A possible synoptic source of water for alluvial fan formation in southern Margaritifer Terra, Mars

The morphometry and occurrence of crater-bound fans in southern Margaritifer Terra that were active from around the Hesperian–Amazonian boundary into the Early Amazonian is consistent with emplacement related to synoptic precipitation. Precipitation, possibly occurring as snow, may have been locally influenced by topography and (or) orbital variations. It is not known how much of the total sediment inventory in the fans relates to this late activity versus possible earlier events where water may have been available from alternate sources such as impact-related melting of ground ice. Winds may have concentrated late occurring precipitation into existing relief and (or) preexisting alcoves that facilitated physical weathering to produce fine sediments later incorporated into fans. Two of the craters containing fan deposits, Holden and Eberswalde, were finalists for the MSL landing site. Results suggest that exposed and accessible fan sediments at both crater sites may record a late period of colder, drier conditions relative to early Mars that was punctuated by ephemeral water-driven activity.