Pluto: Icy Wasteland or A Geological Marvel?

Pluto : Icy Wasteland or Geological Marvel?

By Simeon Wren

Pluto has long been thought of as a tiny, barren dwarf-planet, almost 5 billion kilometres from Earth (Redd, 2016) with no real significance or interesting features. Since it was so far from Earth, before 2015 the only images of Pluto were no more than 10 grey and white pixels. However, all that changed after the New Horizons mission. For the first time, high-definition photographs of Pluto were taken, and they revealed some fascinating mysteries and many new details which completely contradicted previous beliefs. Some of the discoveries may even point towards the possibility of extra-terrestrial life.

When the images were analysed, numerous unusual features were found which raised many questions. A particularly large area of icy plains was found to be craterless (Duncan, 2020), a stark difference to the rest of Pluto’s surface. Scientists knew it was highly unlikely that this area did not receive any meteorite or asteroid impacts because there was no logical reason for it. Therefore, they compared Pluto to Earth. The Earth does not have many craters for two different reasons: one is that lots of geological activity on Earth. The tectonic plates shift, and rock sinks down into Earth’s mantle and melts. Any craters or other landforms in the rock eventually sink and melt and then molten rock rises to form new, craterless land. This was far more likely since Pluto doesn’t have an atmosphere, and there was other evidence to back it up.

Fig. 1&2: The patterns of the plains on Pluto (left). The shapes are very large (the features in the top left corner are mountains) and the boundaries between them are very strong and defined (Radford, 2016). The shapes on Pluto are very similar to those in stratocumulus clouds (right) caused by convection currents. (Lipke, 2017)

Upon closer inspection of these plains, a regular pattern was found across the surface (Fig. 1). Despite being in a completely different environment and in a completely different location, these shapes on Pluto’s surface were very similar to stratocumulus clouds on Earth. Stratocumulus clouds are created by convection currents: warm air, which can be very humid, rises and cools, causing the water in the air to condense. This forms clouds broken up into specific shapes much like the ones created by the plains on Pluto. Convection currents are also found inside the Earth, with rock melting nearer the centre and then cooling and condensing at the crust again. These relatively regular shapes on Pluto’s surface suggest that there are geological processes occurring underneath the surface (Duncan, 2020).

Another interesting landform found in the mountainous regions of Pluto further supports the idea of geological processes occurring beneath the surface (Duncan, 2020). One mountain around 5km tall has a large pit inside it, which is also around 5km deep. By using measurements taken by a spectrograph on board the New Horizons spacecraft, scientists found out that the mountain was in fact made of water ice. The surface of the mountain looked exactly like cooled lava-flows on Earth, as if water were erupting from the centre and cooling to orm ice. This was an incredibly exciting prospect because it meant that there was the possibility of a liquid water ocean below Pluto’s surface.

However, the water would freeze and turn to ice as soon as it reaches the bottom of the pit. So, on Pluto where ice is like rock, how could ice flow like lava? This question baffled scientists since unlike many of the other problems there was no natural example on Earth which they could compare it to. Eventually though, they found the answer. Much of Pluto’s surface is red in colour, a colouring caused partly by ammonia (Duncan, 2020). When ammonia is added to water and the water freezes, the water molecules are physically blocked so they cannot become a rigid shape like usual. Instead of being solid ice it is a slush which can flow just like lava. Therefore, an ice volcano would be a solution to this puzzle which makes sense. However, geological processes need heat and so does maintaining a liquid water ocean beneath the surface. On Earth, most of the required heat comes from radioactive elements underground, but because Pluto is much smaller than Earth scientists thought that there would not be enough elements in it to heat the dwarf-planet. They concluded that

there had to be something trapping the heat from escaping. They eventually discovered that because snow and ice have a crystalline structure gas can get trapped inside it (Fig. 3). When this happens, the substance is called gas hydrate snow (Duncan, 2020) (Tenenbaum, 2012). Gas hydrate snow is found in the Arctic regions on Earth which have very similar conditions to those on Pluto, and it has very little thermal conductivity and therefore acts as an insulator. It traps the heat inside Pluto, allowing a liquid water ocean to exist. There could even be enough heat to drive the geological processes believed to occur in the craterless plains on Pluto.

Fig. 3: A diagram showing gas hydrate snow. In the centre is the gas, in this case methane, which is trapped inside the structure of the ice/snow. Interestingly, gas hydrate ice and snow can be lit on fire because the gas inside burns. (Tenenbaum, 2012)

References

Duncan, A. (Director). (2020). Pluto: Back From the Dead [Motion Picture]. Lipke, K. (2017, July 1st). Stratocumulus Clouds: Lesson for Kids. Moore, J. M. (2016). The Geology of Pluto and Charon Through the Eyes of New Horizons. Science. Radford, T. (2016). Pluto’s Perplexing Polygonal Patterns Caused By Convection, Scientists Suggest. The Guardian. Redd, N. T. (2016). How Far Away is Pluto? Space. Stern, S. A. (2015). The Pluto System: Initial Results From its Exploration by New Horizons. Science. Tenenbaum, D. J. (2012). Melting Methane: New Thermometer for Ancient Ocean? The Why Files.

The high likelihood of a liquid water ocean existing beneath the surface of Pluto also provides exciting new research for astrobiologists. From research on Earth, we know that all lifeforms likely need liquid water to exist (Duncan, 2020), so when searching for extra-terrestrial life the first sign scientists look for is liquid water. Now that we know Pluto has a liquid water ocean beneath the surface, it could open many new doors in the hunt for life beyond our planet.