Liquid water cannot exist in a stable form on the surface of Mars with its present low atmospheric pressure and low temperature, except at the lowest elevations for a few hours.
[165][197] So, a geological mystery commenced when observations from NASA's
Mars Reconnaissance Orbiter revealed
gullydeposits that were not there ten years ago, possibly caused by flowing salty water (
brine) during the warmest months on Mars.
[198][199][200][201][202][201][203][204][205][206] The images were of two craters called
Terra Sirenum and
Centauri Montes which appear to show the presence of liquid water flows on Mars at some point between 1999 and 2001.
[201][207][208][209]
There is disagreement in the scientific community as to whether or not gullies are formed by liquid water. It is also possible that the flows that carve gullies are dry,
[210] or perhaps lubricated by carbon dioxide.
[211][212] Even if gullies are carved by flowing water at the surface, the exact source of the water and the mechanisms behind its motion are not well understood.
[213]
In August 2011, NASA announced the discovery by Nepalese student Lujendra Ojha
[214] of current seasonal changes on steep slopes below rocky outcrops near crater rims in the Southern hemisphere. Dark streaks were seen to grow downslope during the warmest part of the Martian Summer, then to gradually fade through the rest of the year, recurring cyclically between years.
[10] The researchers suggested these marks were consistent with salty water (brines) flowing downslope and then evaporating, possibly leaving some sort of residue.
[215] Because these flows have been the flows form and fade in sync with heat flux into the surface, many scientists feel these
recurrent slope lineae are probably the best candidates for features formed by flowing water on Mars today.
[199][216][217] The rate of growth of these features has been shown to be consistent with shallow groundwater flow downslope through a sandy substrate.
[218]