DEGRADATION OF SELECTED TERRESTRIAL AND MARTIAN IMPACT CRATERS

Degraded craters on Mars record the cumulative effects of the complex interplay between erosion, transport and deposition by competing intermittent processes. Fortunately, impact craters are instantaneous landforms common to both the Earth and Mars that are characterized by similar topography and local lithofacies. Deconvolving the signatures of individual degradation processes around young craters on Earth (e.g., drainage scale and density, changing wall slope, deflation features) allows calibrating degradation signatures around craters on Mars and places first-order limits on controlling processes and perhaps past climates. Such comparisons am valid despite differences in vegetation cover and time scales over which activity occurs on the two planets. Analysis of degradation at Meteor Crater, Lonar Crater, and Talemzane Crater reveals a terrestrial sequence of advancing degradation by fluvial, cohan, and mass wasting processes. At Meteor Crater, fluvial processes currently dominate primary erosion; however, combined deflation from the ejecta and both alluvium and colluvium make eolian processes most important in overall ejecta degradation. Lonar and Talemzane reveal that advancing fluvial degradation is characterized by larger drainages and decreasing wall slopes. At Talemzane, interior drainages breach the rim-crest and pirate headward regions of exterior drainage basins. When comparable degradation signatures resolvable at Viking resolutions are used in selected areas on Mars, important differences from the terrestrial sequence are revealed. All morphologies of craters in southern Ismenius Lacus (SIL) am typically incised by small valleys with low drainage densities, display rare alluvial deposits, possess walls sloped at the angle of repose, and am sometimes partially covered by airfall deposit remnants. Analogy with the terrestrial craters indicates signatures associated with advancing fluvial degradation should be readily detected on Mars. Together with other differences between degraded craters on the two planets, the paucity of fluvial signatures in SIL implies that mass wasting and eolian activity predominated degradation of martian craters preserved since the Hesperian. Crater statistics for SIL indicate most craters were degraded during geologically brief intervals in the Noachian and mid-Hesperian. Therefore, fluvial signatures preserved in SIL imply any runoff since the Hesperian was short lived, locally debouched, and unrelated to rainfall.