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[[Image:Martian conditions in miniature (7494313830) (2).jpg|thumb|Martian conditions in miniature - In the Mars simulation chamber, DLR researchers recreated the atmospheric composition and pressure, the planet's surface, the temperature cycles and the solar radiation incident on the surface. The activity of polar and alpine lichen was investigated under these conditions.]]
A series of experiments by DLR (German aerospace company) in Mars simulation chambers and on the ISS show that some Earth life (Lichens and strains of chrooccocidiopsischroococcidiopsis, a green algae) can survive Mars surface conditions and photosynthesize and metabolize, slowly, in absence of any water at all. They could make use of the humidity of the Mars atmosphere.<ref name="dlrMarsStudy">[http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-3409/ Surviving the conditions on Mars] DLR, 26 April 2012</ref><ref name="sciencedirect.com">{{cite journal|url = http://www.sciencedirect.com/science/article/pii/S1754504812000098 | doi=10.1016/j.funeco.2012.01.008 | volume=5 | issue=4 | title=Lichens as survivors in space and on Mars | year=2012 | journal=Fungal Ecology | pages=472–479 | last1 = de Vera | first1 = Jean-Pierre}}</ref><ref name="norlx51.nordita.org">R. de la Torre Noetzel, F.J. Sanchez Inigo, E. Rabbow, G. Horneck, J. P. de Vera, L.G. Sancho [http://norlx51.nordita.org/~brandenb/astrobiology/EANA2012/single_abstracts/Delatorre.pdf Survival of lichens to simulated Mars conditions] {{webarchive|url=https://web.archive.org/web/20130603191033/http://norlx51.nordita.org/~brandenb/astrobiology/EANA2012/single_abstracts/Delatorre.pdf |date=2013-06-03 }}</ref><ref name="Issue 1 2012, Pages 102">{{cite journal|url = http://www.sciencedirect.com/science/article/pii/S0032063312002425 | doi=10.1016/j.pss.2012.08.005 | bibcode=2012P&SS...72..102S | volume=72 | issue=1 | title=The resistance of the lichen Circinaria gyrosa (nom. provis.) towards simulated Mars conditions—a model test for the survival capacity of an eukaryotic extremophile | year=2012 | journal=Planetary and Space Science | pages=102–110 | last1 = Sánchez | first1 = F.J.}}</ref><ref name="BilliViaggiu2011">{{cite journal|url=https://scholar.google.com/scholar_url?url=http://www.researchgate.net/profile/Charles_Cockell/publication/49810974_Damage_escape_and_repair_in_dried_Chroococcidiopsis_spp._from_hot_and_cold_deserts_exposed_to_simulated_space_and_martian_conditions/links/0c960530543245cde9000000.pdf&hl=en&sa=T&oi=gsb-gga&ct=res&cd=1&ei=M2AqVeLzG-fq0AG5xYGACA&scisig=AAGBfm1aHrkKehQaYpPYGQ9mjRxVTxPS0Q|last1=Billi|first1=Daniela|last2=Viaggiu|first2=Emanuela|last3=Cockell|first3=Charles S.|last4=Rabbow|first4=Elke|last5=Horneck|first5=Gerda|last6=Onofri|first6=Silvano|title=Damage Escape and Repair in DriedChroococcidiopsisspp. from Hot and Cold Deserts Exposed to Simulated Space and Martian Conditions|journal=Astrobiology|volume=11|issue=1|year=2011|pages=65–73|issn=1531-1074|doi=10.1089/ast.2009.0430|pmid=21294638|bibcode=2011AsBio..11...65B}}</ref> Though the absolute humidity is low, the relative humidity at night in winter reaches over 70% even in regions close to the equator, as measured directly in Gale Crater by Curiosity, because of the large day / night swings in atmospheric pressure and temperature. It may well reach much closer to 100% in regions where frosts are seen (including the [[Viking 2]] lander site) and where ground hugging mists form, including equatorial regions in Valles Marineres and the Hellas basin<ref name=Martínez2017/>.
This is relevant to the search for native life on Mars. It is also relevant to planetary protection, the need to protect Mars from Earth life if we wish to study native life in the habitats in its original state.
However, when partially shaded from the UV light, as it is in its natural habitats in Antarctica, both fungus and algae survived, and the algae remained photosynthetically active throughout. Also new growth of the lichen was observed. Photosynthetic activity continued to increase for the duration of the experiment, showing that the lichen adapted to the Mars conditions.<ref name="DLRLichenHabitable"/>
This is remarkable as the fungus is an aerobe, growing in an atmosphere with no appreciable amount of oxygen and 95% CO<sub>2</sub>. It seems that the algae provides it with enough oxygen to survive. The lichen was grown in Sulfatic Mars Regolith Simulant - igneous rock with composition similar to Mars meteorites, consisting of gabbro and olivine, to which quartz and anhydrous iron oxide hematite (the only thermodynamically stable iron oxide under present day Mars conditions) were added. It also contains gypsum and geothite, and was crushed to simulate the martian regolith. This was an ice free environment. They found that photosynthetic activity was strongly correlated with the beginning and the end of the simulated Martian day. Those are times when atmospheric water vapour could condense on the soil and be absorbed by it, and could probably also form cold brines with the salts in the simulated martianMartian regolith. The pressure used for the experiment was 700 - 800 Pa, above the triple point of pure water at 600 Pa and consistent with the conditions measured by Curiosity in Gale crater.<ref name="DLRLichenHabitable">{{cite journal|url=https://core.ac.uk/download/pdf/31019036.pdf|last1=de Vera|first1=Jean-Pierre|last2=Schulze-Makuch|first2=Dirk|last3=Khan|first3=Afshin|last4=Lorek|first4=Andreas|last5=Koncz|first5=Alexander|last6=Möhlmann|first6=Diedrich|last7=Spohn|first7=Tilman|title=Adaptation of an Antarctic lichen to Martian niche conditions can occur within 34 days|journal=Planetary and Space Science|volume=98|year=2014|pages=182–190|issn=0032-0633|doi=10.1016/j.pss.2013.07.014|bibcode=2014P&SS...98..182D|quote=This work strongly supports the interconnected notions (i) that terrestrial life most likely can adapt physiologically to live on Mars (hence justifying stringent measures to prevent human activities from contaminating / infecting Mars with terrestrial organisms); (ii) that in searching for extant life on Mars we should focus on "protected putative habitats"; and (ii) that early-originating (Noachian period) indigenous Martian life might still survive in such micro-niches despite Mars' cooling and drying during the last 4 billion years|}}</ref>
The experimenters concluded that it is likely that some lichens and cyanobacteria can adapt to Mars conditions, taking advantage of the night time humidity, and that it is possible that life from early Mars could have adapted to these conditions and still survive today in microniches on the surface.
The humidity variations on Mars are mainly due to the daily and seasonal variations of temperature of the air. Colder air has a higher relative humidity for the same water content.
The amount of water vapour can be measured as the volume mixing ratio of water vapour (VMR). This varies between around 10 ppm in winter and 70 ppm in summer at the Curiosity site. You might expect the highest relative humidithyhumidity then to be in summer, but no, it's in winter, because it is so much colder then. The summer relative himidityhumidity is about 10% and the winter relative humidity around 70%.
So, it's 10ppm (approx) for 70% RH in winter, and 70 ppm for 10% RH in summer (by a strange coincidence the numbers are reversed).
CurosityCuriosity made these mesaurementsmeasurements of 70% humidity at a height of 1.6 meters above ground level (see Sect 14<ref>[https://link.springer.com/article/10.1007/s11214-017-0360-x#Sec14 Sec 14]</ref>. This is in winter with a temperature range of around 50 C and mean temperature around -63 C, so highest temperature around -38 C and lowest temperature around -88 C (see their figure 5<ref>[https://link.springer.com/article/10.1007/s11214-017-0360-x#Fig5 Fig5]</ref>), and the highest humidity is normally reached between 04:00 and 06:00 Local Mean Solar Time (LMST). The lowest humidity readings are between 10:00 and 18:000 LMST, when they are typically less than 5%.
The Viking landers didn't have humidity sensors. But the humidity can be estimated indirectly, with maximum volume mixing ratio of water vapour of 200 ppm for both spacecraft.
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