Possible present day habitats for life on Mars (Including potential Mars special regions): Difference between revisions
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From the [[Viking program|Viking landers]] in 1976 through to the Phoenix measurements in 2008, conditions on Mars seemed so inhospitable that many scientists believed that water ''"does not and cannot exist on the surface of Mars today" (to quote NASA Astrobiology magazine)''..<ref name="LevinMarsLifeIdea" /> There are only five regions where the atmospheric pressure is high enough for liquid fresh water can form, Amazonis, Chryse and Elysium Planitia, in the Hellas Basin and the Argyre Basin. However, even in those places, it is close to its boiling point of 10 °C and would soon evaporate<ref name="Hellas">{{cite web|title=Extracts from "Making a Splash on Mars"|url=http://lasp.colorado.edu/home/wp-content/uploads/2013/06/Mars_Articles_20130617.pdf}}</ref>. Also there may be little by way of a source of ice to melt to form water as ice exposed to the atmosphere is not long term stable in the equatorial regions within around ± 30° of the equator<ref>Schorghofer, N. and Aharonson, O., 2005. [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2004JE002350 Stability and exchange of subsurface ice on Mars]. Journal of Geophysical Research: Planets, 110(E5)</ref>.
This view of the Martian surface as totally sterile and inhosptiable to present day life changed in 2008 with the [[Phoenix (spacecraft) | Phoenix lander]], the first and so far only spacecraft to land successfully in the martian polar regions, near the north pole. It observed droplet-like features that formed on its landing legs<ref name="phoenix_droplets_2009" />, and also detected the presence of geologically recent surface or near surface brines indirectly, through isotopic measurements of oxygen in the atmosphere that had been exchanged with surface water<ref name="phoenixisotope" /> <ref name="phoenix_droplets_2009" />. Perchlorates and other salts act as an antifreeze, and some mixes of salts can stay liquid on Mars at temperatures within the habitability range of life<ref name="GoughChevrier2014" />. In December 2013, Nilton Renno<ref name="NiltonRennoFaculty" /> and his team using the Michigan Mars Environmental Chamber were able to simulate the conditions at the Phoenix landing site and to simulate the Phoenix leg droplets<ref name="MicheganMars">https://www.researchgate.net/publication/283504377_The_Michigan_Mars_Environmental_Chamber_Preliminary_Results_and_Capabilities</ref>. They formed salty brines within minutes when salt overlaid ice. The team concluded that suitable conditions for brine droplets may be widespread in the polar regions<ref name="salt_ice">[http://www.astrobio.net/news-brief/liquid-water-ice-salt-mars/ Liquid Water from Ice and Salt on Mars], Aaron L. Gronstal -Astrobiology Magazine (NASA), Jul 3, 2014</ref><ref name="salt_ice_paper">Fischer, E., Martínez, G.M., Elliott, H.M. and Rennó, N.O., 2014. [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL060302 Experimental evidence for the formation of liquid saline water on Mars]. Geophysical research letters, 41(13), pp.4456-4462.</ref>.
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If these droplet microhabitats exist, there are many additional challenges. The Mars soil (regolith) and dust contains between 0.5 and 1% of reactive perchlorates<ref name="DavilaPerchlorates">Davila, A.F., Willson, D., Coates, J.D. and McKay, C.P., 2013. [https://www.researchgate.net/publication/242525435_Perchlorate_on_Mars_A_chemical_hazard_and_a_resource_for_humans Perchlorate on Mars: a chemical hazard and a resource for humans]. International Journal of Astrobiology, 12(4), pp.321-325.</ref>. The UV radiation is rapidly lethal to microbes, unless shielded<ref name="RummelBeaty2014" />, and the ionizing radiation will sterilize any dormant life within 500,000 years on the surface of Mars<ref name="KminekBada2006" />.▼
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However, these conditions may not make the surface completely uninhabitable to microbial life. NASA's Mars Exploration Program Analysis Group concluded based on the [[Curiosity (rover)| Curiosity rover]]'s RAD measurements that 500 years of ionizing radiation would kill only 90% of even the most radiation-sensitive bacterium such as E. coli<ref name="RummelBeaty2014SpecialRegionsConclusion" />. The UV is easily blocked by about 0.3 mm of surface soil<ref name="Mateo-Marti2014" />,a millimeter of dust<ref name="RummelBeaty2014" />, or protective pigments such as [[melanin]], [[parietin]] and [[usnic acid]]<ref name="Ustvedt" />. The perchlorates, though harmful to some forms of life, are metabolized by others<ref name="Oren">{{cite journal | doi = 10.2166/wst.2009.635 | volume=60 | issue=1 | pages=75–80 | title=Molecular assessment of salt-tolerant, perchlorate- and nitrate-reducing microbial cultures | year=2009 | journal=Water Science & Technology | last1 = Zuo | first1 = G. | last2 = Roberts | first2 = D. J. | last3 = Lehman | first3 = S. G. | last4 = Jackson | first4 = G. W. | last5 = Fox | first5 = G. E. | last6 = Willson | first6 = R. C.|pmid=24150694}}</ref>. Cassie Conley, the NASA planetary protection officer from 2006 to 2018, said of the perchlorates, ''“The environment on Mars potentially is basically one giant dinner plate for Earth organisms,”''<ref>{{cite news|last1=Chang|first1=Kenneth|title=Mars Is Pretty Clean. Her Job at NASA Is to Keep It That Way.|url=https://www.nytimes.com/2015/10/06/science/mars-catharine-conley-nasa-planetary-protection-officer.html|agency=New York Times|date=October 5, 2015}}</ref>.
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