeks.

Dollfus was keenly interested in historical top-

ics. He completed a major history of the Grande Lunette, richly illustrated with contemporary documents and photographs. In the 1990s the SAF rebuilt one of the great “aerial telescopes” from the time of Cassini, and Dollfus dressed in authentic period costume to make sketches at the eyepiece. There was a strong sense of history, too, at the historic “Maison Dollfus” at Chaville, where a delicious meal would be served upon ancient porcelain plates and dishes, always decorated with a balloon motif.

Many honours came his way. He was made Chevalier of the Legion of Honour in 1989, and received the highest award of the SAF, the Prix Janssen. In 1973 he received the Galabert Astronautical Prize, and in 1988 the Grand Prize of the French Academy of Sciences. In 1980 asteroid 2451 was named in his honour.

In 1995 he celebrated 50 years as a professional astronomer with a commemorative book, 50 Ans d’Astronomie: Comprendre l’Univers.

Dollfus was a warm individual who gave the impression of enormous industry. He was always well dressed in jacket and tie – worn outside the pullover – accompanied in cold weather by a long raincoat and a beret. He was a great humanist, too, who embraced the true “Egalité”. He strongly discouraged smoking.

In late August this year he entered hospital in Versailles for surgery upon his leg, and it was a shock to learn of his death there on 1 October.

Audouin and his wife Catherine had one son and three daughters. Audouin was a remarkable individual whose personal qualities and whose devotion to solar system astronomy, its documentation and history, will long remain his monuments. Following his funeral in Versailles, he was buried in Lyons-la-Forêt.

Richard McKim

Deaths of Fellows Prof. Adriaan Blaauw Born: 12 April 1914 Elected: 11 December 1964 Associate: 11 February 1977 Died: 1 December 2010 Dr Brian G Marsden Born: 5 August 1937 Elected: 8 January 1960 Died: 18 November 2010 Emeritus Professor Wilbur Norman (Chris) Christiansen Born: 9 August 1913 Elected: 13 April 1956 Died: 2007 Dr Allen R Sandage Born: 18 June 1926 Elected: 11 December 1953 Associate: 11 February 1966 Died: 13 November 2010 Jan Hers Born: 4 March 1915 Elected: 12 March 1965 Died: 24 August 2010

A&G • February 2011 • Vol. 52

Society News

Planetarium show sets sail

Astronaut Piers Sellers (above) returned the Society’s Jackson-Gwilt Medal to Earth – and Burlington House – after taking it almost 8 million km on the Space Shuttle Atlantis. The medal flew on mission STS-132 to the International Space Station. “It’s a huge pleasure to be here to return the medal,” said Sellers, “and to address one of the world’s largest astronomical societies. As a scientist and astronaut, I know how much space, astronomy and the wider universe have inspired me throughout my life.” The Jackson-Gwilt Medal is given for achievements in the field of astronomical instrumentation or techniques, so its presence on the Shuttle was especially apt.

New Fellows The following were elected Fellows of the Society on 10 December 2010: Feargus Abernethy, Shipston on Stour, Warwickshire.

Aveesha Ahsan, Barking, Essex.

Poul Alexander, Institute of Astronomy, University of Cambridge.

Rosalind Armytage, Dept of Earth Sciences, University of Oxford.

David Barnes, Harwell, Oxon.

Alex Barrett, Malvern, Worcestershire.

Emma Barton, Colwyn Bay, North Wales.

Carl Bryers, Kendal, Cumbria.

Anne Buckner, Norbury, London.

Richard Busuttil, Walnut Tree, Milton Keynes.

Hannah Calcutt, Brentwood, Essex.

Nathan Case, Lancaster University.

Nicolas Clarke, London.

Jonathan Crass, Institute of Astronomy, University of Cambridge.

Camilla Danielski, London.

Sarah Day, Stoke-on-Trent, Staffordshire.

Elizabeth Day, Bullard Labs, Madingley Rise, Cambridge.

Leon de Sainte Croix, Hayling Island, Hampshire.

Ajith Dharmakeerthi, Enfield, Middlesex.

Zeeshan Ali Dinally, Northfield, Birmingham.

Elizabeth Dubois, Astronomy Centre, University of Sussex.

Kate Dutson, Loughborough, Leicestershire.

Francesca Faedi, Astrophysics Research Centre, Queen’s University Belfast.

Samane Farazian, Richmond, London.

A&G • February 2011 • Vol. 52

A dedicated RAS show for the planetarium on board the Queen Mary 2 has been developed and shown, as part of what is hoped will be a new outreach programme.

In 2006 a successful link between the RAS and Cunard (now part of Carnival plc) saw RAS Fellows giving a series of popular lectures on cruises.

The Queen Mary 2 was designed with a 12 m planet­arium dome, seating 150, which has been used to screen film shows on astronomical topics. With the success of the RAS lectures on board, the opportunity arose to design a dedicated RAS talked-through show.

A group of RAS Fellows descended on Carnival UK’s headquarters in Southampton to put together this outreach show. With expert programming from Skyscan (planet­arium software providers), Francisco Diego, Mark Butterworth and Charlie Barclay, with input from Heather Couper, Nigel Henbest, Ian Ridpath and Robert Massey, devised the brand new show in four days, including a full run through on board ship. The initial show, “The Winter Sky”, takes the audience through the cultural interpretations of constellations and the

importance of asterisms and diurnal motions, explaining common nakedeye observations. The show finishes with a voyage beyond the Milky Way, returning home at 50 million times the speed of light.

Francisco Diego stayed on the QM2 for a voyage to New York and to present the premiere. Despite rough weather and 6 m waves, the show was booked out and demand was such that a repeat was arranged. In time we plan to develop other general shows, for example on the summer sky; there is also scope for new shows.

It is hoped that other RAS Fellows may be able to train on the software; to that end it is hoped to establish a dedicated Skyscan computer in Burlington House (contact Robert Massey for details). RAS Fellows who subsequently talk on QM2 would have this superb extra facility to deliver observational-based outreach, given that deck-based observing is often impossible in wind and bad weather.

Fellows who are interested in lecturing on Cunard ships should contact Anne Vansverry, Entertainment Planning and Sourcing Executive at Cunard, via

Robert Farmer, Monkston Park, Milton Keynes.

Philip Hall, Jesus College, Cambridge.

Alexander Hall, Institute of Astronomy, University of Cambridge.

Dr Patrick Harkness, Lennoxtown, East Dumbartonshire.

Thomas Haworth, School of Physics, University of Exeter.

Kate Hibbert, Salisbury, Wiltshire.

David Higgon, Putney, London.

Tim Higgs, Milton, Southsea.

Morgan Hollis, Lutterworth, Leicestershire.

Karen Hurren, Colchester, Essex.

Ashley Hyde, Astrophyiscs Group, Imperial College London.

Alan Jackson, Institute of Astronomy, University of Cambridge.

Dr David Jess, Dept of Physics & Astronomy, Queen’s University Belfast.

Captain Robert Jocumsen, Takura, Queensland, Australia.

Roser Juanola-Parramon, Dept of Physics & Astronomy, University College London.

Abdul Waheed Khan, Ilford, Essex.

Taichi Kidani, Southsea, Hampshire.

David Kipping, Dept of Physics & Astronomy, University College London.

Donnacha Kirk, Dept of Physics & Astronomy, University College London.

Paula Koelemeijer, Bullard Laboratories, Madingley Rise, Cambridge.

Dali Kong, Mathematics Research Institute, University of Exeter.

Sandor-Iozsef Kruk, University College London.

Shaaron Leverment, Clifton, Bristol.

Yu-Hsuan Liang, Dept of Earth Sciences,

University of Oxford.

Dr Harvey MacGillivray, East Calder, West Lothian.

Anna Mäkinen, Bullard Laboratories, Madingley Rise, University of Cambridge.

Chris Malcolm, Brighton, East Sussex.

Dr Alberto Mancini, Corciano, Italy.

Silvia Martinavarro, Camden Town, London.

Prof. John Mathews, Pennsylvania State College, USA Prof. Michael Merrifield, School of Physics & Astronomy, University of Nottingham.

Andrew Morris, Halling, Rochester.

Dr Andrew Morse, Sandy, Bedfordshire.

Dr Andrew Newsam, Astrophysics Research Institute, Liverpool John Moores University.

David Nixon, Los Angeles, California, USA. Sophie Nixon, Planetary & Space Sciences Research Institute, The Open University, Milton Keynes.

Tom Nordheim, MSSL, Dorking, Surrey.

Angela Occhiogrosso, Wembley, Middlesex.

James O’Donoghue, Leicester.

Dr Paul Olver, Canon Pyon, Hereford.

Adesoji Oninuire, London.

Maia Orsi, Liverpool.

James Owen, Institute of Astronomy, University of Cambridge.

Luke Peck, Norwich, Norfolk.

Alex Pettitt, Exeter.

Luca Porcelli, School of Mathematics & Physical Sciences, University of Sussex, Brighton.

Samantha Rolfe, Bancroft Park, Milton Keynes.

Huri Tugca Sener-Satir, Armagh Observatory, Armagh.

News in Brief Patrick Moore Medal The RAS has created a new medal to recognize excellence in astronomy teaching, and named it after the broadcaster Sir Patrick Moore. Sir Patrick is one of the world’s most famous contributors to public education and outreach in astronomy.

The first Patrick Moore Medal will be awarded at the National Astronomy Meeting in 2012, following nomination in 2011. http://www.ras.org.uk/news-and-press

GJI student winners RAS journal Geophysical Journal International has awarded its prizes for the best student papers in 2010 to three authors: Andrew P Valentine, Judicael Decriem and Yongxin Gao. Each was lead author on their winning paper, on automated data selection for seismic tomography, the 29 May 2008 Iceland earthquake doublet and seismoelectromagnetic waves, respectively. http://www.ras.org.uk/publications/ journals

anne.vansverry@carnivalukgroup.com

Nathalie Skrzypek, Astrophysics Group, Imperial College London.

Phillipa Smith, Stourport-On-Severn, Worcester.

Maayane Soumagnac, Dept of Physics & Astronomy, University College London.

Liam Steele, Dept Physics & Astronomy, Open University, Milton Keynes.

Oliver Steele, Portsmouth.

Charlotte Strege, London.

Catherine Stretch, Exeter.

Farung Surina, Astrophysics Research Institute, Liverpool John Moores University.

Rafal Szepietowski, Portsmouth.

Stephen Taylor, Christ’s College, Cambridge.

Marcell Tessenyi, Dept of Physics & Astronomy, University College London.

Jonathan Thurgood, North Shields, Tyne & Wear.

Dr Giovanna Tinetti, Dept of Physics & Astronomy, University College London.

Sam Tuttle, Highfield, Southampton.

Shyam Vikraman Nair, Astronomy Centre, University of Sussex, Brighton.

Stephen Walker, Ashington, Northumberland.

James Waltom, Beaconsfield, Buckinghamshire.

Lauren Weiss, Emmanuel College, Cambridge.

Philip White, Northwich, Cheshire.

Felicity Williams, New Bradwell, Milton Keynes.

Emma Woodfield, Gravesend.

Shenghua Yu, Armargh Observatory, Armagh.

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