Dry avalanches at seasonal defrosting as a process for current gullies activity

Recent gullies on Mars, discovered by the MOC/MGS instrument are observed on the wallslopes of the mid latitude regions with a preferential orientations on poleward facing slopes. They might sign the presence of fluid flows, likely involving liquid water, in a recent past, or even currently as shown by the recent gullies activity. Our study is an hirise image that was obtained at Ls=145° and it is located at 38.9°S, 196E. The image shows some recent gullies that erode inside the wallslopes of a fresh impact crater. A close-up on the alcoves is possible thanks to the very good spatial resolution (25 cm/pixel). Frost, visible by the very bright tone, is locally present on the image, especially inside the gullied alcoves and on the steepest part of the wallslopes. Our key observation consists of the presence of new streaks formed within the frost blanket. These streaks are visible from their lower albedo in a strong contrast with the surrounding frost layers. We infer that these streaks form over the frost, or cross this frost. Streaks could be highlighted either because the frost is transparent, or because the frost has been sublimated on the streaks only but this is not consistent with most observations, especially because these streaks are also visible on the part of terrain not covered by frost. This observation of streaks, formed over seasonal frost, signs a current activity of mass wasting inside gullies alcove. The streaks shapes appear as typical of granular flows, different from viscous or liquid flows involving liquid water. Thus, the observed flows are small avalanches that are likely dry. They form currently as a consequence of the defrosting by destabilization of the debris aprons material. We exclude liquid water as being responsible of the current gullies activity due to lack of adequate thermodynamic conditions shortly after defrosting and lack of geomorphic landforms typical of water-rich material. Nevertheless, liquid water is not excluded for a past activity that created the large channelled flows, different from the small avalanches observed.