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.
We also found that the Martian gullies were strikingly similar to some "debris flows" that two of us observed on the dry and cold Greenland east coast (a terrestrial environment a little "Mars-like"). Field studies showed that these debris flows are not formed by ground water seepage, but that they result from the thawing of the near surface, which becomes impregnated by liquid water when the ground ice and the snow cover melt.
On this basis, it was tempting to assess if such a process involving the melting of the near-surface could have occurred on Mars. Using a state-of-the-art model of the current Martian climate used to analyse current mission observations, we calculated the temperature of the surface and sub-surface on various locations on Mars and for various obliquities. Obliquity is the inclination of the planet rotation axis on its orbit . Mars orbit specialists believe that its obliquity has varied a lot in the past million years, and these variations have strongly affected the climate.
Our calculations revealed that the only places on Mars where the daily mean temperature has been above the melting point of water during the past obliquity cycles are the mid and high latitudes above 30°, especially on poleward-facing slopes. The corresponding thermal wave could have melted the ground ice over several tens of centimeters. The fact that poleward-facing slopes receive more sunlight and get warmer at high obliquity in the summer is due to the pole being tilted toward the sun. This preferential orientation and the latitudinal distribution of the warmest near-surface temperature coincide with the location of the observed Martian gullies, suggesting a link between near-surface warming and debris flows. In fact, to further test this hypothesis, we performed a more detailed statistical analysis of the observed gullies orientations. We found an almost perfect agreement between the variations of the orientation of the gullies with latitude observed on mars and those predicted by the model ! On this basis, and since Mars at high obliquity is thought to have had a water-rich atmosphere thicker than today (so that liquid water could sometime flow on the surface), we believe that the Mars gullies result from the melting of the near surface ground ice at high obliquity.
François Costard*, François Forget+, Nicolas Mangold*, and Jean-Pierre Peulvast*
* CNRS- UMR8616 OrsayTerreE, Université Paris-Sud, Orsay, France
+ Laboratory for Dynamic Meteorology, CNRS, Paris France, email@example.com