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Do these habitats exist?
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<ref name=Hellas/>. 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.
There's a wide range of views on whether these habitats exist, especially so when it comes to potential surface microhabitats. Some astrobiologists consider that though these brines do exist, they are likely to be uninhabitable. Others treat it it is an open question whether there are temporary habitats that could be recolonized from below,, or inhabited continuously on or near the surface. Others say that it is likely that some parts of the Mars surface are already habitable for some lichens and cyanobacteria ("blue-green algae"), and that they can do this in the absence of liquid water, taking advantage of the night time humidity. Finally, a small minority of astrobiologists say that there is a strong possibility that present day life has already been detected on present day Mars with the Viking Labeled Release experiments. This would mean that much of the Martian surface is not only habitable but actually inhabited by some form of life. See # Views on the possibility of present day life on or near the surface.
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<ref name=Hellas/>. 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. <ref name="RummelBeaty2014SpecialRegionsConclusion"/>
If these habitats do exist they could be inhabited. Life could have evolved on Mars in the past, as there is evidence that it was far more habitable in the past. There is evidence of an early Mars ocean covering most of the northern hemisphere, and in December 2014, Curiosity scientists presented evidence that Gale Crater once contained a huge lake that was filled and evaporated many times.. This lake may have been habitable for life. For more on this see Life on Mars.
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 [[River delta|deltas]] , with seventeen of them found at the altitude of a proposed shoreline for a Martian ocean<ref>{{cite journal | last1 = DiAchille | first1 = G | last2 = Hynek | first2 = B. | year = 2010 | title = Ancient ocean on Mars supported by global distribution of deltas and valleys. nat | url = | journal = Geosci | volume = 3 | issue = 7| pages = 459–463 | doi = 10.1038/ngeo891 | bibcode=2010NatGe...3..459D}}</ref> This is what would be expected if the deltas were all next to a large body of water.<ref>{{cite journal | last1 = DiBiasse | first1 = | last2 = Limaye | first2 = A. | last3 = Scheingross | first3 = J. | last4 = Fischer | first4 = W. | last5 = Lamb | first5 = M. | year = 2013 | title = Deltic deposits at Aeolis Dorsa: Sedimentary evidence for a standing body of water on the northern plains of Mars | url = | journal = [[Journal Of Geophysical Research: Planets]] | volume = 118 | issue = | pages = 1285–1302 }}</ref>, 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.<ref name="NASA-20141208">{{cite web |last=Brown |first=Dwayne |last2=Webster |first2=Guy |title=Release 14-326 - NASA’s Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape |url=http://www.nasa.gov/press/2014/december/nasa-s-curiosity-rover-finds-clues-to-how-water-helped-shape-martian-landscape/ |date=8 December 2014 |work=[[NASA]] |accessdate=8 December 2014}}</ref><ref name="NYT-20141208">{{cite news |last=Kaufmann |first=Marc |title=(Stronger) Signs of Life on Mars |url=https://www.nytimes.com/2014/12/09/science/-stronger-signs-of-life-on-mars.html |date=8 December 2014 |work=[[New York Times]] |accessdate=8 December 2014}}</ref><ref name="sciencedaily2">{{cite web|url=https://www.sciencedaily.com/releases/2014/12/141208122903.htm|archive-url=https://web.archive.org/web/20141213204457/https://www.sciencedaily.com/releases/2014/12/141208122903.htm |archive-date=2014-12-13 |dead-url=yes|title=NASA's Curiosity rover finds clues to how water helped shape Martian landscape -- ScienceDaily|accessdate=4 July 2015}}</ref><ref name="nasa">{{cite web|url=http://www.jpl.nasa.gov/video/details.php?id=1346|title=JPL | Videos | The Making of Mount Sharp|publisher=jpl.nasa.gov|accessdate=4 July 2015}}</ref><ref name="nasa2">{{cite web|url=http://www.jpl.nasa.gov/news/news.php?feature=4398|title=JPL | News | NASA's Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape|publisher=jpl.nasa.gov|accessdate=4 July 2015}}</ref><ref name="usgs">{{cite web|url=https://pubs.er.usgs.gov/publication/70047207|title=Martian fluvial conglomerates at Gale Crater|publisher=pubs.er.usgs.gov|accessdate=4 July 2015}}</ref><ref>{{cite journal |last1=Williams |first1=R. |display-authors=etal |year=2013 |title=Martian fluvial conglomerates at Gale Crater |url=|journal=Science |volume= 340| issue=|pages= 1068–1072| doi=10.1126/science.1237317 |pmid=23723230|bibcode=2013Sci...340.1068W}}</ref>, 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 (Mars|Amazonian]] period which continues to the present day.
The habitats could also exist and be uninhabited, a possibility investigated by Charles Cockell in a series of papers. See Uninhabited habitats
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<ref name=Hellas/>. 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. <ref name="RummelBeaty2014SpecialRegionsConclusion"/> 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<ref name=Hellas/>. 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<ref name="RummelBeaty2014SpecialRegionsConclusion"/>. 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".<ref name=Oren>{{cite journal | doi = 10.2166/wst.2009.635 | volume=60 | title=Molecular assessment of salt-tolerant, perchlorate- and nitrate-reducing microbial cultures | year=2009 | journal=Water Science & Technology | page=1745 | 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 Connley, planetary protection officer for NASA from 2006 to 2018, put it like this, when interviewed by the New York Times:<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>:
Conferences on the topic of present day habitats for life on Mars
{{quote|"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.{{cn|date=August 2018}}<!--will be adding cites-->
2013, February 4-6, conference on the Present Day Habitability of Mars was held in 2013 in UCLA..
2017, April 24-29 conference sub session on Modern Mars Habitability, Mesa, Arizona, organized by the NASA Ames Research Center, and LPL, University of Arizona, as part of the Astrobiology Science Conference 2017 .
2019, January 20 - February 1: Mars Extant Life: What’s Next?" to discuss the "numerous extant life hypotheses that have been advanced over the years and that have evolved in response to discoveries by on-going Mars missions."
Mars surface conditions simulation chambers
These chambers simulate the Martian day night cycle and other conditions of the Martian surface, with the goal to investigate the present day habitability of Mars. It's especially important to simulate the temperature and pressure variations because, though the amount of water vapour in the Mars atmosphere is low, at night the atmosphere becomes so cold that the relative humidity approaches 100%, which is of significance for any life that may be there. The chambers also have to simulate the Martian sunlight which has much less UV light filtered out than Earth sunlight. This is sterilizing over short timescales to any unprotected life directly exposed to the sunlight.
The Michigan Mars Environmental Chamber is run by Nilton Renno and his team:
Introduction: We have developed the Michigan Mars Environmental Chamber (MMEC) to simulate the entire range of Martian surface and shallow subsurface conditions with respect to temperature, pressure, relative humidity, solar radiation and soil wetness. Our goal is to simulate the Martian diurnal cycle for equatorial as well as polar Martian conditions and test the hypothesis that salts known to exist in the Martian regolith can deliquesce and form brine pockets or layers by freeze-thaw cycles. Motivation: Liquid water is one of the necessary ingredients for the development of life as we know it. ... It has been shown that liquid brines are ubiquitous in the Martian polar regions and microbial communities have been seen to survive under similar conditions in Antarctica's Dry Valleys.
The Mars Simulation Facility-Laboratory at the German Aerospace facilities (DLR) in Berlin is run by Jean-Pierre de Vera used for numerous astrobiological Mars habitability studies. as part of HOME (Habitability of Mars Environments)
"used for different astrobiological and physical experiments to simulate the key environmental conditions like pressure, temperature, radiation, gas composition, and primarily also the diurnally varying atmospheric humidity in a range from earth conditions to similar to those at the near-surface atmosphere of Martian mid- and low latitude" run by Jean-Pierre de Vera used for numerous astrobiological Mars habitability studies.
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