Ερευνητική Εργασία «Είμαστε Μόνοι; Η αναζήτηση για την εξωγήινη ζωή» Η εργασία εκπονήθηκε στο ΓΕΛ Κρεμαστής κατά τη διάρκεια του Α` Τετραμήνου του σχολικού έτους 2012-2013, υπό την επίβλεψη του κ. Κωνσταντίνου Πράπα, καθηγητή κλάδου ΠΕ0401. Η εργασία αναφέρεται σε όλες τις επιστημονικές απόπειρες που έχουν πραγματοποιηθεί προκειμένου η ανθρωπότητα να βρει την απάντηση σε ένα από τα μεγαλύτερα υπαρξιακά της ερωτήματα: «Είμαστε μόνοι στο Σύμπαν;» Είναι χωρισμένη σε τέσσερεις ενότητες: Στην πρώτη ενότητα παρουσιάζεται η επιστήμη της Αστροβιολογίας, δηλαδή του διεπιστημονικού κλάδου που μελετάει θεωρητικά την πιθανότητα ύπαρξης ζωής στο Σύμπαν. Στη δεύτερη ενότητα γίνεται αναφορά στα προγράμματα SETI, δηλαδή στις προσπάθειές μας να ανιχνεύσουμε σήματα από εξωγήινους τεχνικούς πολιτισμούς. Στην τρίτη ενότητα σκιαγραφούνται οι προσπάθειές του ανθρώπου να επικοινωνήσει ο ίδιος με πιθανούς εξωγήινους πολιτισμούς. Τέλος, στην τέταρτη ενότητα παρουσιάζονται διάφορες μαθηματικές και φιλοσοφικές υποθέσεις για την εξωγήινη ζωή.
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Να σημειωθεί ότι το μεγαλύτερο κομμάτι της εργασίας είναι γραμμένο στα Αγγλικά, σύμφωνα και με τις σχετικές οδηγίες του Υπουργείου.
Στην εργασία συμμετείχαν οι παρακάτω μαθητές της Α` Λυκείου:
Αλάβανος Θωμάς Βόλας Τσαμπίκος Γαβαλάς Μιχαήλ- Άγγελος Γιαννούλης Ηλίας Διακολουκάς Στέργος Τσαμπίκος Καλαϊτζίδης Ιωακείμ Καρπαθάκης Χριστόφορος - Παναγιώτης Κιτσαής Παύλος Μαυρουδής Ηλίας Μιχαήλ Μιχαήλ Ορφανίδης Κωνσταντίνος Πάττας Ελευθέριος Πούλιος Βασίλειος Ρουσάκης Δημήτριος Σταύρας Ιωάννης- Μιχαήλ Συγκελλάκης Γεώργιος - Μάρτιν Τολακίδης Γεώργιος Χρήστος Χατζηδημητρίου Ιωάννης Χρονιάρης Μιχαήλ
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ΕΝΟΤΗΤΑ ΠΡΩΤΗ
ΑΣΤΡΟΒΙΟΛΟΓΙΑ
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ASTROBIOLOGY
Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe: extraterrestrial life and life on Earth. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry, laboratory and field research into the origins and early evolution of life on Earth, and studies of the potential for life to adapt to challenges on Earth and in outer space. Astrobiology addresses the question of whether life exists beyond Earth, and how humans can detect it if it does. (The term exobiology is similar but more specific — it covers the search for life beyond Earth, and the effects of extraterrestrial environments on living things.) Astrobiology makes use of physics, chemistry, astronomy, biology, molecular biology, ecology, planetary science, geography, and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from the biosphere on Earth. Astrobiology concerns itself with interpretation of existing scientific data; given more detailed and reliable data from other parts of the universe, the roots of astrobiology itself— physics, chemistry and biology—may have their theoretical bases challenged. Although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories. Earth is the only place in the universe known to harbor life. However, recent advances in planetary science have changed fundamental assumptions about the possibility of life in the universe, raising the estimates of habitable zones around other stars and the search for extraterrestrial microbial life. The possibility of life on Mars, either currently or in the past, is an active area of research. On 17 March 2013, researchers reported data that suggested microbial life forms thrive in the Mariana Trench, the deepest spot on the Earth. Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet below the sea floor under 8500 feet of ocean off the coast of the northwestern United States. According to one of the researchers, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are. -4-
In a National Institutes of Health study, the authors hypothesize that if biological complexity increased exponentially during evolution, life in the universe may have begun "10 billion years ago" - more than 5 billion years before the Earth existed. Astrobiology is etymologically derived from the Greek ἄστρον, astron, "constellation, star"; βίος, bios, "life"; and -λογία, -logia, study. The synonyms of astrobiology are diverse; however, the synonyms were structured in relation to the most important sciences implied in its development: astronomy and biology. A close synonym is exobiology from the Greek Έξω, "external"; Βίος, bios, "life"; and λογία, -logia, study. The term exobiology was first coined by molecular biologist Joshua Lederberg. Exobiology is considered to have a narrow scope limited to search of life external to earth, whereas subject area of astrobiology is wider and investigates the link between life and the universe, which includes the search for extraterrestrial life, but also includes the study of life on earth, its origin, evolution and limits. Exobiology as a term has tended to be replaced by astrobiology. Another term used in the past is xenobiology, ("biology of the foreigners") a word coined in 1954 by science fiction writer Robert Heinlein in his work The Star Beast. The term xenobiology has also been used in a more specialized sense, to mean "biology based on foreign chemistry", whether of extraterrestrial or terrestrial (possibly synthetic) origin. Since alternate chemistry analogs to some life-processes have been created in the laboratory, xenobiology can be said to be an extant subject. While it is an emerging and developing field, the question of whether life exists elsewhere in the universe is a verifiable hypothesis and thus a valid line of scientific inquiry. Though once considered outside the mainstream of scientific inquiry, astrobiology has become a formalized field of study. Planetary scientist David Grinspoon calls astrobiology a field of natural philosophy, grounding speculation on the unknown, in known scientific theory. NASA's interest in exobiology first began with the development of the U.S. Space Program. In 1959, NASA funded its first exobiology project, and in 1960, NASA founded an Exobiology Program; Exobiology research is now one of four elements of NASA's current Astrobiology Program. In 1971, NASA funded the Search for Extra-Terrestrial Intelligence (SETI) to search radio frequencies of the electromagnetic spectrum for signals being transmitted by extraterrestrial life outside the Solar System. NASA's Viking missions to Mars, launched in 1976, included three biology experiments designed to look for possible signs of present life on Mars. The Mars Pathfinder lander in 1997 carried a scientific -5-
payload intended for exopaleontology in the hopes of finding microbial fossils entombed in the rocks. In the 21st century, astrobiology is a focus of a growing number of NASA and European Space Agency Solar System exploration missions. The first European workshop on astrobiology took place in May 2001 in Italy, and the outcome was the Aurora programme. Currently, NASA hosts the NASA Astrobiology Institute and a growing number of universities in the United States (e.g., University of Arizona, Penn State University, Montana State University, University of Washington, and Arizona State University), Britain (e.g., The University of Glamorgan), Canada, Ireland, and Australia (e.g., The University of New South Wales) now offer graduate degree programs in astrobiology. The International Astronomical Union regularly organizes international conferences through its Bioastronomy Commission. Advancements in the fields of astrobiology, observational astronomy and discovery of large varieties of extremophiles with extraordinary capability to thrive in the harshest environments on Earth, have led to speculation that life may possibly be thriving on many of the extraterrestrial bodies in the universe. A particular focus of current astrobiology research is the search for life on Mars due to its proximity to Earth and geological history. There is a growing body of evidence to suggest that Mars has previously had a considerable amount of water on its surface, water being considered an essential precursor to the development of carbon-based life. Missions specifically designed to search for life include the Viking program and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive, and Beagle 2 failed to transmit from the surface and is assumed to have crashed. A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been cancelled. In late 2008, the Phoenix lander probed the environment for past and present planetary habitability of microbial life on Mars, and to research the history of water there. In November 2011, NASA launched the Mars Science Laboratory (MSL) rover, nicknamed Curiosity, which continues the search for past or present life on Mars. Curiosity landed on Mars at Gale Crater in August 2012. The European Space Agency is developing the ExoMars astrobiology rover, which is to be launched in 2018. -6-
LIFE IN THE SOLAR SYSTEM
INTRODUCTION Alien life, such as bacteria, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth. This argument is embodied in the Copernican principle, which states that the Earth does not occupy a unique position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth. Life may have emerged independently at many places throughout the Universe. Alternatively life may form less frequently, then spread between habitable planets through panspermia or exogenesis. In any case, complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth based on computer model studies. According to these studies, this same process may also occur around other stars that acquire planets. (Also see Extraterrestrial organic molecules.) Suggested locations at which life might have developed include the planets Venus and Mars, Jupiter's moon Europa, and Saturn's moons Titan and Enceladus.
LIFE ON MARS Introduction For centuries people have speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Serious searches for evidence of life began in the 19th century, and continue via telescopic investigations and landed missions. While early work focused on phenomenology and bordered on fantasy, modern scientific inquiry has emphasized the search for chemical biosignatures of life in the soil and rocks at the planet's -7-
surface, and the search for biomarker gases in the atmosphere. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, and it remains an open question whether life currently exists on Mars, or has existed there in the past.
Historical perspective Mars' polar ice caps were observed as early as the mid-17th century, and they were first proven to grow and shrink alternately, in the summer and winter of each hemisphere, by William Herschel in the latter part of the 18th century. By the mid-19th century, astronomers knew that Mars had certain other similarities to Earth, for example that the length of a day on Mars was almost the same as a day on Earth. They also knew that its axial tilt was similar to Earth's, which meant it experienced seasons just as Earth does — but of nearly double the length owing to its much longer year. These observations led to the increase in speculation that the darker albedo features were water, and brighter ones were land. It was therefore natural to suppose that Mars may be inhabited by some form of life. In 1854, William Whewell, a fellow of Trinity College, Cambridge, who popularized the word scientist, theorized that Mars had seas, land and possibly life forms. Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals — which were however soon found to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilization. This idea led British writer H. G. Wells to write The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet’s desiccation. Spectroscopic analysis of Mars' atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen were present in the Martian atmosphere. By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal theory.
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Unmanned missions Mariner 4
Mariner Crater, as seen by Streamlined Islands seen by Mariner 4 in 1965. Pictures like Viking orbiter showed that this suggested that Mars is too dry large floods occurred on Mars. for any kind of life. Image is located in Lunae Palus quadrangle.
Mariner 4 probe performed the first successful flyby of the planet Mars, returning the first pictures of the Martian surface in 1965. The photographs showed an arid Mars without rivers, oceans, or any signs of life. Further, it revealed that the surface (at least the parts that it photographed) was covered in craters, indicating a lack of plate tectonics and weathering of any kind for the last 4 billion years. The probe also found that Mars has no global magnetic field that would protect the planet from potentially life-threatening cosmic rays. The probe was able to calculate the atmospheric pressure on the planet to be about 0.6 kPa (compared to Earth's 101.3 kPa), meaning that liquid water could not exist on the planet's surface. After Mariner 4, the search for life on Mars changed to a search for bacteria-like living organisms rather than for multicellular organisms, as the environment was clearly too harsh for these.
Viking orbiters Liquid water is necessary for known life and metabolism, so if water was present on Mars, the chances of it having supported life may have been determinant. The Viking orbiters found evidence of possible river valleys in many areas, erosion and, in the southern hemisphere, branched streams. -9-
Carl Sagan poses next to a replica of the Viking landers.
Viking experiments The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil because the favorable conditions for the evolution of multicellular organisms ceased some four billion years ago on Mars. The tests were formulated to look for microbial life similar to that found on Earth. Of the four experiments, only the Labeled Release (LR) experiment returned a positive result, showing increased
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CO2 production on first exposure of soil to water and nutrients. All scientists
agree on two points from the Viking missions: that radiolabeled
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CO2 was evolved in the
Labeled Release experiment, and that the GC-MS detected no organic molecules. However, there are vastly different interpretations of what those results imply. One of the designers of the Labeled Release experiment, Gilbert Levin, believes his results are a definitive diagnostic for life on Mars. However, this result is disputed by many scientists, who argue that superoxidant chemicals in the soil could have produced this effect without life being present. An almost general consensus discarded the Labeled Release data as evidence of life, because the gas chromatograph & mass spectrometer, designed to identify natural organic matter, did not detect organic molecules. The results of the Viking mission concerning life are considered by the general expert community, at best, as inconclusive. In 2007, during a Seminar of the Geophysical Laboratory of the Carnegie Institution (Washington, D.C., USA), Gilbert Levin's investigation was assessed once more. Levin still maintains that his original data were correct, as the positive and negative control experiments were in order. Moreover, Levin's team, on 12 April 2012, reported a statistical speculation, based on old data —reinterpreted mathematically through complexity analysis— of the Labeled Release experiments, that may suggest evidence of "extant microbial life on Mars." Critics counter that the method has not yet been proven effective - 10 -
for differentiating between biological and non-biological processes on Earth so it is premature to draw any conclusions. Ronald Paepe, an edaphologist (soil scientist), communicated to the European Geosciences Union Congress that the discovery of the recent detection of silicate minerals on Mars may indicate pedogenesis, or soil development processes, extended over the entire surface of Mars. Paepe's interpretation views most of Mars surface as active soil, colored red by eons of widespread wearing by water, vegetation and microbial activity. A research team from the National Autonomous University of Mexico headed by Rafael Navarro-González, concluded that the equipment (TV-GC-MS) used by the Viking program to search for organic molecules, may not be sensitive enough to detect low levels of organics. Because of the simplicity of sample handling, TV–GC–MS is still considered the standard method for organic detection on future Mars missions, so Navarro-González suggests that the design of future organic instruments for Mars should include other methods of detection. Gillevinia straata
The claim for life on Mars, in the form of Gillevinia straata, is based on old data reinterpreted as sufficient evidence of life, mainly by professors Gilbert Levin, Rafael Navarro-González and Ronalds Paepe. The evidence supporting the existence of Gillevinia straata microorganisms relies on the data collected by the two Mars Viking landers that searched for biosignatures of life, but the analytical results were, officially, inconclusive. In 2006, Mario Crocco, a neurobiologist at the Neuropsychiatric Hospital Borda in Buenos Aires, Argentina, proposed the creation of a new nomenclatural rank that classified the Viking landers' results as 'metabolic' and therefore belonging to a form of life. Crocco proposed to create new biological ranking categories (taxa), in the new kingdom system of life, in order to be able to accommodate the genus of Martian microorganisms. Crocco proposed the following taxonomical entry:
Organic life system: Solaria
Biosphere: Marciana
Kingdom: Jakobia (named after neurobiologist Christfried Jakob)
Genus et species: Gillevinia straata - 11 -
As a result, the hypothetical Gillevinia straata would not be a bacterium (which rather is a terrestrial taxon), but a member of the kingdom 'Jakobia' in the biosphere 'Marciana' of the 'Solaria' system. The intended effect of the new nomenclature was to reverse the burden of proof concerning the life issue, but the taxonomy proposed by Crocco has not been accepted by the scientific community and is considered a single nomen nudum. Further, no Mars mission has found traces of biomolecules.
An artist's concept of the Phoenix spacecraft
Phoenix lander, 2008 The Phoenix mission landed a robotic spacecraft in the polar region of Mars on May 25, 2008 and it operated until November 10, 2008. One of the mission's two primary objectives was to search for a "habitable zone" in the Martian regolith where microbial life could exist, the other main goal being to study the geological history of water on Mars. The lander has a 2.5 meter robotic arm that was capable of digging shallow trenches in the regolith. There was an electrochemistry experiment which analysed the ions in the regolith and the amount and type of antioxidants on Mars. The Viking program data indicate that oxidants on Mars may vary with latitude, noting that Viking 2 saw fewer oxidants than Viking 1 in its more northerly position. Phoenix landed further north still. Phoenix's preliminary data revealed that Mars soil contains perchlorate, and thus may not be as life-friendly as thought earlier. The pH and salinity level were viewed as benign from the standpoint of biology. The analysers also indicated the presence of bound water and CO2.
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Mars Science Laboratory
Curiosity rover self-portrait at "Rocknet" (October 31, 2012), with the rim of Gale Crater and the slopes of Aeolis Mons in the distance.
The Mars Science Laboratory mission is a NASA spacecraft launched on November 26, 2011 that deployed the Curiosity rover, a nuclear-powered robot bearing instruments designed to look for past or present conditions relevant to biological activity (planetary habitability. The Curiosity rover landed on Mars on Aeolis Palus in Gale Crater, near Aeolis Mons (a.k.a.Mount Sharp), on August 6, 2012.
Future missions 
ExoMars
is
a
European-led
multi-spacecraft
programme
currently
under
development by the European Space Agency (ESA) and Russian Space Agency for launch in 2016 and 2018. Its primary scientific mission will be to search for possible biosignatures on Mars, past or present. A rover with a 2 metres (6.6 ft) core drill will be used to sample various depths beneath the surface where liquid water may be found and where microorganisms might survive cosmic radiation 
Mars Sample Return Mission — The best life detection experiment proposed is the examination on Earth of a soil sample from Mars. However, the difficulty of providing and maintaining life support over the months of transit from Mars to Earth remains to be solved. Providing for still unknown environmental and nutritional - 13 -
requirements is daunting. Should dead organisms be found in a sample, it would be difficult to conclude that those organisms were alive when obtained.
Presence of molecules and conditions necessary for life Liquid water
A series of artist's conceptions of hypothetical past water coverage on Mars.
No Mars probe since Viking has tested the Martian regolith specifically for metabolism which is the ultimate sign of current life. NASA's recent missions have focused on another question: whether Mars held lakes or oceans of liquid water on its surface in the ancient past. Scientists have found hematite, a mineral that forms in the presence of water. Thus, the mission of the Mars Exploration Rovers of 2004 was not to look for present or past life, but for evidence of liquid water on the surface of Mars in the planet's ancient past. Liquid water, necessary for Earth life and for metabolism as generally conducted by species on Earth, cannot exist on the surface of Mars under its present low atmospheric pressure and temperature, except at the lowest shaded elevations for short periods and liquid water does not appear at the surface itself. In June 2000, evidence for water currently under the surface of Mars was discovered in the form of flood-like gullies. Deep subsurface water deposits near the planet's liquid core might form a present-day habitat for life. However, in March 2006, astronomers announced the discovery of similar gullies on the Moon, which is believed never to have had liquid water on its surface. The astronomers suggest that the gullies could be the result of micrometeorite impacts. - 14 -
In March 2004, NASA announced that its rover Opportunity had discovered evidence that Mars was, in the ancient past, a wet planet. This had raised hopes that evidence of past life might be found on the planet today. ESA confirmed that the Mars Express orbiter had directly detected huge reserves of water ice at Mars' south pole in January 2004. On July 28, 2005, ESA announced that they had recorded photographic evidence of surface water ice near Mars' North pole. In December 2006, NASA showed images taken by the Mars Global Surveyor that suggested that water occasionally flows on the surface of Mars. The images did not actually show flowing water. Rather, they showed changes in craters and sediment deposits, providing the strongest evidence yet that water coursed through them as recently as several years ago, and is perhaps doing so even now. Some researchers were skeptical that liquid water was responsible for the surface feature changes seen by the spacecraft. They said other materials such as sand or dust can flow like a liquid and produce similar results. Recent analysis of Martian sandstones, using data obtained from orbital spectrometry, suggests that the waters that previously existed on the surface of Mars would have had too high a salinity to support most Earth-like life. Tosca et al. found that the Martian water in the locations they studied all had water activity, aw ≤ 0.78 to 0.86—a level fatal to most Terrestrial life. Haloarchaea, however, are able to live in hypersaline solutions, up to the saturation point. The Phoenix Mars lander from NASA, which landed in the Mars Arctic plain in May 2008, confirmed the presence of frozen water near the surface. This was confirmed when bright material, exposed by the digging arm of the lander, was found to have vaporized and disappeared in 3 to 4 days. This has been attributed to sub-surface ice, exposed by the digging and sublimated on exposure to the atmosphere. Cosmic radiation In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery. Scientists speculate that the lack of magnetic shielding helped the solar wind blow away much of Mars's atmosphere over the course of several billion years. - 15 -
After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation. In 2007, it was calculated that DNA and RNA damage by cosmic radiation would limit life on Mars to depths greater than 7.5 metres below the planet's surface. Therefore, the best potential locations for discovering life on Mars may be at subsurface environments that have not been studied yet. Life on Earth under Martian conditions On 26 April 2012, scientists reported that lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).
LIFE ON TITAN Whether there is life on Titan, the largest moon of Saturn, is at present an open question and a topic of scientific evaluation and research. Titan is far colder than Earth, and its surface seems to lack liquid water; factors which have led some scientists to consider life there unlikely. On the other hand, the following points have been made in favor of Titan's suitability to sustain some form of life: 
Titan appears to have lakes of liquid ethane and/or liquid methane on its surface, as well as rivers and seas, which some scientific models (still tentative and debated) suggest could support non-water-based life.
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It has also been suggested that life may exist in a sub-surface ocean consisting of water and ammonia. Recent data from NASA's Cassini spacecraft have strengthened evidence that Titan likely harbors a layer of liquid water under its ice shell.
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Titan is the only known natural satellite (moon) in the Solar System that is known to have a fully developed atmosphere that consists of more than trace gases. Titan's atmosphere is thick, chemically active, and is known to be rich in organic compounds; this has led to speculation about whether chemical precursors of life may have been generated there. - 16 -
The atmosphere also contains hydrogen gas, which is cycling through the atmosphere and the surface environment, and which living things comparable to Earth methanogens could combine with some of the organic compounds (such as acetylene) to obtain energy.
In June 2010, scientists analysing data from the Cassini–Huygens mission reported anomalies in the atmosphere near the surface which could be consistent with the presence of methane-producing organisms, but may alternatively be due to non-living chemical or meteorological processes. The Cassini–Huygens mission was not equipped to provide direct evidence for biology or complex organics.
LIFE ON EUROPA Europa, is the sixth closest moon of the planet Jupiter, and the smallest of its four Galilean satellites, but still one of the largest moons in the Solar System. Europa was discovered in 1610 by Galileo Galilei and possibly independently by Simon Marius around the same time. Progressively more in-depth observation of Europa has occurred over the centuries by Earth-bound telescopes, and by space probe flybys starting in the 1970s. Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of water ice and is one of the smoothest in the Solar System. This surface is striated by cracks and streaks, while cratering is relatively infrequent. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life. This hypothesis proposes that heat energy from tidal flexing causes the ocean to remain liquid and drives geological activity similar to plate tectonics. The Galileo mission, launched in 1989, provided the bulk of current data on Europa. Although only fly-by missions have visited the moon, the intriguing characteristics of Europa have led to several ambitious exploration proposals. The next mission to Europa is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022.
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Europa has emerged as one of the top locations in the Solar System in terms of potential habitability and the possibility of hosting extraterrestrial life. Life could exist in its underice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents. Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean. So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there. Until the 1970s, life, at least as the concept is generally understood, was believed to be entirely dependent on energy from the Sun. Plants on Earth's surface capture energy from sunlight to photosynthesize sugars from carbon dioxide and water, releasing oxygen in the process, and are then eaten by oxygen-respiring animals, passing their energy up the food chain. Even life in the deep ocean, far below the reach of sunlight, was believed to obtain its nourishment either from the organic detritus raining down from the surface, or by eating animals that in turn depend on that stream of nutrients. An environment's ability to support life was thus thought to depend on its access to sunlight. However, in 1977, during an exploratory dive to the Galapagos Rift in the deep-sea exploration submersible Alvin, scientists discovered colonies of giant tube worms, clams, crustaceans, mussels, and other assorted creatures clustered around undersea volcanic features known as black smokers. These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent food chain. Instead of plants, the basis for this food chain was a form of bacterium that derived its energy from oxidization of reactive chemicals, such as hydrogen or hydrogen sulfide, that bubbled up from the Earth's interior. This chemosynthesis revolutionized the study of biology by revealing that life need not be sun-dependent; it only requires water and an energy gradient in order to exist. It opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats. While the tube worms and other multicellular eukaryotic organisms around these hydrothermal vents respire oxygen and thus are indirectly dependent on photosynthesis, anaerobic chemosynthetic bacteria and archaea that inhabit these ecosystems provide a possible model for life in Europa's ocean. The energy provided by tidal flexing drives active geological processes within Europa's interior, just as they do to a far more obvious degree on its sister moon Io. While Europa, like the Earth, may possess an internal energy source from radioactive decay, the energy generated by tidal flexing would be several orders of - 18 -
magnitude greater than any radiological source. However, such an energy source could never support an ecosystem as large and diverse as the photosynthesis-based ecosystem on Earth's surface. Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths are known to inhabit on Earth. Alternatively, it could exist clinging to the lower surface of the moon's ice layer, much like algae and bacteria in Earth's polar regions, or float freely in Europa's ocean. However, if Europa's ocean were too cold, biological processes similar to those known on Earth could not take place. Similarly, if it were too salty, only extreme halophiles could survive in its environment. In September 2009, planetary scientist Richard Greenberg calculated that cosmic rays impacting on Europa's surface convert some water ice into free oxygen (O2) which could then be absorbed into the ocean below as water wells up to fill cracks. Via this process, Greenberg estimates that Europa's ocean could eventually achieve an oxygen concentration greater than that of Earth's oceans within just a few million years. This would enable Europa to support not merely anaerobic microbial life but potentially larger, aerobic organisms such as fish. In 2006, Robert T. Pappalardo, an assistant professor in the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder said, We’ve spent quite a bit of time and effort trying to understand if Mars was once a habitable environment. Europa today, probably, is a habitable environment. We need to confirm this … but Europa, potentially, has all the ingredients for life … and not just four billion years ago … but today. In November 2011, a team of researchers presented evidence in the journal Nature suggesting the existence of vast lakes of liquid water entirely encased in the moon's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell. If confirmed, the lakes could be yet another potential habitat for life. A paper published in March 2013 suggests that hydrogen peroxide is abundant across much of the surface of Jupiter's moon Europa. The authors argue that if the peroxide on the surface of Europa mixes into the ocean below, it could be an important energy supply for simple forms of life, if life were to exist there. The scientists think hydrogen peroxide is an important factor for the habitability of the global liquid water ocean under Europa's icy crust because hydrogen peroxide decays to oxygen when mixed into liquid water. - 19 -
Exploration
Europa in 1973 by Pioneer 10
Europa in 1979 by Voyager 1
Much human knowledge of Europa has been derived from a series of flybys beginning in the 1970s. Pioneer 10 and 11 visited Jupiter in 1973 and 1974 respectively; these first closeup photos of Jupiter's largest moons produced by the Pioneers were fuzzy compared to later missions. The two Voyager probes traveled through the Jovian system in 1979 providing more detailed images of Europa's icy surface. The images caused many scientists to speculate about the possibility of a liquid ocean underneath. Starting in 1995, Galileo probe began a Jupiter orbiting mission that lasted for eight years, until 2003, and provided the most detailed examination of the Galilean moons to date. It included, Galileo Europa Mission and Galileo Millennium Mission, with numerous close flybys of Europa. New Horizons imaged Europa in 2007, as it flew by the Jovian system while on its way to Pluto. - 20 -
Future missions Conjecture on extraterrestrial life has ensured a high profile for the moon and has led to steady lobbying for future missions. The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its hypothesized subsurface oceans. Missions to Europa need to survive the high radiation environment around itself and Jupiter. Europa receives about 540 rem of radiation per day. In 2012, Jupiter Icy Moon Explorer was selected by the ESA as a planned mission. That mission includes some flybys of Europa, but is more focused on Ganymede. In 2011, a Europa mission was recommended by the latest Planetary Science Decadal Survey. Missions to study Europa in response to this recommendation included concepts for an orbiter or flyby spacecraft, and for a lander. Instrument payload for an orbiter could include a radio subsystem, laser altimeter, magnetometer, Langmuir probe, and a mapping camera.
LIFE ON ENCELADUS Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Enceladus seems to have liquid water under its icy surface. Cryovolcanoes at the south pole shoot large jets of water ice particles into space. Some of this water falls back onto the moon as "snow", some of it adds to Saturn's rings, and some of it reaches Saturn. The whole of Saturn's E ring is believed to have been made from these ice particles. Because of the apparent water at or near the surface, Enceladus may be one of the best places for humans to look for extraterrestrial life. By contrast, the water thought to be on Jupiter's moon Europa is locked under a very thick layer of surface ice. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface. The Voyagers showed that the diameter of Enceladus is only 500 kilometers (310 mi), about a tenth of that of Saturn's largest moon, Titan, and that it reflects almost all of the sunlight that strikes it. Voyager 1 found that Enceladus orbited in the densest part of Saturn's diffuse E ring, - 21 -
indicating a possible association between the two, while Voyager 2 revealed that despite the moon's small size, it had a wide range of terrains ranging from old, heavily cratered surfaces to young, tectonically deformed terrain, with some regions with surface ages as young as 100 million years old. In 2005 the Cassini spacecraft performed several close flybys of Enceladus, revealing the moon's surface and environment in greater detail. In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity; proximity to the planet can then lead to tidal heating of the satellite's interior, offering a possible explanation for the activity. Enceladus is one of only three outer Solar System bodies, with Jupiter's moon Io's sulfur volcanoes and Neptune's moon Triton's nitrogen "geysers" where active eruptions have been observed. Analysis of the outgassing suggests that it originates from a body of subsurface liquid water, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology.The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E ring. In May 2011 NASA scientists at an Enceladus Focus Group Conference reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".
Possible water ocean In late 2008, scientists observed water vapor spewing from Enceladus's surface, and it was later discovered that this vapor trails into Saturn. This could indicate the presence of liquid water, which might also make it possible for Enceladus to support life. Candice Hansen, a scientist with NASA's Jet Propulsion Lab, headed up a research team on the plumes after they were found to be moving at ~2,189 kilometers per hour (1,360 miles per hour). Since that speed is difficult to attain unless liquids are involved, they decided to investigate the compositions of the plumes. - 22 -
Eventually it was discovered that in the E-ring about 6% of particles contain 0.5–2% of sodium salts by mass, which is a significant amount. In the parts of the plume close to Enceladus the fraction of "salty" particles increases to 70% by number and >99% by mass. Such particles presumably are frozen spray from the salty underground ocean. On the other hand, the small salt-poor particles form by homogenous nucleation directly from the gas phase. The sources of salty particles are uniformly distributed along the tiger stripes, whereas sources of "fresh" particles are closely related to the high-speed gas jets. The "salty" particles move slowly and mostly fall back onto the surface, while the fast "fresh" particles escape to the E-ring, explaining its salt-poor composition. The "salty" composition of the plume strongly suggests that its source is a subsurface salty ocean or subsurface caverns filled with salty water. Alternatives such as the clathrate sublimation hypothesis can not explain how "salty" particles form. Additionally, Cassini found traces of organic compounds in some dust grains. Enceladus is therefore a candidate for harboring extraterrestrial life. The presence of liquid water under the crust implies that there is an internal heat source. It is now thought to be a combination of radioactive decay and tidal heating, as tidal heating alone is not sufficient to explain the heat. Mimas, another of Saturn's moons, is closer to the planet and has a much more eccentric orbit, meaning it should be exposed to far greater tidal forces than Enceladus, and yet its old and scarred surface implies that it is geologically dead.
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EXTREMOPHILES INTRODUCTION
Thermophiles, a type of extremophile, produce some of the bright colors of Grand Prismatic Spring, Yellowstone National Park
An extremophile (from Latin extremus meaning "extreme" and Greek philiā (φιλία) meaning "love") is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles. In the 1980s and 1990s, biologists found that microbial life has an amazing flexibility for surviving in extreme environments — niches that are extraordinarily hot, or acidic, for example — that would be completely inhospitable to complex organisms. Some scientists even concluded that life may have begun on Earth in hydrothermal vents far under the ocean's surface. According to astrophysicist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth — and we know they're very hardened to radiation." On 6 February 2013, scientists reported that bacteria were found living in the cold and dark in a lake buried a half-mile deep under the ice in Antarctica. On 17 March 2013, researchers reported data that suggested microbial life forms thrive in the Mariana Trench, the deepest spot on the Earth.Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet below the sea floor under 8500 feet of ocean off the coast of the northwestern United States. According to one of the researchers,"You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are." - 24 -
Most known extremophiles are microbes. The domain Archaea contains renowned examples, but extremophiles are present in numerous and diverse genetic lineages of bacteria and archaeans. Furthermore, it is erroneous to use the term extremophile to encompass all archaeans, as some are mesophilic. Neither are all extremophiles unicellular; protostome animals found in similar environments include the Pompeii worm, the psychrophilic Grylloblattidae (insects), Antarctic krill (a crustacean) and Tardigrades (water bears).
IN ASTROBIOLOGY Astrobiology is the field concerned with forming theories, such as panspermia, about the distribution, nature, and future of life in the universe. In it, microbial ecologists, astronomers, planetary scientists, geochemists, philosophers, and explorers cooperate constructively to guide the search for life on other planets. Astrobiologists are particularly interested in studying extremophiles, as many organisms of this type are capable of surviving in environments similar to those known to exist on other planets. For example, Mars may have regions in its deep subsurface permafrost that could harbor endolith communities. The subsurface water ocean of Jupiter's moon Europa may harbor life, especially at hypothesized hydrothermal vents at the ocean floor. Recent research carried out on extremophiles in Japan involved a variety of bacteria including Escherichia coli and Paracoccus denitrificans being subject to conditions of extreme gravity. The bacteria were cultivated while being rotated in an ultracentrifuge at high speeds corresponding to 403,627 times "g" (the normal acceleration due to gravity). Paracoccus denitrificans was one of the bacteria which displayed not only survival but also robust cellular growth under these conditions of hyperacceleration which are usually found only in cosmic environments, such as on very massive stars or in the shock waves of supernovas. Analysis showed that the small size of prokaryotic cells is essential for successful growth under hypergravity. The research has implications on the feasibility of panspermia. Recently, on 26 April 2012, scientists reported that lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR). - 25 -
EXAMPLES New sub-types of -philes are identified frequently and the sub-category list for extremophiles is always growing. For example, microbial life lives in the liquid asphalt lake, Pitch Lake. Research indicates that extremophiles inhabit the asphalt lake in populations ranging between 106 to 107 cells/gram. Likewise, until recently boron tolerance was known but a strong borophile was undiscovered in bacteria. With the recent isolation of Bacillus boroniphilus, borophiles came into discussion. Studying these borophiles may help illuminate the mechanisms of both boron toxicity and boron deficiency.
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HYPOTHETICAL TYPES OF BIOCHEMISTRY
INTRODUCTION Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. While the kinds of living beings we know on Earth commonly use carbon for basic structural and metabolic functions, water as a solvent and DNA or RNA to define and control their form, it may be possible that undiscovered life-forms could exist that differ radically in their basic structures and biochemistry from that known to science. The possibility of extraterrestrial life being based on these "alternative" biochemistries is a common subject in science fiction, but is also discussed in a non-fiction scientific context.
NON CARBON BASED BIOCHEMISTRIES Introduction On Earth, all living things have a molecular machinery of carbon compounds. Scientists have speculated about the pros and cons of using atoms other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe. Sagan used the term "carbon chauvinism" for such an assumption. Carl Sagan regarded silicon and germanium as conceivable alternatives to carbon; but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos.
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Silicon biochemistry
The most commonly proposed basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information. However, silicon has several drawbacks as a carbon alternative. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms necessary for the chemical versatility required for metabolism. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms. Moreover, where it does interact with other atoms, silicon creates molecules that have been described as "monotonous compared with the combinatorial universe of organic macromolecules". This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds (the double bonded carbon is part of the carbonyl group, a fundamental motif of bio-organic chemistry). Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations. Complex long-chain silicone molecules are still less stable than their carbon counterparts, though. Another obstacle is that silicon dioxide (a common ingredient of many sands), the analog of carbon dioxide, is a non-soluble solid at the temperature range where water is liquid, making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it. Another - 28 -
problem with silicon dioxide is that it would be the product of aerobic respiration. If a silicon-based life form were to breathe using oxygen, as life on Earth does, it would possibly produce silicon dioxide as a by-product of this, assuming that the only difference between the two types of life is silicon in place of carbon. This implies that the exhaled product, silicon dioxide, would be a solid, thus filling the respiratory organs of the organism with sand. This however would be solved if the organism lives in temperatures of several hundred to thousand degrees, where the silicon dioxide becomes a liquid. Oxygen-breathing silicon life, if it exists, is therefore most likely to exist in environments with very high temperatures or pressure. Finally, of the varieties of molecules identified in the interstellar medium as of 1998, 84 are based on carbon while only 8 are based on silicon. Moreover, of those 8 compounds, four also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets. Somewhat in support, in September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics - "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". (Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks.") Also, even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (the relative abundance of silicon to carbon in the Earth's crust is roughly 925:1), terrestrial life is carbon-based. The fact that carbon, though rare, has proven to be much more successful as a life base than the much more abundant silicon, may be evidence that silicon is poorly suited for biochemistry on Earth-like planets. For example: silicon is less versatile than carbon in forming compounds; the compounds formed by silicon are unstable and it blocks the flow of heat. Even so, biogenic silica is used by some Earth life, such as the silicate skeletal structure of diatoms. This suggests that extraterrestrial life forms may have silicon-based structure molecules and carbon-based proteins for metabolic
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purposes, therefore enabling the ability to feed on a common resource on a terrestrial planet like Earth for building up the silicon-based part of their body. Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon. A. G. Cairns-Smith has proposed that the first living organisms to exist on Earth were clay minerals—which were probably based on silicon. In cinematic and literary science fiction, a moment when man-made machines cross from nonliving to living, it is often posited, this new form would be the first example of noncarbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as computers (or computer-guided robots) and filed under "silicon-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".
Other exotic element-based biochemistries
Boron's chemistry is possibly even more variable than that of carbon, since it has the ability to form polyhedral clusters and three-center two-electron bonds. Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. However, boron's low cosmic abundance makes it less likely as a base for life than carbon.
Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds; the heteropoly acids are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond-like crystals (such as cubic zirconia). Titanium, aluminium, magnesium, and iron are all more abundant in the Earth's crust than carbon. Metal-oxide-based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon-based life would be unlikely.
Sulfur is also able to form long-chain molecules, but suffers from the same highreactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely theoretical, especially because sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an - 30 -
electron acceptor is widespread and can be traced back 3.5 billion years on Earth, thus predating the use of molecular oxygen. Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.) Chlorine as an alternative to oxygen A number of alternatives to molecular oxygen as a terminal electron acceptor are known from anaerobic life forms on Earth. However, it has been proposed that chlorine might serve as a more general biological alternative to oxygen, either in carbon-based biologies or hypothetical non-carbon-based ones. But chlorine is much less abundant than oxygen in the universe, and so planets with a sufficiently chlorine-rich atmosphere are likely to be rare, if they exist at all. Chlorine will, instead, likely be bound up as salts and other inert compounds. Arsenic as an alternative to phosphorus
Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy. It has been speculated that the earliest life forms on Earth may have used arsenic in place of phosphorus in the backbone of their DNA. A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic would not be suitable for this function. The authors of a 2010 geomicrobiology study, supported in part by NASA, have postulated that a bacterium, named GFAJ-1, collected in the sediments of Mono Lake in eastern California, can employ such 'arsenic DNA' when cultured without phosphorus. They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate or other means to reduce the effective concentration of water and stabilize its arsenate esters. This claim was heavily criticized almost immediately after publication for the perceived lack of - 31 -
appropriate controls Science writer Carl Zimmer contacted several scientists for an assessment: "I reached out to a dozen experts ... Almost unanimously, they think the NASA scientists have failed to make their case". Other authors were unable to reproduce their results and showed that the NASA scientists had issues with phosphate contamination (3 microM), which could sustain extremophile lifeforms. Selenium or tellurium as an alternative to protein sulfur Some organisms are already known to feature selenoproteins, in which sulfur is replaced by selenium. Some fungi also can produce telluro-methionine and telluro-cysteine. Selenomethionine is commonly included in media used to culture transformed bacteria for the purposes of X-ray diffraction studies of a protein target. Non-water solvents In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons, and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid. Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist; however on another occasion he said he was a carbon chauvinist but "not that much of a water chauvinist". He considered hydrocarbons, hydrofluoric acid, and ammonia
as
possible alternatives to water. Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity (useful for temperature regulation), a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H + ion, allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives. - 32 -
Additionally, water has the unusual property of being less dense as a solid (ice) than as a liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds), then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life. Not all properties of water are necessarily advantageous for life, however. For instance, water ice has a high albedo, meaning that it reflects a lot of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased. There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Because boiling points vary with the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 92 bars (9.2 MPa) of pressure, it can indeed exist in liquid form over a wide temperature range.
Ammonia
Artist's conception of how a planet with ammonia-based life may look.
The ammonia molecule (NH3), like the water molecule, is abundant in the universe, being a compound of hydrogen (the simplest and most common element) with another very common element, nitrogen. The possible role of liquid ammonia as an alternative solvent
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for life is an idea that goes back at least to 1954, when J.B.S. Haldane raised the topic at a symposium about life's origin. Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water. Ammonia can dissolve most organic molecules at least as well as water does and, in addition, it is capable of dissolving many elemental metals. Haldane made the point that various common water-related organic compounds have ammonia-related analogs; for instance the ammonia-related amine group (-NH2) is analogous to the water-related alcohol group (-OH). Ammonia, like water, can either accept or donate an H+ ion. When ammonia accepts an H+, it forms the ammonium cation (NH4+), analogous to hydronium (H3O+). When it donates an H+ ion, it forms the amide anion (NH2−), analogous to the hydroxide anion (OH−). Compared to water, however, ammonia is more inclined to accept an H + ion, and less inclined to donate one; it is a stronger nucleophile. Ammonia added to water functions as Arrhenius base: it increases the concentration of the anion hydroxide. Conversely, using a solvent system definition of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium. The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogous imine group (C=N) could be used instead. However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be a third, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. Gerald Feinberg and Robert Shapiro have questioned whether ammonia could hold prebiotic molecules together well enough to allow the emergence of a self-reproducing system. Ammonia is also flammable in oxygen, and could not exist sustainably in an environment suitable for aerobic metabolism.
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Titan's theorized internal structure, subsurface ocean shown blue.
A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth usually exists within the melting point and boiling point of water at normal pressure, between 0 °C (273 K) and 100 °C (373 K); at normal pressure ammonia's melting and boiling points are between −78 °C (195 K) and −33 °C (240 K). Chemical reactions generally proceed more slowly at a lower temperature, therefore liquid-ammonia life, if it exists, might metabolize more slowly and evolve more slowly than life on Earth. On the other hand, lower temperatures could also enable living systems to use chemical species which at Earth temperatures would be too unstable to be useful. Ammonia could be a liquid at Earth-like temperatures, but at much higher pressures; for example, at 60 atm, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K). Ammonia and ammonia–water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn's largest moon Titan.
Methane and other hydrocarbons Methane (CH4) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos, hydrogen and carbon. It has a cosmic abundance comparable with ammonia. Hydrocarbons could act as a solvent over a wide range of temperatures, but - 35 -
would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane. Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on Titan by the Cassini spacecraft.
Titan's surface lakes of liquid methane and ethane. False-color Cassini radar mosaic of Titan's north polar region. Blue coloring indicates low radar reflectivity, caused by the lakes.
There is debate about the effectiveness of methane and other hydrocarbons as a medium for life compared to water or ammonia. Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. However, water is also more chemically reactive, and can break down large organic molecules through hydrolysis. A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way. Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules. Life with a hydrocarbon
solvent
could
make
more
use
of
hydrogen
bonds
within
its
biomolecules.Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low temperature biochemistry. Astrobiologist Chris McKay has argued, on thermodynamic grounds, that if life does exist on Titan's surface, using hydrocarbons as a solvent, it is likely also to use the more complex hydrocarbons as an energy source by reacting them with hydrogen, reducing ethane and acetylene to methane. Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University; a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 1025 molecules per second and disappearance of hydrogen near Titan's surface. As Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present. The same year, - 36 -
another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane. While restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are to be considered more likely: the possibilities of yet unidentified physical or chemical processes (e.g., a non-living surface catalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow. He noted that even a non-biological catalyst, effective at 95 Kelvin, would in itself be a startling discovery. (While Mars is not known to have liquid methane, methane gas in its atmosphere is of astrobiological interest as a substance that might be produced by living organisms. See Life on Mars (planet).)
Hydrogen fluoride Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is −84 °C and its boiling point is 19.54 °C (at atmospheric pressure); the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbor molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath and Carl Sagan. The biota in an HF ocean could use the fluorine as an electron acceptor to photosynthesize energy. HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as paraffin waxes, are stable with it. Like water and ammonia, liquid hydrogen fluoride supports an acid-base chemistry. Using a solvent system definition of acidity and basicity, nitric acid functions as a base when it is added to liquid HF. However, hydrogen fluoride, unlike water, ammonia and methane, is cosmically rare.
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Other solvents or cosolvents
Model of sulfuric acid molecule (H2SO4).
Other solvents sometimes proposed:
Simple hydrogen compounds: hydrogen sulfide, hydrogen chloride,
More complex compounds: sulfuric acid, formamide, methanol,
Very-low-temperature fluids: liquid nitrogen, hydrogen in the form of a supercritical fluid.
Hydrogen sulfide is the closest chemical analog to water, but is less polar and a weaker inorganic solvent. Hydrogen sulfide and hydrogen chloride are cosmically rarer than water and ammonia. Sulfuric acid in liquid form is strongly polar. It is known to be abundant in the clouds of Venus, in the form of as aerosol droplets. In a biochemistry that used sulfuric acid as a solvent, the alkene group (C=C), with two carbon atoms joined by a double bond, could function analogously to the carbonyl group (C=O) in water-based biochemistry. A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent. A 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.
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Other types of speculations Non-green photosynthesizers Physicists have noted that, while photosynthesis on Earth generally involves green plants, a variety of other colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than that received on Earth. These studies indicate that, while blue photosynthetic plants would be unlikely (because absorbed blue light provides some of the highest photosynthetic yields in the light spectrum), yellow or red plants are plausible. These conclusions are, in part, based on the luminosity spectra of different types of stars, the transmission characteristics of hypothetical planetary atmospheres, and the absorption spectra of various photosynthetic pigments from organisms on Earth.
Alternative atmospheres The gases present in the atmosphere on Earth have varied greatly over its history. Traditional plant photosynthesis transformed the atmosphere by sequestering carbon from carbon dioxide, increasing the proportion of molecular oxygen, and by participating in the nitrogen cycle. Modern oxygen breathing animals would have been biochemically impossible until earlier photosynthetic life transformed Earth's atmosphere. The first dramatic rise in atmospheric oxygen on Earth, to about a tenth of its present-day value, occurred approximately 2.5 billion years ago, and that level did not change significantly until the Cambrian era approximately 600 million years ago. Changes in the gas mixture in the atmosphere, even in an atmosphere made up predominantly of the same molecules of Earth's atmosphere, impacts the biochemistry and morphology of life. For example, periods of high oxygen concentrations (determined from ice core samples) have been associated with fauna of a larger scale in the fossil record, while periods of low oxygen concentrations have been associated with fauna of a smaller scale in the fossil record. Also, while it is customary to think of plants on one side of the oxygen and nitrogen cycles as being sessile, and of animals on the other side as being motile, this is not a biological imperative. There are animals which are sessile for all or most of their lives (such as corals), - 39 -
and there are plants (such as tumbleweeds, and venus fly traps) that exhibit more mobility than is customarily associated with plants. On a slowly rotating planet, for example, it might be adaptive for photosynthesis to be performed by "plants" that can move to remain in the light, like Earth's sunflowers; while non-photosynthetic "animals", much like Earth's fungi, might have a lesser need to move from place to place on their own. This would be a mirror image of Earth's ecology.
Variable environments Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a spore or hibernation state that can be sustained for years or even millennia between more active life stages. Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it. For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state, whereas desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods. Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.
Nonplanetary life Dust and plasma-based In 2007, V. N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.Computer models showed that, when the dust became charged, the particles could self-organize into microscopic helical structures capable of replicating themselves, interacting with other neighboring structures, and evolving into more stable forms. Similar forms of life were described in Fred Hoyle's classic novel The Black Cloud.
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POSSIBILITY OF LIFE IN EXOPLANETS Extrasolar planet
2 January 2013: Astronomers state that the Milky Way galaxy may contain as many as 400 billion exoplanets, with almost every star hosting at least one planet.
Artist's view gives an impression of how commonly planets revolve around the stars in the Milky Way Galaxy.
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of 861 such planets (in 677 planetary systems, including 128 multiple planetary systems) have been identified as of March 22, 2013. The Kepler mission has detected over 18,000 additional transit events, including 262 that may be habitable planets. In the Milky Way galaxy, it is expected that there are many billions of planets (at least one planet, on average, orbiting around each star, resulting in 100–400 billion exoplanets), with many more free-floating planetary-mass bodies orbiting the galaxy directly. The nearest known exoplanet is Alpha Centauri Bb. Almost all of the planets detected so far are within our home galaxy the Milky Way; however, there have been a small number of possible detections of extragalactic planets. Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) reported in
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January 2013, that "at least 17 billion" Earth-sized exoplanets are estimated to reside in the Milky Way galaxy. For centuries, many philosophers and scientists supposed that extrasolar planets existed, but there was no way of knowing how common they were or how similar they might be to the planets of the Solar System. Various detection claims, starting in the nineteenth century, were all eventually rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Due to improved observational techniques, the rate of detections has increased rapidly since then. Some exoplanets have been directly imaged by telescopes, but the vast majority have been detected through indirect methods such as radial velocity measurements. Besides exoplanets, "exocomets", comets beyond our solar system, have also been detected and may be common in the Milky Way galaxy. Most known exoplanets are giant planets believed to resemble Jupiter or Neptune, but this reflects a sampling bias, as massive planets are more easily observed. Some relatively lightweight exoplanets, only a few times more massive than Earth (now known by the term Super-Earth), are known as well; statistical studies now indicate that they actually outnumber giant planets while recent discoveries have included Earth-sized and smaller planets and a handful that appear to exhibit other Earth-like properties. There also exist planetary-mass objects that orbit brown dwarfs and other bodies that "float free" in space not bound to any star; however, the term "planet" is not always applied to these objects. The discovery of extrasolar planets, particularly those that orbit in the habitable zone where it is possible for liquid water to exist on the surface (and therefore also life), has intensified interest in the search for extraterrestrial life. Thus, the search for extrasolar planets also includes the study of planetary habitability, which considers a wide range of factors in determining an extrasolar planet's suitability for hosting life. On January 7, 2013, astronomers from the Kepler Mission space observatory announced the discovery of KOI-172.02, an Earth-like exoplanet candidate orbiting a star similar to our Sun in the habitable zone and possibly a "prime candidate to host alien life".
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PLANETARY HABITABILITY
Understanding planetary habitability is partly an extrapolation of the Earth's conditions, as this is the only planet currently known to support life.
Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia. As the existence of life beyond Earth is currently uncertain, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favourable to life's flourishing—in particular those factors that have sustained complex, multicellular organisms and not just simpler, unicellular creatures. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology. An absolute requirement for life is an energy source, and the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body can support life. In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favourable for the assembly of complex organic molecules, and energy sources to sustain metabolism." In determining the habitability potential of a body, studies focus on its bulk composition, orbital properties, atmosphere, and potential chemical interactions. Stellar characteristics of importance include mass and luminosity, stable variability, and high metallicity. Rocky, terrestrial-type planets and moons with the potential for Earth-like chemistry are a primary - 43 -
focus of astrobiological research, although more speculative habitability theories occasionally examine alternative biochemistries and other types of astronomical bodies. The idea that planets beyond Earth might host life is an ancient one, though historically it was framed by philosophy as much as physical science. The late 20th century saw two breakthroughs in the field. The observation and robotic spacecraft exploration of other planets and moons within the Solar System has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies. The discovery of extrasolar planets, beginning in the early 1990s and accelerating thereafter, has provided further information for the study of possible extraterrestrial life. These findings confirm that the Sun is not unique among stars in hosting planets and expands the habitability research horizon beyond the Solar System. In 1964 Stephen H. Dole estimated the number of habitable planets in our galaxy to be about 600 million.
SUITABLE STAR SYSTEMS Introduction An understanding of planetary habitability begins with stars. While bodies that are generally Earth-like may be plentiful, it is just as important that their larger system be agreeable to life. Under the auspices of SETI's Project Phoenix, scientists Margaret Turnbull and Jill Tarter developed the "HabCat" (or Catalogue of Habitable Stellar Systems) in 2002. The catalogue was formed by winnowing the nearly 120,000 stars of the larger Hipparcos Catalogue into a core group of 17,000 "HabStars," and the selection criteria that were used provide a good starting point for understanding which astrophysical factors are necessary to habitable planets.
Spectral class The spectral class of a star indicates its photospheric temperature, which (for main-sequence stars) correlates to overall mass. The appropriate spectral range for "HabStars" is presently considered to be "early F" or "G", to "mid-K". This corresponds to temperatures of a little more than 7,000 K down to a little more than 4,000 K; the Sun, a G2 star, is well within these bounds. "Middle-class" stars of this sort have a number of characteristics considered important to planetary habitability: - 44 -
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They live at least a few billion years, allowing life a chance to evolve. More luminous main-sequence stars of the "O", "B", and "A" classes usually live less than a billion years and in exceptional cases less than 10 million.
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They emit enough high-frequency ultraviolet radiation to trigger important atmospheric dynamics such as ozone formation, but not so much that ionisation destroys incipient life.
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Liquid water may exist on the surface of planets orbiting them at a distance that does not induce tidal locking. K Spectrum stars may be able to support life for long periods, far longer than the Sun.
This spectral range probably accounts for between 5% and 10% of stars in the local Milky Way galaxy. Whether fainter late K and M class red dwarf stars are also suitable hosts for habitable planets is perhaps the most important open question in the entire field of planetary habitability given their ubiquity (habitability of red dwarf systems). Gliese 581 c, a "superEarth", has been found orbiting in the "habitable zone" of a red dwarf and may possess liquid water. Alternately, a greenhouse effect may render it too hot to support life, while its neighbor, Gliese 581 d, may in fact be a more likely candidate for habitability. In September 2010, the discovery was announced of another planet, Gliese 581 g, in an orbit between these two planets. However, reviews of the discovery have placed the existence of this planet in doubt, and it is currently listed as "unconfirmed". In September 2012, the discovery of two planets orbiting Gliese 163 was announced. One of the planets, Gliese 163 c, about 6.9 times the mass of Earth and somewhat hotter, was considered to be within the habitable zone.
A stable habitable zone The habitable zone (HZ, categorized by the Planetary Habitability Index) is a theoretical shell surrounding a star in which any planet present would have liquid water on its surface. After an energy source, liquid water is considered the most important ingredient for life, considering how integral it is to all life-systems on Earth. This may reflect the bias of humanity's water-dependent biology, however, and if life is discovered in the absence of water (for example, in a liquid-ammonia solution), the notion of an HZ may have to be greatly expanded or else discarded altogether as too restricting.
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A "stable" HZ denotes two factors. First, the range of an HZ should not vary greatly over time. All stars increase in luminosity as they age and a given HZ naturally migrates outwards, but if this happens too quickly (for example, with a super-massive star), planets may only have a brief window inside the HZ and a correspondingly weaker chance to develop life. Calculating an HZ range and its long-term movement is never straightforward, given that negative feedback loops such as the CNO cycle will tend to offset the increases in luminosity. Assumptions made about atmospheric conditions and geology thus have as great an impact on a putative HZ range as does Solar evolution; the proposed parameters of the Sun's HZ, for example, have fluctuated greatly. Secondly, no large-mass body such as a gas giant should be present in or relatively close to the HZ, thus disrupting the formation of Earth-like bodies. The mass of the asteroid belt, for example, appears to have been unable to accrete into a planet due to orbital resonances with Jupiter; if the giant had appeared in the region that is now between the orbits of Venus and Mars, Earth would almost certainly not have developed its present form. This is somewhat ameliorated by suggestions that a gas giant inside the HZ might have habitable moons under the right conditions. In the Solar System, the inner planets are terrestrial, the outer ones gas giants, but discoveries of extrasolar planets suggest this arrangement may not be at all common: numerous Jupiter-sized bodies have been found in close orbit about their primary, disrupting potential HZs. However, present data for extrasolar planets is likely to be skewed towards these types (large planets in close orbits) because they are far easier to identify; thus, it remains to be seen which type of planetary system is the norm, or indeed if there is one.
Low stellar variation Changes in luminosity are common to all stars, but the severity of such fluctuations covers a broad range. Most stars are relatively stable, but a significant minority of variable stars often experience sudden and intense increases in luminosity and consequently the amount of energy radiated toward bodies in orbit. These are considered poor candidates for hosting life-bearing planets as their unpredictability and energy output changes would negatively impact organisms. Particularly, living things adapted to a specific temperature range would probably be unable to survive too great a temperature deviation. Further, upswings in luminosity are generally accompanied by massive doses of gamma ray and X-ray radiation - 46 -
which might prove lethal. Atmospheres do mitigate such effects, but atmosphere retention might not occur on planets orbiting variables, because the high-frequency energy buffeting these bodies would continually strip them of their protective covering. The Sun, in this respect as in many others, is relatively benign: the variation between solar max and minimum is roughly 0.1% over its 11-year solar cycle. There is strong (though not undisputed) evidence that even minor changes in the Sun's luminosity have had significant effects on the Earth's climate well within the historical era; the Little Ice Age of the midsecond millennium, for instance, may have been caused by a relatively long-term decline in the Sun's luminosity. Thus, a star does not have to be a true variable for differences in luminosity to affect habitability. Of known "solar analogs," one that closely resembles the Sun is considered to be 18 Scorpii; unfortunately for the prospects of life existing in its proximity, the only significant difference between the two bodies is the amplitude of the solar cycle, which appears to be much greater for 18 Scorpii.
High metallicity While the bulk of material in any star is hydrogen and helium, there is a great variation in the amount of heavier elements (metals) stars contain. A high proportion of metals in a star correlates to the amount of heavy material initially available in the protoplanetary disk. A low amount of metal significantly decreases the probability that planets will have formed around that star, under the solar nebula theory of planetary system formation. Any planets that did form around a metal-poor star would probably be low in mass, and thus unfavorable for life. Spectroscopic studies of systems where exoplanets have been found to date confirm the relationship between high metal content and planet formation: "Stars with planets, or at least with planets similar to the ones we are finding today, are clearly more metal rich than stars without planetary companions." This relationship between high metallicity and planet formation also means that habitable systems are more likely to be found around younger stars, since stars that formed early in the universe's history have low metal content.
PLANETARY CHARACTERISTICS
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The moons of some gas giants could potentially be habitable. [18]
The chief assumption about habitable planets is that they are terrestrial. Such planets, roughly within one order of magnitude of Earth mass, are primarily composed of silicate rocks and have not accreted the gaseous outer layers of hydrogen and helium found on gas giants. That life could evolve in the cloud tops of giant planets has not been decisively ruled out, though it is considered unlikely given that they have no surface and their gravity is enormous. The natural satellites of giant planets, meanwhile, remain perfectly valid candidates for hosting life. In February 2011 the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the habitable zone. Six of the candidates in this zone are smaller than twice the size of Earth. A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported. Based on the findings, the Kepler Team estimated there to be "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone. In analyzing which environments are likely to support life, a distinction is usually made between simple, unicellular organisms such as bacteria and archaea and complex metazoans (animals). Unicellularity necessarily precedes multicellularity in any hypothetical tree of life and where single-celled organisms do emerge there is no assurance that this will lead to greater complexity. The planetary characteristics listed below are considered crucial for life generally, but in every case habitability impediments should be considered greater for multicellular organisms such as plants and animals versus unicellular life.
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Mass
Mars, with its rarefied atmosphere, is colder than the Earth would be if it were at a similar distance from the Sun
Low-mass planets are poor candidates for life for two reasons. First, their lesser gravity makes atmosphere retention difficult. Constituent molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision. Planets without a thick atmosphere lack the matter necessary for primal biochemistry, have little insulation and poor heat transfer across their surfaces (for example, Mars, with its thin atmosphere, is colder than the Earth would be if it were at a similar distance from the Sun), and provide less protection against meteoroids and high-frequency radiation. Further, where an atmosphere is less than 0.006 Earth atmospheres, water cannot exist in liquid form as the required atmospheric pressure, 4.56 mm Hg (608 Pa) (0.18 inch Hg), does not occur. The temperature range at which water is liquid is smaller at low pressures generally. Secondly, smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide. Plate tectonics appear particularly crucial, at least on Earth: not only does the process recycle important chemicals and minerals, it also fosters bio-diversity through continent creation and increased environmental complexity and helps create the convective cells necessary to generate Earth's magnetic field.
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"Low mass" is partly a relative label; the Earth is considered low mass when compared to the Solar System's gas giants, but it is the largest, by diameter and mass, and densest of all terrestrial bodies. It is large enough to retain an atmosphere through gravity alone and large enough that its molten core remains a heat engine, driving the diverse geology of the surface (the decay of radioactive elements within a planet's core is the other significant component of planetary heating). Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere. Thus, it would be fair to infer that the lower mass limit for habitability lies somewhere between that of Mars and Earth or Venus; 0.3 Earth masses has been offered as a rough dividing line for habitable planets. However, a 2008 study by the Harvard-Smithsonian Center for Astrophysics suggests that the dividing line may be higher. Earth may in fact lie on the lower boundary of habitability, since if it were any smaller, plate tectonics would be impossible. Venus, which has 85 percent Earth's mass, shows no signs of tectonic activity. Conversely, "super-Earths", terrestrial planets with higher masses than Earth, would have higher levels of plate tectonics and thus be firmly placed in the habitable range. Exceptional circumstances do offer exceptional cases: Jupiter's moon Io (which is smaller than any of the terrestrial planets) is volcanically dynamic because of the gravitational stresses induced by its orbit, and its neighbor Europa may have a liquid ocean or icy slush underneath a frozen shell also due to power generated from orbiting a gas giant. Saturn's Titan, meanwhile, has an outside chance of harbouring life, as it has retained a thick atmosphere and has liquid methane seas on its surface. Organic-chemical reactions that only require minimum energy are possible in these seas, but whether any living system can be based on such minimal reactions is unclear, and would seem unlikely. These satellites are exceptions, but they prove that mass, as a criterion for habitability, cannot necessarily be considered definitive at this stage of our understanding. A larger planet is likely to have a more massive atmosphere. A combination of higher escape velocity to retain lighter atoms, and extensive outgassing from enhanced plate tectonics may greatly increase the atmospheric pressure and temperature at the surface compared to Earth. The enhanced greenhouse effect of such a heavy atmosphere would tend to suggest that the habitable zone should be further out from the central star for such massive planets. - 50 -
Finally, a larger planet is likely to have a large iron core. This allows for a magnetic field to protect the planet from stellar wind and cosmic radiation, which otherwise would tend to strip away planetary atmosphere and to bombard living things with ionized particles. Mass is not the only criterion for producing a magnetic field—as the planet must also rotate fast enough to produce a dynamo effect within its core—but it is a significant component of the process.
Orbit and rotation As with other criteria, stability is the critical consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between a planet's farthest and closest approach to its parent star divided by the sum of said distances. It is a ratio describing the shape of the elliptical orbit. The greater the eccentricity the greater the temperature fluctuation on a planet's surface. Although they are adaptive, living organisms can only stand so much variation, particularly if the fluctuations overlap both the freezing point and boiling point of the planet's main biotic solvent (e.g., water on Earth). If, for example, Earth's oceans were alternately boiling and freezing solid, it is difficult to imagine life as we know it having evolved. The more complex the organism, the greater the temperature sensitivity. The Earth's orbit is almost wholly circular, with an eccentricity of less than 0.02; other planets in the Solar System (with the exception of Mercury) have eccentricities that are similarly benign. Data collected on the orbital eccentricities of extrasolar planets has surprised most researchers: 90% have an orbital eccentricity greater than that found within the Solar System, and the average is fully 0.25. This means that the vast majority of planets have highly eccentric orbits and of these, if their average distance from their star is deemed to be within the HZ they would nonetheless only be spending a small portion of their time within the zone. A planet's movement around its rotational axis must also meet certain criteria if life is to have the opportunity to evolve. A first assumption is that the planet should have moderate seasons. If there is little or no axial tilt (or obliquity) relative to the perpendicular of the ecliptic, seasons will not occur and a main stimulant to biospheric dynamism will disappear. The planet would also be colder than it would be with a significant tilt: when the greatest
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intensity of radiation is always within a few degrees of the equator, warm weather cannot move poleward and a planet's climate becomes dominated by colder polar weather systems. If a planet is radically tilted, meanwhile, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. The axial tilt of the Earth is higher now (in the Quaternary) than it has been in the past, coinciding with reduced polar ice, warmer temperatures and less seasonal variation. Scientists do not know whether this trend will continue indefinitely with further increases in axial tilt (see Snowball Earth). The exact effects of these changes can only be computer modelled at present, and studies have shown that even extreme tilts of up to 85 degrees do not absolutely preclude life "provided it does not occupy continental surfaces plagued seasonally by the highest temperature." Not only the mean axial tilt, but also its variation over time must be considered. The Earth's tilt varies between 21.5 and 24.5 degrees over 41,000 years. A more drastic variation, or a much shorter periodicity, would induce climatic effects such as variations in seasonal severity. Other orbital considerations include:
The planet should rotate relatively quickly so that the day-night cycle is not overlong. If a day takes years, the temperature differential between the day and night side will be pronounced, and problems similar to those noted with extreme orbital eccentricity will come to the fore.
The planet should also rotate quickly enough so that a magnetic dynamo may be started in its iron core to produce a magnetic field.
Change in the direction of the axis rotation (precession) should not be pronounced. In itself, precession need not affect habitability as it changes the direction of the tilt, not its degree. However, precession tends to accentuate variations caused by other orbital deviations; see Milankovitch cycles. Precession on Earth occurs over a 26,000-year cycle.
The Earth's Moon appears to play a crucial role in moderating the Earth's climate by stabilising the axial tilt. It has been suggested that a chaotic tilt may be a "deal-breaker" in terms of habitability—i.e. a satellite the size of the Moon is not only helpful but required to produce stability. This position remains controversial. - 52 -
Geochemistry It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental biochemistry as found on Earth, as the four elements most vital for life, carbon, hydrogen, oxygen, and nitrogen, are also the most common chemically reactive elements in the universe. Indeed, simple biogenic compounds, such as very simple amino acids such as glycine, have been found in meteorites and in the interstellar medium. These four elements together comprise over 96% of Earth's collective biomass. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, compose the solvent in which biological processes take place and in which the first reactions occurred that led to life's emergence. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available by oxidizing organic compounds, is the fuel of all complex life-forms. These four elements together make up amino acids, which in turn are the building blocks of proteins, the substance of living tissue. In addition, neither sulfur, required for the building of proteins, nor phosphorus, needed for the formation of DNA, RNA, and the adenosine phosphates essential to metabolism, are rare. Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only oxygen is present in any abundance in the Earth's crust. This can be partly explained by the fact that many of these elements, such as hydrogen and nitrogen, along with their simplest and most common compounds, such as carbon dioxide, carbon monoxide, methane, ammonia, and water, are gaseous at warm temperatures. In the hot region close to the Sun, these volatile compounds could not have played a significant role in the planets' geological formation. Instead, they were trapped as gases underneath the newly formed crusts, which were largely made of rocky, involatile compounds such as silica (a compound of silicon and oxygen, accounting for oxygen's relative abundance). Outgassing of volatile compounds through the first volcanoes would have contributed to the formation of the planets' atmospheres. The Miller-Urey experiment showed that, with the application of energy, amino acids can form from the synthesis of the simple compounds within a primordial atmosphere. Even so, volcanic outgassing could not have accounted for the amount of water in Earth's oceans. The vast majority of the water —and arguably carbon— necessary for life must - 53 -
have come from the outer Solar System, away from the Sun's heat, where it could remain solid. Comets impacting with the Earth in the Solar System's early years would have deposited vast amounts of water, along with the other volatile compounds life requires (including amino acids) onto the early Earth, providing a kick-start to the origin of life. Thus, while there is reason to suspect that the four "life elements" ought to be readily available elsewhere, a habitable system probably also requires a supply of long-term orbiting bodies to seed inner planets. Without comets there is a possibility that life as we know it would not exist on Earth.
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ΕΝΟΤΗΤΑ ΔΕΥΤΕΡΗ
ΠΡΟΓΡΑΜΜΑΤΑ SETI
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SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE (S.E.T.I.) .
Screen shot of the screensaver for SETI@home, a distributed computing project in which volunteers donate idle computer power to analyze radio signals for signs of extraterrestrial intelligence
INTRODUCTION The search for extraterrestrial intelligence (SETI) is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. SETI projects use scientific methods in this search. For example, electromagnetic radiation is monitored for signs of transmissions from civilizations on other worlds. Some of the most well known projects are run by Harvard University, the University of California, Berkeley, and the SETI Institute. Since the United States government withdrew funding for SETI projects in 1995, projects have been primarily funded by private sources. There are great challenges in searching the cosmos for signs of intelligent life, including their identification and interpretation. SETI projects necessarily make assumptions to narrow the search, the foremost being that electromagnetic radiation would be a medium of communication for advanced extraterrestrial life.
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RADIO EXPERIMENTS
Microwave window as seen by a ground based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence
Many radio frequencies penetrate our atmosphere quite well, and this led to radio telescopes that investigate the cosmos using large radio antennas. Furthermore, human endeavors emit considerable electromagnetic radiation as a byproduct of communications such as television and radio. These signals would be easy to recognize as artificial due to their repetitive nature and narrow bandwidths. If this is typical, one way of discovering an extraterrestrial civilization might be to detect non-natural radio emissions from a location outside our Solar System.
Early work As early as 1896, Nikola Tesla suggested that radio could be used to contact extraterrestrial life. In 1899 while investigating atmospheric electricity using a Tesla coil receiver in his Knob Hill lab, Tesla observed repetitive signals, substantially different from the signals noted from storms and Earth noise, that he interpreted as being of extraterrestrial origin. He later recalled the signals appeared in groups of one, two, three, and four clicks together. - 57 -
Tesla thought the signals were coming from Mars. Analysis of Tesla's research has ranged from suggestions that Tesla detected nothing, he simply was misunderstanding the new technology he was working with, to claims that Tesla may have been observing naturally occurring Jovian plasma torus signals. In the early 1900s, Guglielmo Marconi, Lord Kelvin, and David Peck Todd also stated their belief that radio could be used to contact Martians, with Marconi stating that his stations had also picked up potential Martian signals. On August 21–23, 1924, Mars entered an opposition closer to Earth than any time in a century before or since. In the United States, a "National Radio Silence Day" was promoted during a 36-hour period from the 21–23, with all radios quiet for five minutes on the hour, every hour. At the United States Naval Observatory, a radio receiver was lifted 3 kilometers above the ground in a dirigible tuned to a wavelength between 8 and 9 kilometers, using a "radio-camera" developed by Amherst College and Charles Francis Jenkins. The program was led by David Peck Todd with the military assistance of Admiral Edward W. Eberle (Chief of Naval Operations), with William F. Friedman (chief cryptographer of the US Army), assigned to translate any potential Martian messages. A 1959 paper by Philip Morrison and Giuseppe Cocconi first pointed out the possibility of searching the microwave spectrum, and proposed frequencies and a set of initial targets In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma", after the Queen of Oz in L. Frank Baum's fantasy books. Drake used a radio telescope 26 meters in diameter at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency, a region of the radio spectrum dubbed the "water hole" due to its proximity to the hydrogen and hydroxyl radical spectral lines. A 400 kilohertz band was scanned around the marker frequency, using a single-channel receiver with a bandwidth of 100 hertz. The information was stored on tape for off-line analysis. He found nothing of great interest, but has continued a pro-active involvement in the search for life beyond Earth for 50 years. The first SETI conference took place at Green Bank, West Virginia in November 1961. The ten attendees were conference organiser Peter Pearman, Frank Drake, Philip Morrison, businessman and radio amateur Dana Atchley, chemist Melvin Calvin, astronomer Su-Shu Huang, neuroscientist John C. Lilly, inventor Barney Oliver, astronomer Carl Sagan and radio astronomer Otto Struve. From the agenda points of the conference Drake derived the - 58 -
Drake equation by multiplying the various factors that were discussed at the conference. The Drake equation is an estimation of how many planets in the Milky Way are inhabited by intelligent life forms. The Soviet scientists took a strong interest in SETI during the 1960s and performed a number of searches with omnidirectional antennas in the hope of picking up powerful radio signals. Soviet astronomer Iosif Shklovskii wrote the pioneering book in the field Universe, Life, Intelligence (1962), which was expanded upon by American astronomer Carl Sagan as the best-selling Intelligent Life in the Universe (1966). The first Kraus-style radio telescope was powered up in 1963. It was 360 feet (110 m) wide, 500 feet (150 m) long, and 70 feet (21 m) high. In the March 1955 issue of Scientific American, John D. Kraus described a concept to scan the cosmos for natural radio signals using a flat-plane radio telescope equipped with a parabolic reflector. Within two years, his concept was approved for construction by Ohio State University. With $71,000 total in grants from the National Science Foundation, construction began on a 20-acre plot in Delaware, Ohio. This Ohio State University radio telescope was called Big Ear. Later, it began the world's first continuous SETI program, called the Ohio State University SETI program.
View of Arecibo Observatory in Puerto Rico with its 300 m dish- the world's largest. A small fraction of its observation time is devoted to SETI searches.
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In 1971, NASA funded a SETI study that involved Drake, Bernard Oliver of HewlettPackard Corporation, and others. The resulting report proposed the construction of an Earthbased radio telescope array with 1,500 dishes known as "Project Cyclops". The price tag for the Cyclops array was $10 billion USD. Cyclops was not built, but the report formed the basis of much SETI work that followed.
The WOW! Signal Credit: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO).
The OSU SETI program gained fame on August 15, 1977, when Jerry Ehman, a project volunteer, witnessed a startlingly strong signal received by the telescope. He quickly circled the indication on a printout and scribbled the phrase “Wow!� in the margin. Dubbed the Wow! signal, it is considered by some to be the best candidate for a radio signal from an artificial, extraterrestrial source ever discovered, but it has not been detected again in several additional searches. In 1979 the University of California, Berkeley, launched a SETI project named "Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations (SERENDIP)". In 1986, UC Berkeley initiated their second SETI effort, SERENDIP II, and has continued with four more SERENDIP efforts to the present day. The latest incarnation of the SERENDIP project is SERENDIP V.v, a commensal all-sky survey using the Arecibo radio telescope began in June 2009.
Sentinel, META, and BETA In 1980, Carl Sagan, Bruce Murray, and Louis Friedman founded the U.S. Planetary Society, partly as a vehicle for SETI studies. In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer specifically intended to search for SETI - 60 -
transmissions. Traditional desktop spectrum analyzers were of little use for this job, as they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit digital signal processing (DSP) technology could be used to build autocorrelation receivers to check far more channels. This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrow band channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the 26-meter Harvard/Smithsonian radio telescope at Harvard, Massachusetts. This project was named "Sentinel", and continued into 1985. Even 131,000 channels weren't enough to search the sky in detail at a fast rate, so Suitcase SETI was followed in 1985 by Project "META", for "Megachannel Extra-Terrestrial Assay". The META spectrum analyzer had a capacity of 8.4 million channels and a channel resolution of 0.05 hertz. An important feature of META was its use of frequency doppler shift to distinguish between signals of terrestrial and extraterrestrial origin. The project was led by Horowitz with the help of the Planetary Society, and was partly funded by movie maker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990 to search the southern sky. META II is still in operation, after an equipment upgrade in 1996. The follow-on to META was named "BETA", for "Billion-channel Extraterrestrial Assay", and it commenced observation on October 30, 1995. The heart of BETA's processing capability consisted of 63 dedicated fast Fourier transform (FFT) engines, each capable of performing a 222-point complex FFTs in two seconds, and 21 general-purpose personal computers equipped with custom digital signal processing boards. This allowed BETA to receive 250 million simultaneous channels with a resolution of 0.5 hertz per channel. It scanned through the microwave spectrum from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation per hop. An important capability of the BETA search was rapid and automatic re-observation of candidate signals, achieved by observing the sky with two adjacent beams, one slightly to the east and the other slightly to the west. A successful candidate signal would first transit the east beam, and then the west beam and do so with a speed consistent with Earth's sidereal rotation rate. A third receiver observed the horizon to veto signals of obvious terrestrial origin. On March 23, 1999, the 26-meter radio telescope on which Sentinel, META and BETA were based was blown over by strong winds and seriously damaged. This forced the BETA project to cease operation. - 61 -
MOP and Project Phoenix
Sensitivity vs range for SETI radio searches. The diagonal lines show transmitters of different effective powers. The x-axis is the sensitivity of the search. The y-axis on the right is the range in light years, and on the left is the number of sun-like stars within this range. The vertical line labeled SS is the typical sensitivity achieved by a full sky search, such as BETA above. The vertical line labeled TS is the typical sensitivity achieved by a targeted search such as Phoenix. [17]
In 1978, the NASA SETI program was heavily criticized by Senator William Proxmire, and funding for SETI research was removed from the NASA budget by Congress in 1981, however, funding was restored in 1982, after Carl Sagan talked with Proxmire and convinced him of the program's value. In 1992, the U.S. government funded an operational SETI program, in the form of the NASA Microwave Observing Program (MOP). MOP was planned as a long-term effort to conduct a general survey of the sky and also carry out targeted searches of 800 specific nearby stars. MOP was to be performed by radio antennas associated with the NASA Deep Space Network, as well as the 140-foot (43 m) radio telescope of the National Radio Astronomy Observatory at Green Bank, West Virginia and the 1,000-foot (300 m) radio telescope at the Arecibo Observatory in Puerto Rico. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be grouped together to obtain greater capacity. Those used in the targeted search had a bandwidth of 1 hertz per channel, while those used in the sky survey had a bandwidth of 30 hertz per channel.
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MOP drew the attention of the U.S. Congress, where the program was ridiculed and canceled a year after its start. SETI advocates continued without government funding, and in 1995 the nonprofit SETI Institute of Mountain View, California resurrected the MOP program under the name of Project "Phoenix", backed by private sources of funding. Project Phoenix, under the direction of Jill Tarter, is a continuation of the targeted search program from MOP and studies roughly 1,000 nearby Sun-like stars. From 1995 through March 2004, Phoenix conducted observations at the 64-meter Parkes radio telescope in Australia, the 140-foot (43 m) radio telescope of the National Radio Astronomy Observatory in Green Bank, West Virginia, and the 1,000-foot (300 m) radio telescope at the Arecibo Observatory in Puerto Rico. The project observed the equivalent of 800 stars over the available channels in the frequency range from 1200 to 3000 MHz. The search was sensitive enough to pick up transmitters with 1 GW EIRP to a distance of about 200 light years. According to Miss Tarter, in 2012 it costs around "$2 million a year to keep SETI research going at the SETI Institute" and approximately 10 times that to support "all kinds of SETI activity around the world."
The SETI League and Project Argus Founded in 1994 in response to the US Congress cancellation of the NASA SETI program, The SETI League, Inc. is a membership-supported nonprofit organization with 1,500 members in 62 countries. This grass-roots alliance of amateur and professional radio astronomers is headed by executive director emeritus Prof. H. Paul Shuch, the engineer credited with developing the world's first commercial home satellite TV receiver. Many SETI League members are licensed radio amateurs and microwave experimenters. Others are digital signal processing experts and computer enthusiasts. The SETI League pioneered the conversion of backyard satellite TV dishes 3 to 5 meters in diameter into research-grade radio telescopes of modest sensitivity.[21] The organization concentrates on coordinating a global network of small, amateur-built radio telescopes under Project Argus, an all-sky survey seeking to achieve real-time coverage of the entire sky. Project Argus was conceived as a continuation of the all-sky survey component of the late NASA SETI program (the targeted search having been continued by the SETI Institute's Project Phoenix). There are currently 143 Project Argus radio telescopes operating in 27 countries. Project Argus instruments typically exhibit sensitivity on the order of 10−23
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Watts/square metre, or roughly equivalent to that achieved by the Ohio State University Big Ear radio telescope in 1977, when it detected the landmark "Wow!" candidate signal. The name "Argus" derives from the mythical Greek guard-beast who had 100 eyes, and could see in all directions at once. In the SETI context, the name has been used for radio telescopes in fiction (Arthur C. Clarke, "Imperial Earth"; Carl Sagan, "Contact"), was the name initially used for the NASA study ultimately known as "Cyclops," and is the name given to an omnidirectional radio telescope design being developed at the Ohio State University.
SETI@home
SETI@home logo
SETI@home was conceived by David Gedye along with Craig Kasnoff and is a popular volunteer distributed computing project that was launched by the University of California, Berkeley, in May 1999. It was originally funded by The Planetary Society and Paramount Pictures, and later by the state of California. The project is run by director David P. Anderson and chief scientist Dan Werthimer. Any individual can become involved with SETI research by downloading the Berkeley Open Infrastructure for Network Computing (BOINC) software program, attaching to the SETI@home project, and allowing the program to run as a background process that uses idle computer power. The SETI@home program itself runs signal analysis on a "work unit" of data recorded from the central 2.5 MHz wide band of the SERENDIP IV instrument. After computation on the work unit is complete, the results are then automatically reported back to SETI@home servers at UC Berkeley. By June 28, 2009, the SETI@home project had over 180,000 active participants volunteering a total of over 290,000 computers. These computers give SETI@home an average computational power of 617 teraFLOPS. Radio source SHGb02+14a is the most interesting signal analyzed to date.
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As of 2010, after 10 years of data collection, SETI@home has listened to that one frequency at every point of over 67 percent of the sky observable from Arecibo with at least 3 scans (out of the goal of 9 scans), which covers about 20 percent of the full celestial sphere.
Allen Telescope Array The SETI Institute has been collaborating with the Radio Astronomy Laboratory at UC Berkeley to develop a specialized radio telescope array for SETI studies, something like a mini-cyclops array. The array concept is named the "Allen Telescope Array" (ATA) (formerly, One Hectare Telescope [1HT]) after the project's benefactor Paul Allen. Its sensitivity will be equivalent to a single large dish more than 100 meters in diameter. The array is being constructed at the Hat Creek Observatory in rural northern California. [24] The full array is planned to consist of 350 or more Gregorian radio dishes, each 6.1 meters (20 ft) in diameter. These dishes are the largest producible with commercially available satellite television dish technology. The ATA was planned for a 2007 completion date, at a very modest cost of $25 million USD. The SETI Institute provides money for building the ATA while UC Berkeley designs the telescope and provides operational funding. Berkeley astronomers will use the ATA to pursue other deep space radio observations. The ATA is intended to support a large number of simultaneous observations through a technique known as "multibeaming", in which DSP technology is used to sort out signals from the multiple dishes. The DSP system planned for the ATA is extremely ambitious. The first portion of the array became operational in October 2007 with 42 antennas. Completion of the full 350 element array will depend on funding and the technical results from the 42-element subarray. CNET published an article and pictures about the Allen Telescope Array (ATA) on December 12, 2008. In April 2011, the ATA was forced to enter "hibernation" due to funding shortfalls. Regular operation of the ATA was resumed on December 5, 2011.
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SETI Net SETI Net is a private search system created by a single individual. It is closely affiliated with the SETI League and is one of the project Argus stations (DM12jw). The SETI Net station consists of off-the-shelf, consumer-grade electronics to minimize cost and to allow this design to be replicated as simply as possible. It has a 3-meter parabolic antenna that can be directed in azimuth and elevation, an LNA that covers the 1420 MHz spectrum, a receiver to reproduce the wideband audio, and a standard PC as the control device and for deploying the detection algorithms. The antenna can be pointed and locked to one sky location, enabling the system to integrate on it for long periods. Currently the Wow! signal area is being monitored when it is above the horizon, but all search data are collected and made available on the internet archive. SETI Net started operation in the early 1980s as a way to learn about the science of the search, and has developed several software packages for the amateur SETI community. It has provided an astronomical clock, a file manager to keep track of SETI data files, a spectrum analyzer optimized for amateur SETI, remote control of the station from the internet, and other packages.
OPTICAL EXPERIMENTS While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested by R. N. Schwartz and Charles Hard Townes in a 1961 paper published in the journal Nature titled "Interstellar and Interplanetary Communication by Optical Masers". In 1983, Townes, one of the inventors of the laser, published a detailed study of the idea in the US journal Proceedings of the National Academy of Sciences. Most SETI researchers agreed with the idea.
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The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central star of a remote star system would be too difficult. Some SETI advocates, such as Frank Drake, have suggested that such a judgment was too conservative; early 21st-century humans have no means of knowing how a superior technology is communicating or would communicate, and negative results may simply mean humans are making the wrong searches. There are two problems with optical SETI. The first problem is that lasers are highly "monochromatic", that is, they emit light only on one frequency, making it troublesome to figure out what frequency to look for. However, according to the uncertainty principle, emitting light in narrow pulses results in a broad spectrum of emission; the spread in frequency becomes higher as the pulse width becomes narrower, making it easier to detect an emission. The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. This means that a laser beam could be easily blocked by clouds of interstellar dust, and Earth would have to cross its direct line of fire by chance to receive it. Optical SETI supporters have conducted paper studies of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, focused into a narrow beam by such a mirror, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see. Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a constant rate. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting contact.
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In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing. During much of the 1990s, the optical SETI cause was kept alive through searches by Stuart Kingsley, a dedicated British researcher living in the US state of Ohio. Several optical SETI experiments are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort. Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group is now working with Princeton University to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise. The Harvard-Smithsonian group is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts. The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches. One is being directed by Geoffrey Marcy, an extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal. The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Werthimer of Berkeley, who built the laser detector for the HarvardSmithsonian group. The Berkeley survey uses a 76-centimeter (30-inch) automated telescope at Leuschner Observatory and an older laser detector built by Werthimer.
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GAMMA-RAY BURSTS Gamma-ray bursts (GRBs) are candidates for extraterrestrial communication. These highenergy bursts are observed about once per day and originate throughout the observable universe. SETI currently omits gamma ray frequencies in their monitoring and analysis because they are absorbed by the Earth's atmosphere and difficult to detect with groundbased receivers. In addition, the wide burst bandwidths pose a serious analysis challenge for modern digital signal processing systems. Still, the continued mysteries surrounding gamma-ray bursts have encouraged hypotheses invoking extraterrestrials. John A. Ball from the MIT Haystack Observatory suggests that an advanced civilization that has reached a technological singularity would be capable of transmitting a two-millisecond pulse encoding 1Ă—1018 bits of information. This is "comparable to the estimated total information content of Earth's biosystem-genes and memes and including all libraries and computer media."
PROBE SETI AND SETA EXPERIMENTS The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960 (see Bracewell probe), and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals. In recognition that any sufficiently advanced interstellar probe in the vicinity of Earth could easily monitor our terrestrial Internet, Invitation to ETI was established by Prof. Allen Tough in 1996, as a Web-based SETI experiment inviting such spacefaring probes to establish contact with humanity. The project's 100 Signatories includes prominent physical, biological, and social scientists, as well as artists, educators, entertainers, philosophers and futurists. Prof. H. Paul Shuch, executive director emeritus of The SETI League, serves as the project's Principal Investigator. - 69 -
In a 2004 paper, C. Rose and G. Wright showed that inscribing a message in matter and transporting it to an interstellar destination can be enormously more energy efficient than communication using electromagnetic waves if delays larger than light transit time can be tolerated. That said, for simple messages such as "hello," radio SETI could be far more efficient. If energy requirement is used as a proxy for technical difficulty, then a solarcentric Search for Extraterrestrial Artifacts (SETA) may be a useful supplement to traditional radio or optical searches. Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects. In 1979, Freitas and Valdes conducted a photographic search of the vicinity of the EarthMoon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude. The authors conducted a second, more comprehensive photographic search for probes in 1982 that examined the five Earth-Moon Lagrangian positions and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near L1/L2, Earth-Moon L3, and also L2 in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17–19th magnitude near L3/L4/L5, 10–18th magnitude for L1/L2, and 14– 16th magnitude for Sun-Earth L2. In June 1983, Valdes and Freitas used the 26 m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5–14 x 10−21 W/m²/channel and 0.7-2 x 10−24 W/m²/channel, respectively, but no detections were made. - 70 -
SEARCH FOR PHYSICAL TELESIGNATURES Introduction Besides directed messages by alien civilizations, another way of detecting their presence is the search for physical technosignatures. Advanced techniques could be used to uncover signs of alien technology, industrial activity or Megascale engineering. It is possible that even a civilisation at the same technology level as human beings might be detectable in this way and the presence more advanced civilisations may be more detectable.
SETI's search for Dyson spheres A Dyson Sphere, constructed by life forms not dissimilar to humans, who dwelled in proximity to a Sun-like star, made with materials similar to those available to humans, would most likely cause an increase in the amount of infrared radiation in the star system's emitted spectrum. Hence, Dyson selected the title "Search for Artificial Stellar Sources of Infrared Radiation" for his published paper. SETI has adopted these assumptions in their search, looking for such "infrared heavy" spectra from solar analogs. As of 2005 Fermilab has an ongoing survey for such spectra by analyzing data from the Infrared Astronomical Satellite (IRAS). Identifying one of the many infra-red sources as a Dyson Sphere would require improved techniques for discriminating between a Dyson Sphere and natural sources. Fermilab discovered 17 potential "ambiguous" candidates of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which are however unconfirmed. As of October 2012 astronomer Geoff Marcy, one of the pioneers of the search for extrasolar planets, was given research grant to search data from Kepler telescope, with the aim of detecting possible signs of Dyson Spheres
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Night Lights Some astronomers including Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and Edwin Turner of Princeton University (2011) have proposed that stable night light such as those emanating from cities, industry and transport networks on Earth could be detectable and indicate the presence of ETI using artificial light. However such signals may not be conclusive. For example NASA's 2012 "Black Marble" experiment showed that significant stable light sources on Earth originate from uninhabited areas and are naturally occurring (such as steady burning wildfires of Western Australia's deserts). Extraterrestrial lightning, aurora, combustion and other natural phenomenon may also be misidentified as being the activity of an ETI. In addition, this approach is somewhat anthropocentric as it assumes that such an ETI uses the visible spectrum like humans do. This technique would be unlikely to detect ETIs that use, for example, infrared or sonar for communication and navigation.
Atmospheric Analysis Atmospheric analysis of planet (by means such as powerful spectroscopy) may reveal ETI activity. Similar techniques are currently used to indirectly study the atmospheres of objects in the Solar System as well as transiting those of extrasolar objects. Atmospheric chemistry could be analysed for complex chemicals that may by the byproducts of technology. Atmospheric emissions from industry on Earth including Nitrogen dioxide and Chlorofluorocarbons are detectable from space. Artificial pollution may therefore be detectable on extrasolar planets. SETI has proposed searching for artificial atmospheres created by planetary engineering to produce habitable environments for colonisation by an ETI. Similarly the signatures of a terraformed Martian environment (by introducing complex warming gases) might be detectable from outside the Solar System. Likewise neighbouring planets with strong similarities in atmospheric breathing gas composition could indicate an ETI's expansion within its home system, particularly if complex gases were also present. For example, a - 72 -
fully terraformed Mars (and Venus) would likely have unexpectedly similar concentrations of oxygen and nitrogen to the Earth which could be observable from outside the Solar System and deduced when taking into account the respective planetary properties (age, mass, distance from the Sun) to be artificial.
Asteroid mining Traces of targeted asteroid mining on asteroids and comets could also be utilized for the search of ETI.
Search for Extraterrestrial Starships or Starprobes Robert Zubrin has pointed out that interstellar vehicles have the potential to be detectable from scores and even hundreds of light years.
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ΕΝΟΤΗΤΑ ΤΡΙΤΗ
ΠΡΟΓΡΑΜΜΑΤΑ ΕΝΕΡΓΟΥ SETI
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ACTIVE SETI
A representation of the 1679-bit Arecibo message.
Active SETI (Active Search for Extra-Terrestrial Intelligence) is the attempt to send messages to intelligent aliens. Active SETI messages are usually in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message. Active SETI is also known as METI (Messaging to Extra-Terrestrial Intelligence), or positive SETI. Active SETI is contrasted to passive SETI, which only searches for signals, without any attempt to send them. The term METI was coined by Russian scientist Alexander Zaitsev, who denoted the clearcut distinction between Active SETI and METI - 75 -
The science known as SETI deals with searching for messages from aliens. METI science deals with the creation of messages to aliens. Thus, SETI and METI proponents have quite different perspectives. SETI scientists are in a position to address only the local question “does Active SETI make sense?” In other words, would it be reasonable, for SETI success, to transmit with the object of attracting ETI’s attention? In contrast to Active SETI, METI pursues not a local and lucrative impulse, but a more global and unselfish one – to overcome the Great Silence in the Universe, bringing to our extraterrestrial neighbors the long-expected annunciation “You are not alone!” .
RADIO MESSAGE CONSTRUCTION The lack of an established communications protocol is a challenge for METI. First of all, while trying to synthesize an Interstellar Radio Message (IRM), we should bear in mind that Extraterrestrials will first deal with a physical phenomenon and, only after that, perceive the information. At first, ET's receiving system will detect the radio signal; then, the issue of extraction of the received information and comprehension of the obtained message will arise. Therefore, above all, the Constructor of an IRM should be concerned about the ease of signal determination. In other words, the signal should have maximum openness, which is understood here as an antonym of the term security. This branch of signal synthesis can be named anticryptography. Also characteristics of the radio signal such as wavelength, type of polarization, and modulation have to be considered. Over galactic distances, the interstellar medium induces some scintillation effects and artificial modulation of electromagnetic signals. This modulation is higher at lower frequencies and is a function of the sky direction. Over large distances, the depth of the modulation can exceed 100%, making any METI signal very difficult to decode.
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Error correction In METI research it is implied that any message must have some redundancy, although the exact amount of redundancy and message formats are still in great dispute. Using ideograms instead of binary sequence already offers some improvement against noise resistance. In faxlike transmissions ideograms will be spread on many lines. This increases its resistance against short bursts of noise like radio frequency interference or interstellar scintillation. One format approach proposed for interstellar messages was to use the product of two prime numbers to construct an image. Unfortunately, this method works only if all the bits are present. As an example, the message sent by Frank Drake from the Arecibo Observatory in 1974 did not have any feature to support mechanisms to cope with the inevitable noise degradation of the interstellar medium. Error correction tolerance rates for previous METI messages
Arecibo Message (1974) : 8.9% (one page)
Yevpatoria message (1999) : 44% (23 separate pages)
Yevpatoria message (2003) : 46% (one page, estimated)
Examples The 1999 Cosmic Call transmission was far from being optimal (from our terrestrial point of view) as it was essentially a monochromatic signal spiced with a supplementary information. Additionally, the message had a very small modulation index overall, a condition not viewed as being optimal for interstellar communication.
Over the 370,967 bits (46,371 bytes) sent, some 314,239 were “1” and 56,768 were “0”, creating a ratio of 5.54 times more “1” than “0”.
Since frequency shift keying modulation scheme was used, most of the time the signal was on the “0” frequency.
In addition, “0” tended to be sent in long stretches (white lines in the message).
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REALIZED PROJECTS These projects have targeted stars between 20 and 69 light-years from the Earth. The exception is the Arecibo message, which targeted globular cluster M13, approximately 24,000 light-years away. The first message to reach its destination will be A Message From Earth, which should reach Gliese 581 in Libra in 2029.
The Morse Message (1962)
Arecibo message (1974)
Cosmic Call 1 (1999)
Teen Age Message (2001)
Cosmic Call 2 (2003)
Across the Universe (2008)
A Message From Earth (2008)
Hello From Earth (2009)
RuBisCo Stars (2009)
Wow! Reply (2012)
TRANSMISSIONS Stars to which messages were sent, are the following: Name
Designation
Constellation
Messier 13
NGC 6205
Hercules
Altair
Alpha Aql
Aquila
Spica
Alpha Vir
Virgo
16 Cyg A
HD 186408
Cygnus
15 Sge
HD 190406
Sagitta
HD 178428
Sagitta
HD 190360 HD 197076
Cygnus Delphinus
Gl 777
Date sent November 16, 1974 August 15 1983 August 1997
Arrival date approx. 27000 1999 2247
May 24, November 1999 2069 June 30, February 1999 2057 June 30, October 1999 2067 July 1, 1999 April 2051 August 29, February
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Message Arecibo Message Altair (Morimoto Hirabayashi) Message NASDA CosmicCollege Cosmic Call 1 Cosmic Call 1 Cosmic Call 1 Cosmic Call 1 Teen Age Message
Name
Designation
47 UMa
HD 95128
37 Gem
HD 50692 HD 126053 HD 76151 HD 193664 HIP 4872
Constellation
Date sent
2001 September 3, Ursa Major 2001 September 3, Gemini 2001 September 3, Virgo 2001 September 4, Hydra 2001 September 4, Draco 2001 Cassiopeia July 6, 2003
HD 245409
Orion
July 6, 2003
HD 75732
Cancer
July 6, 2003
HD 10307
Andromeda
July 6, 2003
47 UMa
HD 95128
Ursa Major
Polaris
HIP 11767
Gliese 581
HIP 74995
Gliese 581
HIP 74995
GJ 83.1
GJ 83.1
55 Cnc
Teegarden's SO Star J025300.5+165258 Kappa Ceti
GJ 137 HIP 34511
37 Gem
HD 50692
55 Cnc
HD 75732
July 6, 2003 February 4, Ursa Minor 2008 October 9, Libra 2008 August 28, Libra 2009 November 7, Aries 2009 November 7, Aries 2009 November 7, Cetus 2009 August 15, Gemini 2012 August 15, Gemini 2012 August 15, Cancer 2012
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Arrival date 2070 July 2047 December 2057 January 2059 May 2057 January 2059 April 2036 August 2040 May 2044 September 2044 May 2049
Message
Teen Age Message Teen Age Message Teen Age Message Teen Age Message Teen Age Message Cosmic Call 2 Cosmic Call 2 Cosmic Call 2 Cosmic Call 2 Cosmic Call 2
2439
Across the Universe
2029
A Message From Earth
2030
Hello From Earth
2024
RuBisCo Stars
2022
RuBisCo Stars
2039
RuBisCo Stars
2163
Wow! Reply
2069
Wow! Reply
2053
Wow! Reply
POTENTIAL RISK Active SETI has been heavily criticized due to the perceived risk of revealing the location of the Earth to alien civilizations, without some process of prior international consultation. Notable among its critics is scientist and science fiction author David Brin, particularly in his article/"expose" Shouting at the Cosmos. However, Russian and Soviet radio engineer and astronomer Alexander L. Zaitsev has argued against these fears: see Sending and Searching for Interstellar Messages and Detection Probability of Terrestrial Radio Signals by a Hostile Super-civilization. Indeed, Zaitsev argues that we should consider the risks of NOT reaching out to extraterrestrial civilizations: see Making a Case for METI. To lend a quantitative basis to discussions of the risks of transmitting deliberate messages from Earth, the SETI Permanent Study Group of the International Academy of Astronautics adopted in 2007 a new analytical tool, the San Marino Scale. Developed by Prof. Ivan Almar and Prof. H. Paul Shuch, the San Marino Scale evaluates the significance of transmissions from Earth as a function of signal intensity and information content. Its adoption suggests that not all such transmissions are created equal, thus each must be evaluated on a case-by-case basis before establishing blanket international policy regarding Active SETI.
BEACON PROPOSAL One proposal for a 10 billion watt interstellar SETI beacon was dismissed by Robert A. Freitas Jr. to be infeasible for a pre-Type I civilization on the Kardashev scale. As a result it has been suggested that civilizations must advance into Type I before mustering the energy required for reliable contact with other civilizations. However, this 1980s technical argument assumes omni-directional beacons which may not be the best way to proceed on many technical grounds. Advances in consumer electronics have made possible transmitters that simultaneously transmit many narrow beams, covering - 80 -
the million or so nearest stars but not the spaces between. This multibeam approach can reduce the power and cost to levels that are reasonable with current mid-2000s Earth technology. Once civilizations have discovered each others' locations, the energy requirements for maintaining contact and exchanging information can be significantly reduced through the use of highly directional transmission technologies. In 1974, the Arecibo Observatory transmitted a message toward the M13 globular cluster about 25,000 light-years away, for example, and the use of larger antennas or shorter wavelengths would allow transmissions of the same energy to be focused on even more remote targets, such as those attempted by Active SETI.
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ΕΝΟΤΗΤΑ ΤΕΤΑΡΤΗ
ΦΙΛΟΣΟΦΙΚΕΣ ΥΠΟΘΕΣΕΙΣ ΓΙΑ ΤΗΝ ΕΞΩΓΗΙΝΗ ΖΩΗ ΚΑΙ ΝΟΗΜΟΣΥΝΗ
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Η ΘΕΩΡΙΑ ΤΗΣ ΠΑΝΣΠΕΡΜΙΑΣ ΣΕ ΕΡΓΑΣΤΗΡΙΟ ΤΟΥ ΕΘΝΙΚΟΥ ΙΔΡΥΜΑΤΟΣ ΕΡΕΥΝΩΝ Του ΠΑΝΑΓΙΩΤΗ ΓΕΩΡΓΟΥΔΗ e-net.gr Η θεωρία της πανσπερμίας, ότι δηλαδή η ζωή στη Γη προέρχεται από μικροοργανισμούς που έφτασαν από το Διάστημα και εξελίχθηκαν, αποδείχτηκε πρόσφατα πειραματικά στο εργαστήριο Νανοφωτονικής του Ινστιτούτου Θεωρητικής και Φυσικής Χημείας του Εθνικού Ιδρύματος Ερευνών (ΕΙΕ), αφού ανακάλυψαν ότι η πιθανότητα επιβίωσης ορισμένων μυκήτων εκτεθειμένων στην ηλιακή υπεριώδη ακτινοβολία, σε χαμηλές θερμοκρασίες και μηδενική πίεση κατά τη διάρκεια ενός διαπλανητικού ταξιδιού από τον Αρη στη Γη φτάνει στο 60%.
Η συναρπαστική αυτή ανακάλυψη δημοσιεύτηκε πρόσφατα σε διεθνές επιστημονικό περιοδικό. Μάλιστα, ερευνητές του ΕΙΕ πήραν μέρος σε σχετικά διαστημικά ευρωπαϊκά - 83 -
προγράμματα, όπως μας είπε ο υπεύθυνος ερευνητής του προαναφερόμενου εργαστηρίου, Κώστας Κεφαλάς. * Κύριε Κεφαλά, ποιες είναι συνοπτικά οι θεωρίες για την προέλευση της ζωής στη Γη; * Από την απαρχή της σκέψης δύο ήταν τα μεγάλα ερωτήματα που βασάνισαν τους μεγάλους στοχαστές και ουσιαστικά καθόρισαν την ιστορική πορεία της ανθρωπότητας, αφού πάνω τους βασίστηκε η οικοδόμηση της επιστήμης. Το πρώτο έχει να κάνει με την αρχή και την ουσία του κόσμου και το δεύτερο, με το μυστήριο της ζωής και την προέλευσή της. Οι μηχανισμοί Πέρα από τις θρησκευτικές και φιλοσοφικές αντιλήψεις, η θεωρία της πανσπερμίας επιχειρεί να ερμηνεύσει την προέλευση της ζωής στη Γη (χωρίς να επιχειρεί να ερμηνεύσει την αρχή της ζωής στο Σύμπαν), βασιζόμενη στην αρχή ότι η ζωή υπάρχει παντού στο Διάστημα και μεταφέρεται μέσω του χώρου και του χρόνου από το ένα σημείο του αχανούς Διαστήματος στο άλλο. Οι μηχανισμοί της πανσπερμίας βασίζονται στη μεταφορά της βιολογικής ύλης είτε μέσω της γαλαξιακής σκόνης, η οποία μπορεί να ταξιδέψει στο χώρο λόγω της πίεσης της ακτινοβολίας των γειτονικών άστρων, ή μέσω του γαλαξιακού ταξιδιού αστεροειδών ή κομητών, οι οποίοι φέρουν μικροοργανισμούς ικανούς να επιβιώσουν στις ακραίες συνθήκες θερμοκρασίας, πίεσης και ακτινοβολίας που επικρατούν στο αχανές σύμπαν. Η θεωρία της πανσπερμίας δεν είναι προϊόν των καιρών μας. Ο πρώτος που τη διατύπωσε ήταν ο Αναξαγόρας (500-428 π.Χ.). Ο Γάλλος φυσιοδίφης Benoit de Maillet το 1743 απέρριψε τη μέχρι τότε επικρατούσα θεωρία της αβιογένεσης, ισχυριζόμενος ότι η ζωή μεταφέρθηκε στη Γη από το Διάστημα και αναπτύχθηκε στη θάλασσα αρχικά, ενώ οι πατέρες της σύγχρονης θερμοδυναμικής Lord Kelvin (William Thomson) (1824-1907) και Hermann von Helmholtz (1821-1894) υπήρξαν οι κύριοι υπέρμαχοί της τον 19ο αιώνα.
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* Σημερινές ενδείξεις υπάρχουν; * Το 1984 ευρέθη στην Ανταρκτική ένας μετεωρίτης που εκτινάχτηκε από την επιφάνεια του Αρη πριν από 15 εκατ. χρόνια από μια αποστολή Αμερικανών επιστημόνων που έψαχναν για μετεωρίτες. Ο μετεωρίτης ονομάστηκε Allan Hills 84001 (ALH84001). Το 1991 στο εσωτερικό του ALH84001 εντοπίστηκαν νανοβακτήρια και αμινοξέα. Η σημαντική αυτή ανακάλυψη δημοσιεύτηκε από τον David McKay της NASA στην επιστημονική επιθεώρηση «The Journal Science» και προκάλεσε τέτοια αίσθηση ώστε ο πρόεδρος Κλίντον ανακοίνωσε την έναρξη της εξερεύνησης του πλανήτη Αρη από τις ΗΠΑ. Στις 29 Απριλίου του 2001 επιστήμονες από την Αγγλία και τις Ινδίες ανακοίνωσαν την ύπαρξη μικροοργανισμών στη στρατόσφαιρα, ενώ στις 11 Μαΐου του ίδιου έτους ο γεωλόγος Bruno d' Argenio και ο βιολόγος Giuseppe Geraci από το Πανεπιστήμιο της Νάπολης ανακοίνωσαν την ύπαρξη εξωγήινων βακτηρίων σε μετεωρίτη ηλικίας 4,5 δισ. ετών. Οι ανακαλύψεις αυτές πυροδότησαν την έναρξη ενός εκτεταμένου προγράμματος ερευνών ταυτόχρονα από τη NASA και τον Ευρωπαϊκό Οργανισμό Διαστήματος (ESA), οι οποίες αποσκοπούν στην επιβεβαίωση της θεωρίας της πανσπερμίας. Τα αποτελέσματα των ερευνών συνεχίζουν να εκπλήττουν τους επιστήμονες ως προς την ανθεκτικότητα των μικροοργανισμών σε ακραίες συνθήκες. * Η ελληνική συμμετοχή στο όλο εγχείρημα ποια είναι; * Μέρος αυτής της προσπάθειας υπήρξε και η κοινή προσπάθεια της Ευρωπαϊκής Ενωσης και του ESA μέσω του προγράμματος SURE, στο οποίο συμμετείχε και το Εθνικό Ιδρυμα Ερευνών, με επικεφαλής τη δρα Ευαγγελία Σαραντοπούλου και μέλη της ομάδας τούς δρες Αλκιβιάδη-Κωνσταντίνο Κεφαλά και Ζωή Κόλλια. Η ερευνητική προσπάθεια περιλάμβανε την εκτέλεση πειραμάτων στο Διεθνή Διαστημικό Σταθμό, παράλληλα με εργαστηριακές προσομοιώσεις συνθηκών Διαστήματος, με σκοπό την εκτίμηση της επίδρασης της έλλειψης βαρύτητας, της υπεριώδους ακτινοβολίας, του διαστημικού κενού και των χαμηλών θερμοκρασιών στη βιωσιμότητα διάφορων μικροοργανισμών.
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Νανοτεχνολογία Χρησιμοποιώντας τις πλέον εξελιγμένες συσκευές νανοτεχνολογίας οι ερευνητές του εργαστηρίου Νανοφωτονικής του Ινστιτούτου Θεωρητικής και Φυσικής Χημείας του Εθνικού Ιδρύματος Ερευνών ανακάλυψαν ότι η πιθανότητα επιβίωσης των κοινών μυκήτων Gladosporium, Ulocladium και Aspergilus niger, εκτιθέμενων στην ηλιακή υπεριώδη ακτινοβολία, σε χαμηλές θερμοκρασίες και μηδενική πίεση κατά τη διάρκεια ενός διαπλανητικού ταξιδιού από τον Αρη στη Γη είναι 60%. Τα αποτελέσματα της έρευνας, που πρόσφατα δημοσιεύτηκαν στην επιθεώρηση Planetary and Space Science, έδειξαν ότι οι μικροοργανισμοί είναι εξαιρετικά ανθεκτικοί στην ιονίζουσα και υπεριώδη ακτινοβολία, στις θερμοκρασίες κοντά στο απόλυτο μηδέν και στις μεγάλες μεταβολές της πίεσης. Τέτοιοι μικροοργανισμοί, θεωρητικά, θα μπορούσαν να εξουδετερώσουν την επίδραση της υπεριώδους ακτινοβολίας ακόμη και πλησίον των άστρων μέσω ενός βιολογικού αμυντικού μηχανισμού, ο οποίος ενεργοποιείται κάτω από ακραίες συνθήκες, με το να καλύπτει την εξωτερική μεμβράνη του με πρωτεΐνες, προστατεύοντάς
τους
έτσι
από
την
παρατεταμένη
ακτινοβολία.
Περαιτέρω έρευνα αναμένεται ότι θα αποκωδικοποιήσει τους μηχανισμούς προστασίας και επιδιορθώσεων του DNA που φαίνεται ότι διαθέτουν διάφοροι μικροοργανισμοί όταν εκτίθενται σε ακραίες συνθήκες, προσθέτοντας ένα μικρό λίθο στη θεωρία της πανσπερμίας.
ΤΟ ΠΑΡΑΔΟΞΟ ΤΟΥ FERMI KAI ΟΙ ΠΙΘΑΝΕΣ ΛΥΣΕΙΣ ΤΟΥ του Δημήτρη Νικολαΐδη Ο πλανήτης μας είναι ένας ενιαίος τόπος, για κάθε μορφή ζωής, η οποία έχει βρει στην Γη τις
απαραίτητες συνθήκες και προϋποθέσεις για την ανάπτυξη και εξέλιξη της, μέσα στο πέρασμα των αιώνων. Ο άνθρωπος δεν είναι παρά ένας πολύ κρίκος της τεράστιας αλυσίδας, η οποία συμπληρώνεται από χιλιάδες άλλα είδη. Αν ένας χώρος τόσο περιορισμένος όσο ο πλανήτης σφύζει από ζωή και μάλιστα από τόσες πολλές διαφορετικές - 86 -
μορφές ζωής, γιατί το απέραντο σύμπαν να είναι ερημικό και να μας δίνει την αίσθηση ότι είμαστε μόνοι;
Η άποψη ότι το σύμπαν δημιουργήθηκε μόνο για εμάς φαντάζει πολύ εγωιστική. Είναι δυνατόν τα δισεκατομμύρια δισεκατομμυρίων άστρων που βρίσκονται διάσπαρτα εκεί έξω να δημιουργήσουν και να συντηρήσουν ζωή; Ασφαλώς! Υπάρχουν δύο πολύ σημαντικοί παράγοντες οι οποίοι συνηγορούν στην άποψη αυτή. Πρώτον, υπάρχει ένας πολύ μεγάλος αριθμός πλανητών, που θα μπορούσαν να έχουν ευνοϊκές συνθήκες για την δημιουργία και την ανάπτυξη ζωής. Δεύτερον, θεωρείται εξαιρετικά μεγάλη η ηλικία του σύμπαντος, η οποία αγγίζει τα 13 δισεκατομμύρια χρόνια , σε σχέση με την ηλικία της ζωής στον πλανήτης μας. Ας αναλογιστούμε μόνον το παράδειγμα που ακολουθεί: αν συμπιέσουμε τον χρόνο από την εμφάνιση της ζωής στον πλανήτη μας στο χρονικό διάστημα των 365 ημερών που διαρκεί ένα γήινο έτος, τότε η δική μας εμφάνιση σαν είδος, δεν έγινε παρά μόνο 1 ώρα πριν την λήξη αυτού του υποθετικού έτους.
Παρά τις ανωτέρω αμφισβητήσεις όμως, το ερώτημα παραμένει: Είμαστε πράγματι μόνοι; Και δυστυχώς το μοναδικό στοιχείο που διαθέτουμε ως τώρα για την παρουσία εξωγήινης ζωής είναι η απουσία της. Ασφαλώς όμως δεν πρέπει η απουσία της παρουσίας να αποτελεί απόδειξη ανυπαρξίας. Το μόνο βέβαιο στην όλη ιστορία είναι ο ότι ο άνθρωπος θα συνεχίσει την αναζήτηση, αφού η δίψα για γνώση και η διαρκής αναζήτηση είναι στοιχεία της φύσης του και του χαρακτήρα του.
Οι προσπάθειες μας μέχρι σήμερα για να έρθουμε σε επαφή με κάποιο εξωγήινο πολιτισμό ή να εντοπίσουμε κάποια ίχνη του έχουν αποβεί άκαρπες και μάταιες. Αν πράγματι υπάρχουν εξωγήινα όντα με νοημοσύνη, τότε θα είχαν φτάσει στη Γη και θα τους “βλέπαμε”. Όμως κάτι τέτοιο δεν έχει συμβεί. Ας υποθέσουμε ακόμα ότι οι εξωγήινοι δεν μπορούν να φτάσουν στον πλανήτη μας. Αλλά γιατί δεν τους ακούμε; έστω και από απόσταση; Εδώ υπεισέρχεται ένα ερώτημα του Ενρίκο Φέρμι (γνωστό ως το παράδοξο του Φέρμι) όπου μέσα σε τρεις λέξεις κατάφερε να εντάξει και να περιγράψει τους προβληματισμούς και τις απορίες ολόκληρης της ανθρωπότητας “ΠΟΥ ΕΙΝΑΙ ΟΛΟΙ;”
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Θα προσπαθήσουμε να δώσουμε μερικές απαντήσεις για το παράδοξο του Φέρμι και τον προβληματισμό στην ύπαρξη της εξωγήινης νοημοσύνης. Είναι ένα μικρό αλλά πολύ αντιπροσωπευτικό δείγμα από τις πολλές απαντήσεις που δίνονται στο βιβλίο του Steven Web “ Where is everybody ?” και απαντούν σε 3 υποθέσεις.
1. Βρίσκονται ανάμεσά μας.
2. Η αλήθεια βρίσκεται εκεί έξω.
3. Σιγά μην υπάρχουν εξωγήινοι!
Αλήθεια, είδα UFO.
Είναι πάρα πολλοί αυτοί που ισχυρίζονται ότι έχουν δει στον ουρανό παράξενα αντικείμενα. Κάποιοι από αυτούς πιστεύουν ότι πράγματι είδαν UFO και κάποιοι άλλοι σκεπτόμενοι ορθολογιστικά, προσπαθούν να εξηγήσουν λογικά το φαινόμενο βασιζόμενοι στα
άπειρα
φαινόμενα
που
παρουσιάζονται
στο
νυκτερινό
ουρανό.
Στις περισσότερες των περιπτώσεων θέασης UFO έχει δοθεί κάποια λογική εξήγηση. Υπάρχουν όμως και περιπτώσεις ανεξιχνίαστες και ανεξήγητες. Είναι άραγε σκάφη εξωγήινων ή πρόκειται για απλές λάμψεις στον ουρανό; Τόσο η θέαση παράξενων διαστημοπλοίων όσο και οι ισχυρισμοί ορισμένων για προσωπική επαφή με τους εξωγήινους είναι προβληματικές, καθώς και στις δύο περιπτώσεις εξωγήινοι και UFO υπήρχαν, αλλά χωρίς αποδείξεις.
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Μήπως καταγόμαστε από εξωγήινους;
Η προέλευση και κυρίως ο μηχανισμός που ανέδειξε την ζωή στη Γη, αποτελεί ένα μεγάλο αίνιγμα και άλυτο γρίφο ακόμα, για τους επιστήμονες. Είναι δυνατό όλα τα όντα να έχουν μια κοινή καταγωγή, από εξωγήινη προέλευση; Στην ερώτηση δίνει απάντηση η θεωρία της πανσπερμίας , με σοβαρές όμως αμφισβητήσεις και αντίλογο, αφού μεταφέρει το πρόβλημα της
προέλευσης
της
ζωής
έξω
από
τα
πλανητικά
μας
σύνορα.
Πολλοί επιστήμονες έχουν ενστερνιστεί την άποψη ότι η ζωή ξεκίνησε στον Άρη και μεταφέρθηκε στη Γη με την μορφή μικροβίων που υπήρχαν σε μετεωρίτες. Ο σκεπτικισμός όμως της συγκεκριμένης άποψης παραμένει έντονος, ακόμα και αν οι συνθήκες στον Άρη που επικρατούσαν παλαιότερα, θα μπορούσαν να ευνοήσουν την ανάπτυξη ζωής.
Οι Κρικ και Οργκελ πρότειναν το 1973 την θεωρία της “κατευθυνόμενης πανσπερμίας”, σύμφωνα με την οποία, ένας αρχαίος εξελιγμένος εξωγήινος πολιτισμός, έστειλε επιλεκτικά την ζωή σε πλανήτες, όπου θα ήταν δυνατόν να αναπτυχθεί, είτε για λόγους μεγάλης καταστροφής που θα έπληττε τον πολιτισμό τους είτε απλά για την δημιουργία αποικιών στο σύμπαν. Η παραπάνω πρόταση αποτελεί ένα εξαιρετικό συνδυασμό σύζευξης της θεωρίας της πανσπερμίας και ευφυΐας, που αν ισχύει τότε είμαστε όλοι απόγονοι των εξωγήινων. Οι φίλοι μας τα ζώα.
Το 1973 επίσης, ο Τζον Μπελ, πρότεινε την υπόθεση του ζωολογικού κήπου. Αυτή λέει απλά ότι οι εξωγήινοι μας αντιμετωπίζουν σαν είδος προς εξαφάνιση. Το σημείο αναφοράς της θεωρίας είναι ότι αν πράγματι υπάρχει εξωγήινη νοημοσύνη, τότε κάποιος πρέπει να είναι ο ηγέτης και να ελέγχει το σύμπαν. Οι άνθρωποι αφήνουν ελάχιστο χώρο στα άλλα είδη του πλανήτη μας για να εξελιχθούν και να αναπτυχθούν. Έτσι ακριβώς και οι εξωγήινοι που βρίσκονται στον πλανήτη μας δεν επεμβαίνουν αλλά παρατηρούν. Για - 89 -
αυτούς η Γη δεν αποτελεί παρά ένα μικρό ζωολογικό κήπο η ισορροπία του οποίου δεν πρέπει να διαταραχθεί.
Πρόκειται για μια πολύ ανθρωποκεντρική ιδέα που ακόμα και αν αληθεύει δεν έχουμε την ικανότητα να την ερευνήσουμε. Στο κάτω – κάτω είμαστε τόσο ξεχωριστοί ώστε να αξίζουμε της προσοχής και της προστασίας των εξωγήινων;
Πόσο διαρκεί ένα ταξίδι μέχρι τη Γη;
Η πιο λογική απάντηση στο παράδοξο του Φέρμι είναι ότι τα ταξίδια στα άστρα είναι ουσιαστικά αδύνατα, καθώς οι μεταξύ τους αποστάσεις είναι τεράστιες. Εκείνο βέβαια που αποτρέπει την πραγματοποίηση ενός τέτοιου εγχειρήματος, δεν είναι ή έλλειψη τεχνολογικών μέσων (άλλωστε πολλές χώρες έχουν αποτολμήσει διαστημικές αποστολές), αλλά ο χρόνος. Με ποιο τρόπο ο άνθρωπος θα ήταν ικανός να νικήσει τον χρόνο, ώστε να ταξιδέψει στο διάστημα και να το εξερευνήσει. Δείτε το παράδειγμα που ακολουθεί και θα καταλάβετε. Το φως (ή καλύτερα μια δέσμη φωτονίων) η οποία ξεκινά από τον Εγγύτατo του Κενταύρου (που είναι το πιο κοντινό άστρο στον Ήλιο μας), χρειάζεται 4,22 χρόνια για να καλύψει την απόσταση αυτή έχοντας ταχύτητα την ταχύτητα του φωτός που ισούται με 299.742.458 χλμ/δευτ. (1c). Αν για παράδειγμα το Voyager I είχε κατεύθυνση τον Εγγύτατο του Κενταύρου θα έφτανε στον προορισμό του μετά από 73.000 χρόνια αφού κινείται μόνο με ταχύτητα ίση με 0,000085c. Θα χρειαζόταν πολλές λοιπόν ανθρώπινες γενιές που θα ήταν επιβάτες σε ένα διαστημικό σκάφος για να επιτευχθεί ένα τέτοιο ταξίδι. Τα εμπόδια που υπάρχουν, τεχνικά, οικονομικά, επιστημονικά και πολιτιστικά δυσκολεύουν τον άνθρωπο και ίσως και κάποιο εξωγήινο πολιτισμό να αποτολμήσει μια τέτοια προσπάθεια και να καταφέρει να δει την αποστολή του μάλιστα να επιστρέφει. Μήπως έχουμε μεγάλη ιδέα για το είδος μας;
Γιατί θεωρούμε ότι η πρώτη επιλογή των εξωγήινων θα ήταν να επισκεφτούν τη Γη; - 90 -
Πιθανότατα στο σύμπαν να υπάρχουν μέρη πολύ πιο ελκυστικά από τη Γη μας, ώστε να αποφασίσει κάποιος να τα επισκεφτεί. Αν κάποιος εξωγήινος πολιτισμός διαθέτει αναπτυγμένη τεχνολογία, η επιλογή του κάλλιστα μπορεί να είναι να “γυρίζει αλητεύοντας” σ΄ όλο το σύμπαν, δημιουργώντας ταυτόχρονα αποικίες παρά να αρκεστεί στην εξερεύνηση και μετοίκηση του σε ένα μόνο πλανήτη. Με τον τρόπο αυτό θα ήταν δυνατό να ταξιδεύει από το ένα άστρο στο άλλο, παίρνοντας ενέργεια από αυτό πριν καταλήξει σε ένα άστρο κόκκινο γίγαντα ή ένα σουπερνόβα.
Γιατί λοιπόν θα επέλεγε το ηλιακό μας σύστημα και τον Ήλιο μας, όταν υπάρχουν παντού και πολλά μάλιστα άστρα, φασματικού τύπου Ο5 .(άστρα που ανήκουν σε αυτό το φασματικό τύπο έχουν επιφανειακή τους θερμοκρασία που αγγίζει τους 30.000 °C) και αποδίδουν
800.000
φορές
περισσότερη
ενέργεια
από
τον
Ήλιο
μας.
Που να πάμε τώρα εκεί πέρα…
Προβλήματα οικονομικής φύσης μπορεί να ταλανίζουν και “αυτούς” όπως κι εμάς άλλωστε, μαζί με ένα γενικότερο κλίμα ωχαδελφισμού και απάθειας. Αλήθεια πότε πέρασαν 38 χρόνια από την μέρα που ο άνθρωπος πάτησε το πόδι του στη Σελήνη; 38 χρόνια και είμαστε ακόμα εδώ, χωρίς σχεδόν καμία άλλη προσπάθεια για επανδρωμένη αποστολή στο διάστημα, έστω στους κοντινούς πλανήτες του ηλιακού μας συστήματος. Γιατί; θα μπορούσε ποτέ αυτό να αλλάξει;
Ο άνθρωπος βέβαια κάποια στιγμή θα θελήσει να επεκταθεί και σε άλλους πλανήτες. Τίποτα όμως προς το παρόν δεν τον ωθεί να προβεί σε μια τόσο επικίνδυνη και πολυδάπανη ταυτόχρονα ενέργεια. Μόνο αν υπήρχε άμεσος κίνδυνος που θα απειλούσε την ύπαρξη του ανθρώπινου γένους, όπως η πρόσκρουση ενός μετεωρίτη, ή η έκρηξη κάποιου ηφαιστείου ή ακόμα η δραματική επιδείνωση των κλιματικών συνθηκών, θα μπορούσαν να οδηγήσουν τον άνθρωπο στην αναζήτηση ενός νέου πλανήτη όπου θα μετοικούσε και παράλληλα θα διασφάλιζε την διαιώνιση του είδους του.
- 91 -
Μέχρι όμως να αισθανθούμε την απειλή του αφανισμού θα παρατείνουμε την παραμονή μας στη Γη. Μήπως με τον ίδιο τρόπο αντιδρούν και οι εξωγήινοι;
Over, μας ακούει κανείς;….
Παραδεχόμαστε λοιπόν ότι πράγματι υπάρχουν εξωγήινοι πολιτισμοί και τα διαστρικά ταξίδια είναι αδύνατα λόγω των τεραστίων αποστάσεων, άρα το γεγονός ότι δεν τους έχουμε δει φαίνεται λογικό. Γιατί δεν τους ακούμε όμως: Ασφαλώς και δεν έχουν να φοβηθούν τίποτα ή να χάσουν κάτι στην προσπάθεια τους για επικοινωνία μαζί μας, σε αντίθεση μάλιστα θα γνώριζαν ένα “εξελιγμένο” πολιτισμό.
Αυτό που σίγουρα δεν είμαστε θέση να γνωρίζουμε είναι το επίπεδο της τεχνολογικής τους εξέλιξης και ικανότητας και μόνο υποθέσεις μπορούμε να κάνουμε, καθώς τα στοιχεία που διαθέτουμε είναι μηδενικά. Αν έχουν αναπτύξει κάποια τεχνολογία και τον τρόπο με τον οποίο θα επέλεγαν να μεταδώσουν τα σήματα τους δεν είμαστε σε θέση να τον γνωρίζουμε. Άρα, δεν ξέρουμε πώς να τους ακούσουμε.
SETI
Από τότε που ξεκίνησε η έρευνα του SETI (Search for Extra Terrestrial Intelligence) ένας τεράστιος όγκος δεδομένων και πληροφοριών έχει συλλεχθεί και επεξεργαστεί. Ο οποιοσδήποτε θα έθετε την εύλογο ερώτηση; μα καλά σε όλες αυτές τις πληροφορίες δεν υπάρχει
το
“σημάδι”
ενός
εξωγήινου
πολιτισμού;
Είναι αρκετές οι φορές που οι αισθητήρες του SETI έχουν ξεγελαστεί από σήματα, που έχουν γήινη προέλευση, προερχόμενα είτε από κινητά τηλέφωνα, είτε από τα ραντάρ κάποιων στρατιωτικών συσκευών. Οι επιστήμονες βέβαια που επεξεργάζονται τα σήματα έχουν την ικανότητα να τα ξεχωρίσουν και να αναγνωρίσουν τις πηγές εκπομπής τους. - 92 -
Υπάρχουν όμως και κάποιες εξαιρέσεις για τις οποίες οι επιστήμονες ψάχνουν ακόμα να δώσουν απαντήσεις. Μεγαλύτερο ενδιαφέρον όμως, παρουσιάζει το μυστήριο όπου κάθε φορά που οι επιστήμονες στρέφουν τα τηλεσκόπιά τους στην κατεύθυνση από την οποία προήλθε
το
προηγούμενο
σήμα,
αυτό
να
μην
επαναλαμβάνεται.
Οι πιο αισιόδοξοι πιστεύουν και ελπίζουν ότι αν κάποιος εξωγήινος πολιτισμός αποφασίσει κάποτε να έλθει σε επικοινωνία μαζί μας, τότε τα σήματα του θα είναι τόσο ευδιάκριτα και σαφή,
ώστε
να
μην
υπάρξει
καμία
αμφισβήτηση
για
την
προέλευσή
τους.
1+1=2, Είμαστε σίγουροι γι΄ αυτό;
Αδιαμφισβήτητα, τα μαθηματικά κατέχουν εξέχουσα θέση στη ζωή μας και στις δραστηριότητές μας. Επηρεάζουν τον τρόπο με τον οποίο αντιλαμβανόμαστε τα πράγματα, ακόμα και την εξέλιξή μας. Μάλιστα, κάποιοι τους προσδίδουν έναν ιδανικό χαρακτήρα, ικανό
να
μας
ανοίξει
το
δρόμο
προς
την
απόλυτη
αλήθεια.
Μια άλλη άποψη όμως «προσγειώνει» τα μαθηματικά στη σφαίρα του ανθρώπινου, θεωρώντας ότι δεν είναι τίποτε περισσότερο από μια εφεύρεση του μυαλού και μέρος του πολιτισμού μας. Με άλλα λόγια, πρόκειται για μια σύμβαση, σκοπός της οποίας είναι να εξυπηρετήσει τις ανάγκες της ανθρωπότητας.
Εάν ισχύει κάτι τέτοιο, τότε οι εξωγήινοι πολιτισμοί ενδέχεται να έχουν αναπτύξει διαφορετικά μαθηματικά, σύμφωνα με τις συνθήκες του περιβάλλοντός τους, τις ανάγκες τους και τον τρόπο ζωής τους. Ίσως τα «εξωγήινα» μαθηματικά να βασίζονται περισσότερο στα σχήματα και στα μεγέθη, παρά στους αριθμούς. Μήπως όμως τελικά τα μαθηματικά των ανθρώπων είναι τα μοναδικά που οδηγούν στην ανάπτυξη της τεχνολογίας; Δεν θέλουν καμιά επαφή μαζί μας.
Ένα από τα μεγαλύτερα παράδοξα που αφορούν στην ύπαρξη της εξωγήινης νοημοσύνης είναι το γεγονός ότι εμείς οι άνθρωποι θεωρούμε δεδομένη την επιθυμία των εξωγήινων να επικοινωνήσουν μαζί μας. Πώς όμως είμαστε τόσο σίγουροι γι’ αυτό, όταν δεν ξέρουμε πώς σκέφτονται και πώς αντιδρούν; - 93 -
Υπάρχουν πραγματικά άπειροι λόγοι που θα κρατούσαν τους εξωγήινους πολιτισμούς μακριά μας. Ο φόβος είναι ένας από αυτούς. Στις κινηματογραφικές ταινίες, οι εξωγήινοι παρουσιάζονται ως εισβολείς που σκορπούν τον τρόμο και τον πανικό στους δρόμους των μεγαλουπόλεων της Γης. Μια ανάλογη ανησυχία μπορεί να τους διακατέχει και αυτούς. Ταυτόχρονα, υποθέτουμε ότι η ύπαρξη εξωγήινης νοημοσύνης θα συνοδευόταν οπωσδήποτε από την επιθυμία για αναζήτηση ζωής στο υπόλοιπο σύμπαν. Αυτό όμως ενδέχεται να αποτελεί ένα από τα ξεχωριστά χαρακτηριστικά μόνο των ανθρώπων. Η συνεχής δίψα για γνώση και η ανάγκη για επικοινωνία δεν πρέπει να θεωρούνται δεδομένες και για τους εξωγήινους.
Τέλος, δεν πρέπει να απορρίπτουμε το ενδεχόμενο να είναι απλώς αδιάφοροι απέναντί μας. Εάν υπάρχουν, θα πρέπει να έχουν εξαιρετικά προχωρημένο πολιτισμό και δεν θα περιμένουν να μάθουν κάτι καινούργιο από εμάς. Μόνο η θρησκεία, η τέχνη και τα έθιμά μας θα τους ενδιέφεραν πραγματικά. Αλλά και πάλι, είναι δυνατόν να επικρατεί τέτοια καθολική αδιαφορία; Γίνεται κανένας από τους εξωγήινους πολιτισμούς που υπάρχουν εκεί έξω να μη θέλει να επικοινωνήσει;
Έχουμε λάθος αντίληψη και δίνουμε διαφορετική ερμηνεία.
Γνωρίζουμε άραγε την αλήθεια των πραγμάτων; Εξηγούμε σωστά τα φαινόμενα που συμβαίνουν τριγύρω μας; Μια απάντηση στο παράδοξο του Φέρμι μπορούμε να δώσουμε, εάν εικάσουμε ότι ο άνθρωπος έχει κάνει κάπου λάθος στον τρόπο με τον οποίο αντιλαμβάνεται το σύμπαν, για παράδειγμα.
Μια πρώτη πρόταση είναι ότι οι εξωγήινοι διαφέρουν ριζικά από εμάς. Εμείς, για παράδειγμα, χρησιμοποιούμε τη γλώσσα για να επικοινωνήσουμε. Οι εξωγήινοι ίσως επικοινωνούν με τηλεπάθεια! Μια άλλη εξήγηση –η οποία βασίζεται περισσότερο στην κβαντομηχανική– είναι αυτή που προτείνει τα παράλληλα σύμπαντα. Ένας παρατηρητής στο σύμπαν Α πειραματίζεται και συμπεραίνει τα ακριβώς αντίθετα από τον παρατηρητή - 94 -
που βρίσκεται στο σύμπαν Β. Βασική κατάληξη όλων αυτών είναι η ύπαρξη άπειρων παράλληλων συμπάντων, τα πλάσματα των οποίων αδυνατούν να συναντηθούν.
Υπάρχουμε γιατί μιλάμε;
Είναι ο άνθρωπος το μοναδικό είδος (από τα 50 δισεκατομμύρια που έχουν υπάρξει) που έχει αναπτύξει τη γλώσσα; Αν ναι, τότε ίσως να μην είμαστε οι μόνοι που μιλούν στη Γη, αλλά σε ολόκληρο το σύμπαν. Είναι γεγονός ότι η γλώσσα αποτελεί εργαλείο προόδου και κοινωνικότητας, με πιο απλά λόγια πρόκειται για ένα μέσο επικοινωνίας. Βέβαια, και τα ζώα επικοινωνούν μεταξύ τους. Μιλούν όμως;
Όταν ένας σκύλος γαυγίζει, θα μπορούσε όντως να μας μιλάει. Εμείς όμως αδυνατούμε να καταλάβουμε και ταυτόχρονα πέφτουμε σε μια αναπόφευκτη παγίδα. Τείνουμε να σκεφτόμαστε τα πάντα ανθρωπομορφικά και να τα εξηγούμε σύμφωνα με τα δικά μας μέτρα και σταθμά.
Ακούμε το σκύλο να γαυγίζει και εμείς αναρωτιόμαστε αν μιλάει. Αν μιλούσε όμως, πολύ απλά δεν θα ήταν σκύλος! Μπορεί όμως να ακούει ήχους, τους οποίους εμείς δεν ακούμε και να εντοπίζει μυρωδιές που εμείς ούτε καν διανοούμαστε πως υπάρχουν. Η βιοποικιλότητα βασίζεται στα μοναδικά χαρακτηριστικά κάθε είδους, που του επιτρέπουν να εξελιχθεί και να προσαρμοστεί στις νέες συνθήκες. Ο έναρθρος λόγος είναι αυτό το χαρακτηριστικό στοιχείο του είδους μας. Αν κάποια μέρα επισκεφθούμε κάποιον άλλο κόσμο, τότε είναι πολύ πιθανό να βρεθούμε αντιμέτωποι με έναν εξωγήινο πολιτισμό με ξεχωριστά χαρακτηριστικά. Μπορεί όμως να μη διαθέτει ότι εμείς: τη γλώσσα. Και προφανώς, πρώτα από όλα η ικανότητα για επικοινωνία είναι απαραίτητη για την ίδια την επικοινωνία.
- 95 -
Δεν έχω αυτοκίνητο καρδιά μου….
Εάν κάποιος εξωγήινος πολιτισμός ήθελε να επικοινωνήσει μαζί μας, θα χρειαζόταν τα απαραίτητα μέσα για να το επιτύχει. Ακόμα όμως και αν υποθέσουμε ότι τα έχει, θα έπρεπε να τα χρησιμοποιήσει με το σωστό τρόπο. Ίσως μόνο ο άνθρωπος έχει βρει το δρόμο προς… την επιστήμη. Εξάλλου, ακόμα και αυτός άργησε πολύ να τα καταφέρει. Οι απαρχές της μοντέρνας επιστήμης εντοπίζονται στην ελληνιστική περίοδο –πριν από 2.500 χρόνια, αλλά για μεγάλο χρονικό διάστημα παρέμεινε περιορισμένη στην παρατήρηση. Αν αναζητήσουμε τους λόγους για τους οποίους η επιστήμη διαφόρων πολιτισμών δεν έφτασε στο απόγειό της, θα διαπιστώσουμε ότι υπάρχουν πολλοί, όπως η τύχη και οι πολιτιστικοί παράγοντες. Αυτοί οι λόγοι είναι αρκετοί για να υποθέσουμε ότι οι εξωγήινοι πολιτισμοί δεν έχουν αναπτύξει τις επιστημονικές τεχνικές που θα τους επέτρεπαν να έρθουν σε επαφή μαζί μας.
Τεκτονικές Πλάκες, τι ρόλο παίζουν;
Πρόκειται για μια ακόμα παράξενη εξήγηση στο παράδοξο του Φέρμι. Ο πλανήτης μας χαρακτηρίζεται από τρία μοναδικά στοιχεία: τη ζωή, το νερό των ωκεανών και τις τεκτονικές πλάκες. Γιατί όμως οι τελευταίες είναι τόσο σημαντικές για την εξέλιξη της ζωής; Οι τεκτονικές πλάκες αποτελούν μηχανισμό αναζωογόνησης και ανανέωσης της Γης, καθώς «καταναλώνουν» και γεννούν νέα στεριά.
Ταυτόχρονα, απελευθερώνουν την
ενέργεια που υπάρχει στο εσωτερικό της Γης και βοηθούν την εξελικτική πορεία και τη βιοποικιλότητα των οργανισμών. Φανταστείτε ένα απλό σενάριο με μακροχρόνιες συνέπειες. Η απομάκρυνση δύο κομματιών στεριάς λόγω τεκτονικών δυνάμεων θα επέφερε μια σημαντική αλλαγή στα είδη των ζώων που θα ζούσαν σε αυτά. Ένα είδος πουλιού, για παράδειγμα, θα ήταν αναγκασμένο να ζει και στην παλιά γη αλλά και στο νησί που μόλις σχηματίστηκε. Κάθε σμήνος θα έπρεπε να αντιμετωπίσει διαφορετικές συνθήκες, με αποτέλεσμα να εξελιχθεί διαφορετικά και με το πέρας των αιώνων να οδηγήσει σε ένα νέο - 96 -
είδος πουλιών. Στην περίπτωση ενός μαζικού αφανισμού, οι τεκτονικές πλάκες θα λειτουργούσαν θετικά, καθώς όσο περισσότερα είδη οργανισμών υπάρχουν τόσο μεγαλύτερες
είναι
οι
πιθανότητες
να
επιζήσει
κάποιο
από
αυτά.
Η γέννηση της ζωής είναι ένα σπάνιο φαινόμενο;
Ο πλανήτης μας θεωρείται ξεχωριστός, χάρη στην ύπαρξη της ζωής σε αυτόν. Πραγματικά, όποτε ο άνθρωπος κάνει λόγο για το στοιχείο της ζωής, αναφέρεται με μοναδικό και θαυμαστό
τρόπο
προς
αυτό
το
μοναδικό
αγαθό.
Είναι
πράγματι
έτσι;
Το απέραντο σύμπαν φαίνεται στείρο από κάθε είδους ζωή. Αν και δεν γνωρίζουμε με ακρίβεια τον τρόπο με τον οποίο ξεπρόβαλε αρχικά η ζωή, υπάρχει ένα μοναδικό στοιχείο που θα μας οδηγούσε στο συμπέρασμα ότι και άλλοι πλανήτες θα μπορούσαν να έχουν δείγματα
ζωής,
έστω
και
αν
πρόκειται
για
απλά
μικρόβια:
το
νερό.
Στο παρελθόν, ο Άρης φαίνεται ότι διέθετε νερό και επομένως θα μπορούσε να υπάρχει σε αυτόν και κάποιο ίχνος ζωής. Μόνο η εύρεση απολιθωμάτων όμως θα μας οδηγούσε σε ασφαλή συμπεράσματα. Εκτός από τη Γη, τρία ακόμα ουράνια σώματα είναι πιθανό να έχουν ωκεανούς. Τα δύο από τα φεγγάρια του Δία –η Ευρώπη και η Καλλιστώ– ενδέχεται να διαθέτουν ωκεανούς, επειδή όμως βρίσκονται μακριά από τη «ζεστασιά» του Ήλιου, τους καλύπτει ένα λεπτό στρώμα πάγου. Το τρίτο ουράνιο σώμα είναι ο Τιτάνας, το φεγγάρι του Κρόνου, που μπορεί να διαθέτει ωκεανούς υγρής αμμωνίας και νερού κάτω από την επιφάνειά του. Και στις τρεις περιπτώσεις, η ύπαρξη ζωής θα σχετίζεται με πολύ αρχικές μορφές ζωντανών οργανισμών, με τους οποίους σε καμιά περίπτωση δεν θα μπορούσαμε να επικοινωνήσουμε. Παρ’ όλα αυτά, δεν θα ήταν αρκετό να γνωρίζουμε ότι η ζωή δεν γεννήθηκε
μόνο
μία φορά
και
άρα
δεν είναι
και
τόσο σπάνια;
Ο μόνος μάστορας είναι ο άνθρωπος !
Ο άνθρωπος ανέκαθεν διέθετε μια μοναδική ικανότητα, την οποία εκμεταλλεύθηκε στο μέγιστο βαθμό, για να εξελιχθεί και να προοδεύσει: τη χρήση και την κατασκευή εργαλείων. Φυσικά, κάποιος θα ισχυριζόταν ότι, εκτός από τον άνθρωπο, υπάρχουν και ορισμένα ζώα που χρησιμοποιούν εργαλεία. Τα ζώα μπορούν όντως να χρησιμοποιήσουν εργαλεία και αυτό αντανακλά ταυτόχρονα την εξυπνάδα και την ικανότητα που διαθέτουν. - 97 -
Ο άνθρωπος όμως όχι μόνο τα χρησιμοποιεί, αλλά και τα κατασκευάζει ο ίδιος. Αν ο άνθρωπος είναι το μόνο είδος ζωντανού οργανισμού που κατέχει το «κατασκευάζειν», τότε δεν θα περίμενε πραγματικά κανείς να δει ή να ακούσει εξωγήινους, αφού χωρίς την κατασκευή των απαραίτητων μέσων (διαστημόπλοιο, σταθμός μετάδοσης σημάτων κ.λπ.) δεν
έχουν
τη
δυνατότητα
να
κάνουν
γνωστή
την
παρουσία
τους.
Πρωταθλητές στη ζωή.
Το SPONCH είναι το κλειδί αυτής της υπόθεσης. Αντιπροσωπεύει τα έξι στοιχεία από τα οποία εξαρτάται η βιοχημεία των γήινων και των εξωγήινων οργανισμών. Το θείο (S), ο φώσφορος (P), το οξυγόνο (Ο), το άζωτο (Ν), ο άνθρακας (C) και το υδρογόνο (Η) είναι τα απαραίτητα υλικά για τη συνταγή της ζωής. Αμέσως όμως μετά τη Μεγάλη Έκρηξη, τα μοναδικά στοιχεία που υπήρχαν στο σύμπαν ήταν το υδρογόνο και το ήλιο, ενώ όλα τα υπόλοιπα σχηματίστηκαν από πυρηνικές αντιδράσεις, που έλαβαν χώρα μέσα σε αστέρια. Τα μέταλλα αυτά απελευθερώθηκαν, όταν έληξε ο παραγωγικός κύκλος των άστρων, ενώ σιγά
-
σιγά
συγκεντρώθηκαν
σε
μεγάλα
ποσοστά
στο
σύμπαν.
Οι πλανήτες γύρω από παλιά άστρα στερούνται των μετάλλων SPONCH, σε αντίθεση με αυτούς που βρίσκονται γύρω από νέα αστέρια, όπως ο Ήλιος, όπου η γέννηση της ζωής είναι εφικτή. Γι’ αυτό και είμαστε τα πρώτα δείγματα ζωής στο σύμπαν!
Είναι επικίνδυνα εκεί έξω….
Τι μπορεί να απειλήσει τον πλανήτη μας, να τον οδηγήσει στην ολοκληρωτική καταστροφή και εμάς στον απόλυτο αφανισμό; Οι κίνδυνοι είναι πολλοί. Μπορούν ανά πάσα στιγμή να χτυπήσουν την πόρτα μας και να έρθουν είτε από το διάστημα (πρόσκρουση ενός τεράστιου μετεωρίτη) είτε από τις δυνάμεις της ίδιας της Γης (έλευση παγετώδους περιόδου, έκρηξη ενός υπέρ-ηφαιστείου).
Η πρόσκρουση μετεωρίτη στη Γη είναι ένας από τους μεγαλύτερους φόβους των - 98 -
επιστημόνων. Μικροί μετεωρίτες πέφτουν στη Γη σχεδόν κάθε μέρα, αλλά ένας μεγάλος αστεροειδής μπορεί να χτυπά τον πλανήτη μας κατά διαστήματα που απέχουν μεταξύ τους μερικές εκατοντάδες εκατομμύρια χρόνια. Ένας τέτοιος αστεροειδής, με μέγεθος 20 χιλιόμετρα, θα αφάνιζε ένα μεγάλο μέρος των ζωντανών οργανισμών από το πρόσωπο της Γης.
Θα μπορούσαν άραγε οι εξωγήινοι πολιτισμοί να αντιμετωπίζουν παρόμοιες απειλές; Αν ναι, τότε αποδεικνύεται ότι το σύμπαν είναι πολύ πιο επικίνδυνο από όσο νομίζουμε. Οι προσκρούσεις μετεωριτών, για παράδειγμα, είναι πολύ συχνό φαινόμενο στα πλανητικά συστήματα. Οι μαζικοί αφανισμοί μοιάζουν αναπόφευκτοι και ίσως συνιστούν μια εξήγηση για την ανυπαρξία ζωής σε άλλους πλανήτες.
Άνθρωπος : ο μοναδικός
Η τελευταία εξήγηση το παράδοξο του Φέρμι είναι ταυτόχρονα η πιο απλή. Ο άνθρωπος είναι το μοναδικό νοήμον πλάσμα. Αυτό όμως δεν συνεπάγεται ότι όλος ο υπόλοιπος γαλαξίας είναι έρημος. Απλούστερες μορφές ζωής είναι πολύ πιθανό να υπάρχουν σε άλλους πλανήτες. Το μόνο αναμφισβήτητο γεγονός είναι ότι ο άνθρωπος διαφέρει από όλους τους ζωντανούς οργανισμούς του πλανήτη μας, χάρη στη γλώσσα, στην ηθική και στη συνείδηση που τον διακρίνουν. Αυτά τα τρία στοιχεία είναι αρκετά για να μας θεωρούμε μοναδικούς!
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ΕΞΙΣΩΣΗ DRAKE
Γενικά Η εξίσωση του Drake ανακοινώθηκε για πρώτη φορά το 1961 στο αστεροσκοπείο Green Βank της Δυτικής Βιρτζίνια των ΗΠΑ στα πλαίσια ενός συνεδρίου για τις πιθανότητες ύπαρξης εξωγήινης νοημοσύνης κατά τις απαρχές του προγράμματος SETI (Search for Extra-Terrestrial Intelligence). Σύμφωνα με αυτήν υπολογίζεται ο αριθμός (Ν) των υπαρχόντων πολιτισμών στον Γαλαξία:
N = R* x fp x ne x fl x fi x fc x L
Όπου: • R* = Ρυθμός δημιουργίας αστέρων • fp = Ποσοστό των αστέρων που χαρακτηρίζεται από πλανητικά συστήματα • ne= Ποσοστό των παραπάνω αστέρων που έχουν πλανήτες όμοιους της Γης • fl= Ποσοστό των πλανητών στους οποίους μπορεί να αναπτυχθεί ζωή • fi= Ποσοστό των πλανητών που έχουν νοήμονα ζωή • fc= Ποσοστό των πλανητών των οποίων οι πολιτισμοί έχουν αναπτύξει τεχνολογία για διαστρικές επικοινωνίες • L = Μέσος χρόνος ζωής ενός προηγμένου τεχνολογικά πολιτισμού από τη στιγμή που βρίσκεται σε θέση να επικοινωνήσει με άλλους πολιτισμούς, μέχρι την αυτοκαταστροφή του
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Σχόλια
Πριν από οτιδήποτε άλλο οι επιστήμονες διαχωρίζουν ποιοι αστέρες είναι υποψήφιοι για τη φιλοξενία ζωής, ώστε στη συνέχεια να μελετήσουν εάν έχουν πλανητικό σύστημα και βραχώδεις πλανήτες, οι οποίοι είναι πιθανότερο (σύμφωνα πάντα με την ανθρώπινη οπτική) να φιλοξενούν ζωή. Οι γίγαντες και υπεργίγαντες θεωρητικά βγαίνουν εκτός της έρευνας επειδή καίνε τόσο γρήγορα τα καύσιμά τους και πεθαίνουν πριν μπορέσει να δημιουργηθεί ζωή στους τυχόν πλανήτες που έχουν δημιουργηθεί γύρω τους. Χρειάζονται αστέρια τα οποία έχουν σταθερότητα και μεγάλο εύρος κατοικήσιμης ζώνης γύρω τους. Τέτοια αστέρια είναι αστέρια της κυρίας ακολουθίας που μοιάζουν με τον δικό μας ήλιο. Ζουν μεγάλο χρονικό διάστημα (της τάξης των 10^10 ετών) κάτι που επιτρέπει τη δημιουργία και ανάπτυξη ζωής και έχουν «πλατιά» κατοικήσιμη (θερμή) ζώνη. Ακόμα, σύμφωνα με τις τελευταίες εκτιμήσεις των αστροφυσικών και εξωβιολόγων, υποψήφια άστρα θα πρέπει να θεωρούνται και οι ερυθροί νάνοι οι οποίοι δεν θεωρούνταν παλιότερα, εξαιτίας της ψυχρότητάς τους και κατ’ επέκταση του μικρού εύρους κατοικήσιμης ζώνης. Θεωρείται ότι, αν και ψυχροί, η σταθερότητα τους και η διάρκεια της ζωής τους (η οποία ξεπερνά κατά πολύ τη διάρκεια ζωής του ήλιου μας) σε συνδυασμό με τον αριθμό τους (αποτελούν τα περισσότερα αστέρια στον Γάλαξία μας) αντισταθμίζουν τα μειονεκτήματα τους και τα θέτουν υποψήφια προς μελέτη.
Γενικά όμως οι παράμετροι της εξίσωσης Drake (εκτός ίσως των τριών-τεσσάρων πρώτων που μπορούμε να κάνουμε εκτιμήσεις βασισμένοι σε παρατηρήσεις) είναι υποκειμενικοί έως αυθαίρετοι, ποικίλουν από προσέγγιση σε προσέγγιση και από τα πιο αισιόδοξα σενάρια (δεκάδες έως εκατοντάδες χιλιάδες πολιτισμοί στον γαλαξία μας μονάχα) στα περισσότερο απαισιόδοξα (1 έως 10 πολιτισμοί). Τα επόμενα χρόνια θα αναδείξουν περισσότερα συμπεράσματα καθώς θα αυξάνονται οι γνώσεις μας για το σύμπαν, τη ζωή στο ηλιακό μας σύστημα (μετά την έρευνα στον Άρη και στους δορυφόρους των Δία, Κρόνου), και τις νέες τεχνολογίες και τηλεσκόπια που θα δημιουργηθούν.
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ΚΛΙΜΑΚΑ ΚΑΡΝΤΑΣΕΦ Η κλίμακα Καρντάσεφ (Kardashev) κατατάσσει έναν πολιτισμό σύμφωνα με την ενέργεια που είναι σε θέση να καταναλώνει και να χειραγωγεί, η οποία και αντιστοιχεί ταυτόχρονα στο βαθμό εξάπλωσης του πολιτισμού αυτού στον Κόσμο. Προτάθηκε το 1964 από τον Ρώσο αστρονόμο Νικολάι Καρντάσεφ (Nikolai Kardashev). Με την επέκταση της κλίμακας Καρντάσεφ και πριν τη μονάδα προκύπτει μέτρο αξιολόγησης και του ανθρώπινου πολιτισμού στην ιστορία του. Η βασική βαθμονόμηση γίνεται με βάση τρεις ενεργειακές θέσεις στην κλίμακα που αντιστοιχούν στην ικανότητα πλήρους διαχείρισης των ενεργειακών πόρων ενός πλανήτη που κατοικείται (τύπος Ι), του άστρου του αντίστοιχου ηλιακού συστήματος (τύπος ΙΙ) και του γαλαξία του (τύπος ΙΙΙ). Δείγματα πολιτισμών που θα μπορούσαν να αντιστοιχιστούν στην επεκταμένη κλίμακα Καρντάσεφ, είναι ο ανθρώπινος και υποθετικοί πολιτισμοί. Εν έτει 2012, ο ανθρώπινος πολιτισμός βρίσκεται στο 0,72 της κλίμακας, ενώ σύμφωνα με σχετικούς υπολογισμούς, πιθανώς να φτάσουμε τον Τύπο I σε 100 – 200 χρόνια, τον Τύπο II σε μερικές χιλιάδες χρόνια (κατά την εκτίμηση του Freeman Dyson, γύρω στο έτος 11.200) και τον Τύπο III σε 100.000 έως 1 εκατομμύριο χρόνια.
Επίπεδα αντιστοίχισης πολιτισμών στην κλίμακα Καρντάσεφ Τύπος μηδέν Ένας πολιτισμός που στηρίζεται στην ενέργεια από τον πλανήτη στον οποίο κατοικεί, αντλώντας την κυρίως από πηγές που οι μορφές ενέργειας είναι ήπια αποθηκευμένες, βρίσκεται πριν των ορισμένων τύπων (Ι, ΙΙ και ΙΙΙ) στην κλίμακα Καρντάσεφ. Έτσι ονομάζεται τύπος μηδέν καθώς βαθμολογείται κάτω της μονάδας στη λογαριθμική αυτή κλίμακα. Ένας τέτοιος πολιτισμός είναι ο σημερινός δικός μας που χρησιμοποιεί ως επί το πλείστον ενέργεια που έχει αποθηκευτεί από νεκρές πλέον μορφές ζωής σε πηγές όπως το - 102 -
κάρβουνο, το πετρέλαιο ή το φυσικό αέριο. Καθίσταται αντιληπτό ότι δεν έχουμε ακόμη φθάσει την μονάδα στην κλίμακα Kardashev. Σύμφωνα με τον Καρλ Σαγκάν, που επέκτεινε και βαθμονόμησε την κλίμακα Καρντάσεφ πριν τον τύπο Ι, κατά τη βιομηχανική επανάσταση, το έτος 1900, ήμασταν στο 0,58 και το 2012 βαθμολογούμαστε στο 0,72. Σύμφωνα με σχετικούς υπολογισμούς, πιθανώς να φτάσουμε τον Τύπο I σε 100 – 200 χρόνια. Τύπος Ι Ο πολιτισμός Τύπου Ι έχει καταφέρει να δαμάσει και καταναλώνει την ενέργεια που του προσφέρει ο ίδιος ο πλανήτης του, τον οποίο και ελέγχει πλήρως. Η θερμοκρασία και το κλίμα του πλανήτη ελέγχονται ή και αλλάζουν κατά βούληση, ενώ τα έντονα καιρικά φαινόμενα (τυφώνες και ισχυρές καταιγίδες) ελέγχονται απόλυτα και προσφέρουν την ενέργειά τους στον πολιτισμό αυτό. Πιθανώς και τα ηφαίστεια ή ακόμα και οι σεισμοί μπορούν να αλλάξουν κατά βούληση από έναν τέτοιο πολιτισμό. Επίσης, εμφανίζεται οίκηση σε απίθανα ως πρότινος μέρη, όπως στους ωκεανούς (με πλωτές πόλεις). Για τον ανθρώπινο πολιτισμό στη Γη, ορατά πλέον σημάδια μετάβασης σε πολιτισμό Τύπου Ι από Τύπου μηδέν, θεωρούνται η τιθάσευση της πυρηνικής ενέργειας, οι προσπάθειες παγκοσμιοποίησης στην οικονομία (Ευρωπαϊκή Ένωση), την γλώσσα, την κουλτούρα, το πολιτικό σύστημα, τη διεπικοινωνία (ίντερνετ), οι επιτυχημένες προσπάθειες πρόγνωσης και πρόκλησης σεισμών, οι τεράστιες κατασκευές που προστατεύουν την ξηρά από την εισβολή της θάλασσας (Κάτω Χώρες) κλπ. Τύπος ΙΙ Ο πολιτισμός Τύπου ΙΙ ελέγχει την ενέργεια του άστρου του και έχει ήδη αποικίσει το αστρικό του σύστημα. Στην περίπτωση του ανθρώπου της Γης θα έλεγχε τον Ήλιο, αντλώντας την ενέργεια του από εκεί, έχοντας πια εξαντλήσει τα αποθέματα ενέργειας του πλανήτη που κατοικεί και των πλανητών του ηλιακού συστήματος που έχει επίσης αποικίσει. Ο πολιτισμός σε αυτό το επίπεδο θεωρείται ήδη αθάνατος και κανένα γνωστό σημερινό όπλο μαζικής καταστροφής ή φυσική διεργασία δε μπορεί να τον αφανίσει, καθώς είτε μπορεί να σταματήσει τις πυρηνικές αντιδράσεις και να αποτρέψει την έκρηξη του άστρου του ως Υπερκαινοφανούς (Supernova), είτε να μετακινήσει τον ίδιο του τον πλανήτη ώστε να διαφύγει της έκρηξης, είτε απλά να μετοικήσει σε άλλο ηλιακό σύστημα.
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Σημειωτέον ότι η ενέργεια που καταναλώνει είναι 10 δισεκατομμύρια φορές περισσότερη από την παραγόμενη ενέργεια ενός πολιτισμού Τύπου I.
Τύπος ΙΙΙ Ο πολιτισμός Τύπου ΙΙΙ ελέγχει την ενέργεια σε γαλαξιακό επίπεδο στο σύνολο του γαλαξία του και πιθανώς να έχει αποικίσει έναν ή περισσότερους γειτονικούς γαλαξίες. Ένας τέτοιος πολιτισμός έχει φτάσει σε επίπεδο να πειραματίζεται ενεργειακά ακόμη και με τις μαύρες τρύπες και τις σκουληκότρυπες. Μπορεί να πραγματοποιήσει ταξίδια στο χώρο και το χρόνο εντός του ιδίου σύμπαντος και έχει τη δυνατότητα να διαφύγει στο παρελθόν σε περίπτωση που στο σύμπαν του επέλθει θερμικός θάνατος. Παρομοίως, η ενέργεια που καταναλώνει είναι 10 δισεκατομμύρια φορές περισσότερη από την παραγόμενη ενέργεια ενός πολιτισμού Τύπου II. Τύπος ΙV Ο Zoltan Galantai έχει ορίσει μία θεωρητική προέκταση της κλίμακας, με πολιτισμό τύπου IV (που βρίσκεται εκτός των ορισμένων τύπων της κλίμακας Καρντάσεφ) αυτόν που μπορεί και ελέγχει την ενέργεια ολόκληρου του ορατού σύμπαντος και πιθανόν και της σκοτεινής ύλης. Πιθανώς να μπορεί να ταξιδέψει σε άλλα σύμπαντα ή να στείλει πληροφορία σε αυτά, ώστε να αναπτυχθεί πολιτισμός Τύπου μηδέν, με προοπτική να εξελιχθεί σε πολιτισμό Τύπου Ι. Ένας τέτοιος πολιτισμός προσεγγίζει ή και υπερβαίνει ακόμα και τα εξωτικά όρια της επιστημονικής φαντασίας, με βάση τις σημερινές μας επιστημονικές γνώσεις και ίσως να είναι αδύνατο να υπάρξει. Ο ίδιος ο Galantai μάλιστα έχει συμφωνήσει ότι ένας τέτοιος πολιτισμός δεν θα ήταν ανιχνεύσιμος, καθώς οι πράξεις του δεν θα διέφεραν από τις ίδιες τις φυσικές διεργασίες. Ο Dr. Michio Kaku, στο βιβλίο του Parallel Worlds (Παράλληλοι Κόσμοι), έχει δώσει έναν εναλλακτικό ορισμό στον Τύπο IV, ονομάζοντας έτσι τον πολιτισμό που μπορεί να ελέγχει και μη γαλαξιακές πηγές ενέργειας, όπως την λεγόμενη σκοτεινή ενέργεια.
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ΠΙΝΑΚΑΣ ΠΕΡΙΕΧΟΜΕΝΩΝ
ΕΝΟΤΗΤΑ ΠΡΩΤΗ: Αστροβιολογία…………………………3
ΕΝΟΤΗΤΑ ΔΕΥΤΕΡΗ: Προγράμματα SETI……………….55
ΕΝΟΤΗΤΑ ΤΡΙΤΗ: Προγράμματα ενεργού SETI…………..74
ΕΝΟΤΗΤΑ ΤΕΤΑΡΤΗ: Φιλοσοφικές υποθέσεις για την εξωγήινη ζωή και νοημοσύνη…………………………………………….82
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