Thermokarst landforms and processes in Ares Vallis, Mars

With a length of 1500 km, Ares Vallis is one of the largest martian outflow channels, and is inferred to have been formed by cataclysmic floods of water conveyed from source areas, which are marked by chaotic terrain, to Chryse Planitia. Near its downstream outlet (∼14°N, 28°W), the floor of Ares widens to 100 km from its average 25 km width. This area of widened channel floor is marked by a complex of irregular terraces, elongated depressions, linear ridges, sinuous ridges, and other indicators of highly irregular dissection of a formerly continuous surface. Thermokarst processes, following either glacial or alluvial histories, seem best to explain these relationships. Various indicators of fluctuating discharge for water and sediment, ponding of debris, and prolonged flow suggest the emplacement of ice-rich debris in the anomalous reach of Ares Vallis. Post-flood or post-glacial thawing of the ice-rich sediments would then generate the thermokarst landscape. These processes, which are consistent with other indicators of anomalously warm climatic conditions, imply a profound change from the modern martian environment.