Timescales of alluvial fan development by precipitation on Mars

Abstract

Dozens of large, low-gradient alluvial fans are present within impact crater basins on the cratered highlands of Mars. The timescales and climate conditions that were required to generate such fans are unknown, but testable through our understanding of terrestrial hill slope erosion in the presence of precipitation. Previous estimates of fan formation time vary from years to millions of years. Here, we use an idealised physical model of 2-D catchment-fan evolution to present a framework within which the development of Martian alluvial fans should be considered. We simplify the erosional and depositional system so that there are only three variables: erodibility due to gravity, amount of water runoff due to precipitation, and catchment-fan boundary elevation. Within this framework, to generate large, low-gradient (<6°) alluvial fans on Mars requires significant periods of erosion due to runoff. We suggest two climate scenarios, either: (1) rates of precipitation that are similar to arid terrestrial climates over timescales of 10 7 to 10 8 yr or (2) a shorter duration of semiarid to temperate climate conditions over a period on the order of 10 6 yr. Hyper-arid conditions generate low-gradient alluvial fans under conditions of a topographically lowered fan-catchment boundary and only over timescales >10 8 yr if the substrate is extremely erodible relative to terrestrial examples. Copyright 2011 by the American Geophysical Union.