LIQUID WATER
ON MARS
Liquid water, necessary for life as we know it, cannot exist on the surface of Mars except at the lowest elevations for minutes or hours.[59][60] Liquid water does not appear at the surface itself,[61] but it could form in minuscule amounts around dust particles in snow heated by the Sun.[62][63] Also, the ancient equatorial ice sheets beneath the ground may slowly sublimate or melt, accessible from the surface via caves.[64][65][66][67]
Water on Mars exists almost exclusively as water ice, located in the Martian polar ice caps and under the shallow Martian surface even at more temperate latitudes.[68][69] A small amount of water vapor is present in the atmosphere.[70] There are no bodies of liquid water on the Martian surface because its atmospheric pressure at the surface averages 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the temperature is far too low, (210 K (−63 °C)) leading to immediate freezing. Despite this, about 3.8 billion years ago,[71] there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface,[72][73][74][75]including large oceans.[76][77][78][79][80] It has been estimated that the primordial oceans on Mars would have covered between 36%[81] and 75% of the planet.[82]
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Analysis of Martian sandstones, using data obtained from orbital spectrometry, suggests that the waters that previously existed on the surface of Mars would have had too high a salinity to support most Earth-like life. Tosca et al. found that the Martian water in the locations they studied all had water activity, aw ≤ 0.78 to 0.86—a level fatal to most Terrestrial life.[83] Haloarchaea, however, are able to live in hypersaline solutions, up to the saturation point.[84]
In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies.[85][86] Additional similar images were published in 2006, taken by the Mars Global Surveyor, that suggested that water occasionally flows on the surface of Mars. The images did not actually show flowing water. Rather, they showed changes in steep crater walls and sediment deposits, providing the strongest evidence yet that water coursed through them as recently as several years ago.
There is disagreement in the scientific community as to whether or not the recent gully streaks were formed by liquid water. Some suggest the flows were merely dry sand flows.[87][88][89][90] Others suggest it may be liquid brine near the surface,[91][92][93] but the exact source of the water and the mechanism behind its motion are not understood.[94]
