THE GLOBAL DISTRIBUTION OF CRATERS WITH ALLUVIAL FANS AND DELTAS ON MARS

Some impact craters on Mars served as local depositional basins that trapped fluvially transported sediment, accumulating deposits such as alluvial fans and deltas. Alluvial fans form when an upland channel debouches onto adjacent, lower-lying terrain, often during ephemeral, high-magnitude flow events. As flow through the channel loses competence and capacity, due to a combination of a reduction in slope and lateral expansion of flow, sediment builds up over time into semi-conical deposits. Fans with arcuate scarp fronts are indicative of subaqueous deposition, implying the presence of a paleolake and a climate that was quite different than modern Mars.

We present the results of a systematic global survey of alluvial fans and deltas using images from the Context Camera (CTX). There are 208 craters that host nearly 900 alluvial fans, 98 craters with 114 scarp-fronted fans (possible deltas), and 38 craters with 45 equivocal fans. Craters with alluvial and scarp-fronted fans are more widespread than previously reported, but are neither randomly nor uniformly distributed. The majority of craters with alluvial fans occur between 10°S and 30°S, with the highest concentrations in Margaritifer Terra, southern Tyrrhena Terra and northern Noachis Terra/southwestern Terra Sabaea. In the northern hemisphere, craters with alluvial fans cluster in Xanthe Terra and northern Terra Cimmeria along the dichotomy boundary. Alluvial fans are preferentially originated along the northern and southern interiors of their host craters, which is indicative of climatic control through solar insolation. Craters with scarp-fronted fans are predominantly located in Xanthe Terra, Arabia Terra, and northern Terra Cimmeria along the dichotomy boundary, and to a lesser extent in the southern highlands between 10°S and 30°S. The timing, source, and duration of aqueous activity that formed alluvial fans and scarp-fronted fans provides geologic constraints on the evolution of martian climate and hydrologic cycle, and insight into the habitability of the planet. The majority of craters hosting alluvial fans and some of the craters with scarp-fronted fans superpose Hesperian or Amazonian terrains, indicative of a significant role for liquid water on post-Noachian Mars.