IMPLICATIONS OF EROSION RATES FOR MARS CLIMATE

A morphometric and morphologic catalog of ~100 small craters imaged by the Opportunity rover over her 33.5 km traverse between Eagle and Endeavour craters on Meridiani Planum has been compiled. Craters in six stages of degradation that range from fresh blocky craters to eroded and shallow depressions ringed by planed off rim blocks have been defined. The age of each morphologic class from <50-200 ka to ~20 Ma has been determined from size-frequency distributions of craters in the catalog, the retention age of small craters on Meridiani Planum, and the age of the latest phase of ripple migration. The rate of degradation and erosion of small craters has been determined from crater depth, rim height and ejecta removal over the age of each morphologic class. These rates show a rapid decrease from ~1 m/Myr for craters <1 Ma to ~0.1 m/Myr for craters 7-20 Ma. This decrease can be explained by a reduction in slope, that has been modeled by non-linear radial diffusion equations that match observed crater slope reduction with time for diffusivities of 10^-6 m²/yr, which are 2-4 orders of magnitude lower than those determined on Earth. Erosion rates of ~1 m/Myr for Meridiani and required for crater free layered deposits on Mars are likely peak short-term eolian erosion rates in the modern martian environment.

In contrast to these relatively fast, short-term erosion rates, average erosion rates on Mars determined mostly from deflation of landing sites over ~100 Myr and 3 Gyr timescales from the literature are of order 0.01 m/Myr or less, which is 3-4 orders of magnitude slower than the slowest terrestrial rates determined over similar timescales. The similarity of these average erosion rates together with the long spatiotemporal scales of averaging involved in their estimation, argues they are representative of the true long-term process rate. Erosion rates during the Middle to Late Noachian based on degradation of craters and denudation associated with valley network formation calculated over ~250 Myr and ~700 Myr average around 1 m/Myr and are comparable to slow terrestrial erosion rates calculated over similar timescales. This argues for a wet climate before ~3 Ga in which liquid water was the erosional agent, followed by a dry environment dominated by slow eolian erosion since.