Evidence for Late Aqueous Activity in Margaritifer Terra and Gale Crater, Mars.

Abstract:

The morphometry, occurrence, and population of superposing craters on alluvial fans within at least seven craters >50 km-diameter in Margaritifer Terra and Gale crater indicates the fans were active after most regional aqueous activity had ended on Mars. It is not known how much the late activity contributed to total fan sediment inventories, but late deposition on the Peace Vallis fan in Gale appears to be responsible for a ~10-20 m veneer of sediment on the upper fan.

In general, fans in Margaritifer Terra and Gale superpose broader surfaces within their host craters: fans in Gale clearly embay rougher and relatively brighter-toned surfaces and the smoother, upper portion of Peace Vallis transitions downslope to a rougher, relatively brighter, and alluvial deposit. Fans typically preserve relatively small-scale morphology as compared to adjacent surfaces, they often slope only a few degrees towards the bounding crater floor, and preserve distributary channels (up to 100s of m wide and kms long) that currently stand meters in relief as a result of later erosional lowering of adjacent surfaces. Fan margins sometimes show embaying relations with the surrounding terrain, with little evidence of significant erosional modification such as isolated remnants.

The size-frequency of craters preserved on the fans helps to constrain their age, but care must be taken to avoid potential uncertainties associated with statistics drawn from small areas that may miss larger, more rarely occurring craters that accurately reflect the age of a surface. Nevertheless, careful mapping and the combination of small, similar morphological units into larger cumulative areas yields interpreted ages ranging from near the Amazonian-Hesperian boundary to the Early Amazonian and is consistent with their superposition and preserved morphology. The wide distribution of fans that are sourced from crater rim alcoves suggests a synoptic source of precipitation, perhaps accumulating as snowfall that later melted. On some fans, such as Peace Vallis, surface runoff may have been limited to upper sections, with water infiltration occurring downfan. Such a scenario is generally in accordance with late aqueous activity inferred from geochronology data form Gale and points to potential habitable settings that occurred later than previously recognized.