DEGRADATION OF SMALL IMPACT CRATERS: EROSION RATES AND MARS CLIMATE

Observations of relatively small impact craters by the Spirit and Opportunity rovers (<200 m diameter) and slightly larger simple craters at the InSight landing site (<600 m diameter) have defined degradational sequences that record the modification and erosional agents at work in the Hesperian and Amazonian environment (and climate) on Mars. Spirit observations show that fresh impact craters were degraded by subsequent impacts and rapidly filled in by eolian processes to become hollows. Opportunity observations show that saltating sand rapidly planed off crater rims of weak, sulfate sandstones and filled in the depressions. At the InSight landing site, rocky ejecta craters were filled in by eolian activity and degraded by mass wasting. At the Spirit and InSight landing sites the sediment that fills the craters is likely locally derived from the ejecta, with sand size particles rapidly saltating into and filling the closest depression. The volume of sediment needed to fill the craters observed by Spirit roughly equals that deflated from the surface. Degradation of the rim and steep interior wall slopes can be modeled by non-linear topographic diffusional processes that slow with time. Model derived erosion rates and erosion rates derived from crater degradation are orders of magnitude slower than the slowest erosion rates on Earth, which are dominated by liquid water. The diffusivity, which characterizes the erodibility of the material and vigor of erosion, determined from diffusion calculations for these craters (~10^-6 m²/yr) are similar to those determined for the Moon and are 2-3 orders of magnitude lower than diffusivities on Earth and thus in agreement with slow, dry erosional processes. In concert with these slow erosion rates and low diffusivities, the only geomorphic processes observed on these landscapes are impact, eolian, and mass wasting.

In contrast to these slow erosion rates, degradation of craters and denudation associated with valley network formation during the Middle to Late Noachian are comparable to slow terrestrial erosion rates calculated over similar timescales (~1 m/Myr). This argues for a wet climate before ~3.9 Ga in which liquid water was the erosional agent, followed by a dry environment dominated by slow eolian erosion and mass wasting since.