Formation of recent gullies and debris-flows on Mars by the melting of near-surface ground ice at high obliquity EurekAlert!

The observation of small gullies on Mars was one of the more unexpected discoveries of the Mars Observer Camera (MOC) aboard the Mars Global Surveyor spacecraft. The characteristics of these gullies suggested that they were formed by flowing water and soil and rocks transported by these flows. They appeared to be surprisingly young, as if they had formed in the last few million years or even more recently. This was a major surprise because the presence of liquid water seemed impossible on Mars in such a recent past. In their initial analysis, the MGS Camera investigators Mike Mallin and Ken Edgett proposed a scenario involving ground water seepage from a sub-surface liquid water reservoir located a few hundred meters or less below the surface. The existence of such an aquifer would have had major consequences for the future of Mars exploration and the possibility of life. However, the process capable of maintaining such a shallow aquifer at temperatures above the freezing point of water remained unclear. Analysing the MGS Camera data archive, we were able to find example of gullies originating from the top of isolated peaks and from dune crests. In these cases, the involvement of a subsurface aquifer was unlikely.