Life in the solar system
bursts of heavier particles and ions during solar flares and coronal mass ejections) and Galactic Cosmic Rays (protons from outside the solar system travelling at relativistic speeds). On Earth, life is shielded by the atmosphere and magnetic field, resulting in a benign surface environment. 6 All life on Earth requires water, as a solvent, transportation medium and a reactant for biochemical reactions. Many organisms dissolve high concentrations of sugars or salts into their internal water to survive at sub- zero temperatures, giving a temperature range wide enough to allow life in the outer solar system. Up until the arrival of Mariner 4 at Mars in 1965, scientific and public opinion was that Mars being habitable and with a well-developed biosphere was likely. Seasonal changes in Mars ’ appearance were attributed to vegetation and cloud formations. However, on the first flybys by space probes, it was found that the Martian surface appeared completely uninhabitable, being blasted by radiation due to the lack of a magnetic field, with an atmospheric pressure of up to 0.7kPa (compared to Earth’s 101kPa) and average surface temperatures of -63°C, although the maximum surface temperature was recorded at 31°C. 7 Additionally, the soils are rich in perchlorates, which are highly toxic to most Earth life, especially when activated by UV radiation. 8 Overall, the surface conditions are generally viewed to be inhospitable to life. The possibility of subsurface life does still remain, and has been bolstered both by Martian observations and discoveries of life in similar environments on Earth. One of the most promising observations has been the detection of methane traces in various locations on Mars. Methane rapidly breaks down in the Martian atmosphere due to the intense solar UV light, so any detected methane must be from an active source. Orbiting spacecraft have determined that atmospheric methane is localized and concentrations have significant seasonal variations. In 2019, the Curiosity rover discovered seasonal methane at the highest ever recorded levels, 21parts per billion by volume (ppbv), much more than the 0.7ppbv baseline. 9 There was no detection of sulphur dioxide alongside the methane so volcanism is unlikely. While other geochemical processes are possible, currently there is no satisfactory explanation that accounts for all the evidence. The question deepened further with the detection of sudden fluctuations in oxygen concentration, which has proved even more difficult to explain. 10 The subsurface environments on Earth and Mars are similar. The Martian subsurface is shielded from radiation, may retain groundwater, and beyond a certain depth has similar temperatures to those found on Earth. Life has been found deep underground on Earth: endolithic organisms are very widespread and have been found at depths of up to 3km. At these depths the underground environment on Earth and Mars is believed to be almost identical. 11 6 https://www.nasa.gov/sites/default/files/atoms/files/space_radiation_ebook.pdf. Consulted 05/08/2020. 7 https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html. Consulted 04/08/2020. 8 Wadsworth J., Cockell C. S. ‘Perchlorates on Mars enhance the bacteriocidal effects of UV light’, Sci Rep 7 , 4662 (2017). 9 https://www.jpl.nasa.gov/news/news.php?feature=7433. Last updated 24/06/2019, consulted 04/08/2020. 10 https://www.nasa.gov/feature/goddard/2019/with-mars-methane-mystery-unsolved-curiosity-serves- scientists-a-new-one-oxygen. Consulted 04/08/2020. 11 Cockell C. S. ‘Life in the lithosphere, kinetics and the prospects for life elsewhere’, Phil. Trans. R. Soc. A.369516 – 537 (2011).
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