Defrosting, dark flow features, and dune activity on Mars: Example in Russell crater

Defrosting processes observed on Mars are among the most unusual features described on high-resolution images. A defrosting sequence has been observed near the crest of the megadune located in the Russell crater on new high-resolution images obtained from the HiRISE instrument and on hyperspectral cubes obtained from CRISM instrument, both on board MRO. Hyperspectral images show that frost overlaps the entire megadune in middle winter. This frost is composed mainly of CO2 and of a small amount of water ice. The deepest ice signatures are mainly located near the crest of the megadune. The defrosting sequence monitored by CRISM reveals spatial heterogeneity and refrosting processes. On the morphological counterpart, the defrosting sequence starts with the development of dark spots, similar to those described in the cryptic regions. After few sols, we observe the emplacement of dark linear flow features that start from the dark spots downward of the main slope of the dune. These linear flow features are 1 to 2 m wide and 50 to 100 m long. They settle on the small rills visible on the frost cover. They are interpreted as avalanches of a mixing of sand, dust, and unstable CO2 gas released under pressure. The avalanches would be triggered by the eruption of the dark spots. The flux of material transported by the flow features has been estimated to 0.25 to 0.5 m(3) by meter width each year on the megadune. This flux is larger than flux transported by wind. These dark flow features are thus very efficient to transport material on slopes located in frosted areas.