Life in the solar system
atmosphere and fall to the surface, providing the precursor compounds for life. 15 The possibility of exotic life using hydrocarbons as a solvent instead of water has been considered, but little is known about how such an alternate biochemistry could function. However, there may be life in the subsurface water ocean. Exchange of the organic compounds on the surface with the ocean would provide a plentiful supply of energy and material for any organism living below. Elsewhere in the solar system, more worlds are being found to harbour potential habitable environments. The dwarf planet Ceres has recently been found to have surface salt deposits from brine eruptions (e.g. in Occator Crater), indicating potential subsurface water. These salt deposits have been found to be very recent (less than 22 million years old and possibly still ongoing). 16 Although all our information on Triton comes from a single flyby by Voyager 2, there is evidence of a subsurface ocean. Potential geysers were observed, which would require a subsurface liquid reservoir and an internal heat source. Since the flyby by New Horizons in 2015, Pluto has garnered the interest of astrobiologists. Previously thought to be an inert, dead world, dynamic processes have been discovered which provide a potential habitable environment. Many objects in the Kuiper belt have reddish surfaces from organic compounds which result frommillions of years of low intensity sunlight and cosmic radiation, which breaks down simple molecules, the reactive atoms then bind to form larger and more complex structures, known as Tholins. This isn’t possible in the inner solar system since the sunlight and radiation environment is too intense for the newly formed compounds to accumulate, they are broken back down into simple molecules. Pluto has these red deposits, and spectral analysis has found a variety of organic compounds including nucleotides, which are the precursors to DNA. 17 Pluto has potential subsurface ocean. Evidence of recent resurfacing in Sputnik Planum with convective zones indicate ongoing heat flow from core, whilst evidence of cryovolcanism has also been found. It is unclear what Pluto’s heat source might be, since it is too small to have retained much heat since its formation, and it does not experience significant tidal heating. It is also believed not to have a high enough metal concentration to sustain much heating from nuclear decay. The subsurface ocean, if it exists, is likely to be comprised of an ammonia-water mix, allowing a very lowmelting point. 18 In the coming years there is set to be major progress towards determining whether life is present on other solar system bodies, as new technologies mean it is now feasible to fly instruments that will be capable of directly determining the presence of life and biological molecules. Previous missions toMars have used a ‘Follow the Water’ strategy to determine past habitability ; in future missions will look for direct evidence of life. The Perseverance rover will prepare for a futureMars Sample Return in the 2026- 2030 timeframe, which will allow for detailed analysis of Martian soil samples, while Exomars rover, in 2022, will attempt a direct search for life by carrying a Raman spectrometer, and a mass spectrometer for chemical analysis and the first microscope to Mars. The rover will also be able to reach the deep subsurface using a 2-metre core drill. 19
15 https://www.nasa.gov/mission_pages/cassini/multimedia/pia17240.html#. Consulted 14/08/2020. 16 https://www.centauri-dreams.org/2020/08/12/ceres-the-lesson-of-occator-crater/. Consulted 15/08/2020. 17 https://ntrs.nasa.gov/citations/20160009761. Cruickshank D. Document ID:20160009761 Pub.30/07/2016. 18 Cruikshank D.P. and Keane J. T. (2019) ‘Recent cryovolcanism in Virgil Fossae on Pluto’, Icarus 330: 155-168. 19 https://exploration.esa.int/web/mars/-/45103-rover-instruments. Consulted 18/08/2020.
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