Small crater modification on Meridiani Planum and implications for erosion rates and climate change on Mars

A morphometric and morphologic catalog of similar to 100 small craters imaged by the Opportunity rover over the 33.5km traverse between Eagle and Endeavour craters on Meridiani Planum shows craters in six stages of degradation that range from fresh and blocky to eroded and shallow depressions ringed by planed off rim blocks. The age of each morphologic class from < 50-200ka to similar to 20Ma has been determined from the size-frequency distribution 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 of the craters has been determined from crater depth, rim height, and ejecta removal over the class age. These rates show a rapid decrease from 1m/Myr for craters < 1Ma to similar to < 0.1m/Myr for craters 10-20Ma, which can be explained by topographic diffusion with modeled diffusivities of similar to 10(-6)m(2)/yr. In contrast to these relatively fast, short-term erosion rates, previously estimated average erosion rates on Mars over 100 Myr and 3 Gyr timescales from the Amazonian and Hesperian are of order < 0.01m/Myr, which is 3-4 orders of magnitude slower than typical terrestrial rates. Erosion rates during the Middle-Late Noachian averaged over similar to 250 Myr, and similar to 700 Myr intervals are around 1m/Myr, comparable to slow terrestrial erosion rates calculated over similar timescales. This argues for a wet climate before similar to 3Ga in which liquid water was the erosional agent, followed by a dry environment dominated by slow eolian erosion.