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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,[4] there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface,[5][6] including large oceans.[7][8][9][10][11] It has been estimated that the primordial oceans on Mars would have covered between 36% [12][13] and 75% of the planet.[14]
There are a number of direct and indirect proofs of water presence either on or under the surface, e.g. dry stream beds,[15][16][17][18] polar caps, glaciers,[19][20][21][22][23] radar and spectroscopic measurements,[24] eroded craters and weathered minerals directly connected to the past existence of liquid water.[25][26][27][28][29] Several Mars orbiters have detected the basins of ancient lakes,[30][31][32][33][34][35][36] ancient river valleys,[16][37] and evidence of widespread glaciations,[20][38][39][40][41] while several landers and rovers directly analyzed soil and water ice from the shallow sub-surface.
Although the surface of Mars was wet and could have been hospitable to microbial life billions of years ago,[42] the present damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life on the surface.[43][44] Therefore, the best potential locations for discovering life on Mars may be at subsurface environments.[45][46][47]
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