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Although the Modern Mars atmosphere is far too thin for fresh liquid water to be stable on its surface, except briefly in the depths of the Hellas Basin[1]. Salty percholrate brines, however, may be stable in Mars surface or near surface conditions. Modern Earth life is able to survive extreme conditions including extremes of aridity, and salinity. The surface of Mars is bathed in UV radiation which is sterilizing for microbial life, however this is easily blocked out by a shadow or a thin layer of dust.

The Mars' surface is exposed to much more ultraviolet radiation than Earth's, and this is lethal to most forms of microbial life. However, this radiation is blocked by shadows or a layer of dust a fraction of a millimeter thick. The levels of ionizing radiation are such that 500 years of exposure would kill only 90% of even highly radiosensitive microbes. At only 0.6% of the pressure of Earth's atmosphere at the surface on average (0.01139 to 0.00030 atm), the Mars' current atmosphere is too thin for fresh water to exist in liquid form anywhere on the planet's surface except the Hellas Basin, and there, only for a short while, as it is close to boiling point[1]. Salty perchlorate brines, however, may remain stable under surface conditions or near-surface conditions, and depending on the composition, may be stable at temperatures warm enough to support Earth based life. [2]

Mars is the fourth planet from the Sun, after Mercury, Venus, and Earth. During the formation of the solar system, it cooled down before Earth did. There is some evidence that Mars once had a large body of water in its northern hemisphere, although it may have been covered with ice for much of the time. This evidence includes fossil deltas , with seventeen of them found at the altitude of a proposed shoreline for a Martian ocean[3] This is what would be expected if the deltas were all next to a large body of water.[4], see Mars ocean hypothesis. In a press conference on December 8, 2014, Curiosity scientists presented evidence that Crater Lake once contained a huge lake that was filled and evaporated many times.[5][6][7][8][9][10][11], about 3 billion years ago. See Lakes on Mars#Gale Crater. From this, researchers have inferred that the thick atmosphere of early Noachian period Mars continued through to the intermediate Hesperian period (although it may have been intermittent, possibly released by volcanic activity [cite]). This created conditions that may have been suitable for life to evolve there in the past, or for life to be transferred from early Earth to inhabit early Mars during the large impacts of the Late Heavy Bombardment. The atmosphere almost vanished, however as it transitioned through to the Amazonian period which continues to the present day.

Most of the search for life on modern Mars has focused on microorganisms, although lichens are also possible there. On Earth, there are many life forms that can survive under extreme conditions, such as very arid, very salty, very acidic, and very hot and cold environments, and most of them are microbial. The Mars' surface is exposed to much more ultraviolet radiation than Earth's, and this is lethal to most forms of microbial life. However, this radiation is blocked by shadows or a layer of dust a fraction of a millimeter thick. The levels of ionizing radiation are such that 500 years of exposure would kill only 90% of even highly radiosensitive microbes. At only 0.6% of the pressure of Earth's atmosphere at the surface on average (0.01139 to 0.00030 atm), the Mars' current atmosphere is too thin for fresh water to exist in liquid form anywhere on the planet's surface except the Hellas Basin, and there, only for a short while, as it is close to boiling point[1]. Salty perchlorate brines, however, may remain stable under surface conditions or near-surface conditions, and depending on the composition, may be stable at temperatures warm enough to support Earth based life. [2] It is also possible that Mars has life adapted to lower temperatures than Earth microbes, with perchlorates and hydrogen peroxide in place of the chlorides of Earth life, as an "antifreeze" The Modern Mars atmosphere is far too thin for fresh liquid water to be stable on its surface, except briefly in the depths of the Hellas Basin[1]. Salty percholrate brines, however, may be stable in Mars surface or near surface conditions. Modern Earth life is able to survive extreme conditions including extremes of aridity, and salinity. The surface of Mars is bathed in UV radiation which is sterilizing for microbial life, however this is easily blocked out by a shadow or a thin layer of dust. The ionizing radiation at the surface has been found to be attenuated to the levels where 500 years of radiation would kill only 90% of even the most radiosensitive microbes[2]. The perchlorate salts themselves are harmful to some forms of life but they serve as an oxidant, as part of their metabolic processes for others, in essence, a form of "food".[12]. Cassie Connley, planetary protection officer for NASA from 2006 to 2018, put it like this, when interviewed by the New York Times:[13]:

"The salts known as perchlorates that lower the freezing temperature of water at the R.S.L.s, keeping it liquid, can be consumed by some Earth microbes. “The environment on Mars potentially is basically one giant dinner plate for Earth organisms,” Dr. Conley said."

This leads to the question of whether there are any habitats for life on Mars. If there are then there may be life there that has survived since the times of early Mars when it was far more habitable than it is now.[citation needed]

  1. ^ a b c d Cite error: The named reference Hellas was invoked but never defined (see the help page).
  2. ^ a b c Cite error: The named reference RummelBeaty2014SpecialRegionsConclusion was invoked but never defined (see the help page).
  3. ^ DiAchille, G; Hynek, B. (2010). "Ancient ocean on Mars supported by global distribution of deltas and valleys. nat". Geosci. 3 (7): 459–463. Bibcode:2010NatGe...3..459D. doi:10.1038/ngeo891.
  4. ^ DiBiasse; Limaye, A.; Scheingross, J.; Fischer, W.; Lamb, M. (2013). "Deltic deposits at Aeolis Dorsa: Sedimentary evidence for a standing body of water on the northern plains of Mars". Journal of Geophysical Research: Planets. 118 (6): 1285–1302. doi:10.1002/jgre.20100. S2CID 2286329.
  5. ^ Brown, Dwayne; Webster, Guy (8 December 2014). "Release 14-326 - NASA's Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape". NASA. Retrieved 8 December 2014.
  6. ^ Kaufmann, Marc (8 December 2014). "(Stronger) Signs of Life on Mars". New York Times. Retrieved 8 December 2014.
  7. ^ "NASA's Curiosity rover finds clues to how water helped shape Martian landscape -- ScienceDaily". Archived from the original on 2014-12-13. Retrieved 4 July 2015.
  8. ^ "JPL | Videos | The Making of Mount Sharp". jpl.nasa.gov. Retrieved 4 July 2015.
  9. ^ "JPL | News | NASA's Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape". jpl.nasa.gov. Retrieved 4 July 2015.
  10. ^ "Martian fluvial conglomerates at Gale Crater". pubs.er.usgs.gov. Retrieved 4 July 2015.
  11. ^ Williams, R.; et al. (2013). "Martian fluvial conglomerates at Gale Crater". Science. 340 (6136): 1068–1072. Bibcode:2013Sci...340.1068W. doi:10.1126/science.1237317. PMID 23723230. S2CID 206548731.
  12. ^ Zuo, G.; Roberts, D. J.; Lehman, S. G.; Jackson, G. W.; Fox, G. E.; Willson, R. C. (2009). "Molecular assessment of salt-tolerant, perchlorate- and nitrate-reducing microbial cultures". Water Science & Technology. 60 (7): 75–80. doi:10.2166/wst.2009.635. PMID 24150694.
  13. ^ Chang, Kenneth (October 5, 2015). "Mars Is Pretty Clean. Her Job at NASA Is to Keep It That Way". New York Times.
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