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Viking 1 Maandur – proovivõtmise robotkäsi kaevas proovide võtmiseks sügavad kraavid(Chryse Planitia).

The current understanding of planetary habitability—the ability of a world to develop environmental conditions favorable to the emergence of life—favors planets that have liquid water on their surface.

This most often requires that the orbit of a planet lie within the habitable zone, which for the Sun extends from just beyond Venus to about the semi-major axis of Mars.[1] During perihelion, Mars dips inside this region, but the planet's thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water demonstrates the planet's potential for habitability. Some recent evidence has suggested that any water on the Martian surface may have been too salty and acidic to support regular terrestrial life.[2]

The lack of a magnetosphere and extremely thin atmosphere of Mars are a challenge: the planet has little heat transfer across its surface, poor insulation against bombardment of the solar wind and insufficient atmospheric pressure to retain water in a liquid form (water instead sublimes to a gaseous state). Mars is also nearly, or perhaps totally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals and minerals between the surface and interior of the planet.[3]

Curiosity rover self-portrait at "Rocknest" (October 31, 2012), with the rim of Gale Crater and the slopes of Aeolis Mons in the distance.

Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there remains unknown. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites and had positive results, including a temporary increase of CO2 production on exposure to water and nutrients. This sign of life was later disputed by some scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life. A re-analysis of the Viking data, in light of modern knowledge of extremophile forms of life, has suggested that the Viking tests were not sophisticated enough to detect these forms of life. The tests could even have killed a (hypothetical) life form.[4] Tests conducted by the Phoenix Mars lander have shown that the soil has a alkaline pH and it contains magnesium, sodium, potassium and chloride.[5] The soil nutrients may be able to support life, but life would still have to be shielded from the intense ultraviolet light.[6] A recent analysis of martian meteorite EETA79001 found 0.6 ppm ClO4, 1.4 ppm ClO3, and 16 ppm NO3, most likely of martian origin. The ClO3 suggests presence of other highly oxidizing oxychlorines such as ClO2 or ClO, produced both by UV oxidation of Cl and X-ray radiolysis of ClO4. Thus only highly refractory and/or well-protected (sub-surface) organics or life forms are likely to survive.[7] In addition, recent analysis of the Phoenix WCL showed that the Ca(ClO4)2 in the Phoenix soil has not interacted with liquid water of any form, perhaps for as long as 600 Myr. If it had, the highly soluble Ca(ClO4)2 in contact with liquid water would have formed only CaSO4. This suggests a severely arid environment, with minimal or no liquid water interaction.[8]

Alga crater – detection of impact glass deposits (green spots) – possible site for preserved ancient life.[9]

Some scientists have proposed that carbonate globules found in meteorite ALH84001, which is thought to have originated from Mars, could be fossilized microbes extant on Mars when the meteorite was blasted from the Martian surface by a meteor strike some 15 million years ago. This proposal has been met with skepticism, and an exclusively inorganic origin for the shapes has also been proposed.[10]

Small quantities of methane and formaldehyde detected by Mars orbiters are both claimed to be possible evidence for life, as these chemical compounds would quickly break down in the Martian atmosphere.[11][12] Alternatively, these compounds may instead be replenished by volcanic or other geological means, such as serpentinization.[13]

Impact glass, formed by the impact of meteors, which on Earth can preserve signs of life, has been found on the surface of the impact craters on Mars.[14][15] Likewise, the glass in impact craters on Mars could have preserved some signs of life if life existed at the site.[16][17][18]

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  1. Viitamistõrge: Vigane <ref>-silt. Viide nimega Nowack on ilma tekstita.
  2. Viitamistõrge: Vigane <ref>-silt. Viide nimega saltlife on ilma tekstita.
  3. Viitamistõrge: Vigane <ref>-silt. Viide nimega hannsson97 on ilma tekstita.
  4. Viitamistõrge: Vigane <ref>-silt. Viide nimega physorg070107 on ilma tekstita.
  5. Viitamistõrge: Vigane <ref>-silt. Viide nimega nutrient on ilma tekstita.
  6. Viitamistõrge: Vigane <ref>-silt. Viide nimega UV on ilma tekstita.
  7. Kounaves, S. P. et al., Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: implications for oxidants and organics, Icarus, 2014, 229, 206–213, DOI:10.1016/j.icarus.2013.11.012,
  8. Kounaves, S. P. et al. (2014). ", Identification of the perchlorate parent salts at the Phoenix Mars landing site and implications". Icarus 232: 226–231. Bibcode:2014Icar..232..226K. doi:10.1016/j.icarus.2014.01.016. 
  9. Staff (8 June 2015). "PIA19673: Spectral Signals Indicating Impact Glass on Mars". NASA. Vaadatud 8 June 2015.  Kontrolli kuupäeva väärtust kohas: |date=, |accessdate= (juhend)
  10. Viitamistõrge: Vigane <ref>-silt. Viide nimega am89 on ilma tekstita.
  11. Viitamistõrge: Vigane <ref>-silt. Viide nimega icarus172 on ilma tekstita.
  12. Viitamistõrge: Vigane <ref>-silt. Viide nimega form on ilma tekstita.
  13. Viitamistõrge: Vigane <ref>-silt. Viide nimega olivine on ilma tekstita.
  14. Nickel, Mark (April 18, 2014). "Impact glass stores biodata for millions of years". Brown University. Vaadatud June 9, 2015.  Kontrolli kuupäeva väärtust kohas: |date=, |accessdate= (juhend)
  15. Schultz, P. H.; Harris, R. Scott; Clemett, S. J.; Thomas-Keprta, K. L.; Zárate, M. (June 2014). "Preserved flora and organics in impact melt breccias". Geology 42 (6): 515–518. Bibcode:2014Geo....42..515S. doi:10.1130/G35343.1.  Kontrolli kuupäeva väärtust kohas: |date= (juhend)
  16. Brown, Dwayne; Webster, Guy; Stacey, Kevin (June 8, 2015). "NASA Spacecraft Detects Impact Glass on Surface of Mars". NASA. Vaadatud June 9, 2015.  Kontrolli kuupäeva väärtust kohas: |date=, |accessdate= (juhend)
  17. Stacey, Kevin (June 8, 2015). "Martian glass: Window into possible past life?". Brown University. Vaadatud June 9, 2015.  Kontrolli kuupäeva väärtust kohas: |date=, |accessdate= (juhend)
  18. Temming, Maria (June 12, 2015). "Exotic Glass Could Help Unravel Mysteries of Mars". Scientific American. Vaadatud June 15, 2015.  Kontrolli kuupäeva väärtust kohas: |date=, |accessdate= (juhend)