The role of rim slumping in the modification of lunar impact craters

Abstract

Wall failure has significantly altered the structure of virtually all large, fresh‐appearing lunar craters. Terrace blocks exposed upon a crater's interior walls are interpreted to be sections of the transient cavity rim that slumped into the cavity during the terminal stages of crater formation. Impact excavation cavities have been reconstructed by restoring the innermost terrace block exposed within a crater to its inferred original position at the cavity rim and accounting for the volume of material that slumped into the cavity. Critical model assumptions include (1) the radial variation of topography near the initial cavity rim crest, (2) the structure of the failure surface along which terrace blocks slumped into the cavity, and (3) the geometric shape of the initial cavity. This terrace restoration model has been applied to 12 fresh lunar craters with observed rim crest diameters D0 ranging from 19 to 137 km. Up to an initial cavity diameter Di of 30 km, reconstructed cavity depths are comparable to or greater than cavity depths extrapolated from nonterraced craters of less than 15 km in diameter. For a wide range of model parameters the reconstructed depths of impact cavities 15–30 km in diameter are significantly greater than depths predicted by small‐crater morphometry, implying that cavity depths inferred from depth/diameter ratios observed for small craters may substantially underestimate the depth of excavation of impact cratering events in this size class. Reconstructed cavity depths for Di > 70 km, however, are consistently less than cavity depths extrapolated from smaller craters. This indicates that the morphometric transition from small, relatively unmodified, bowl‐shaped craters (D0 < 15 km) to large, terraced, saucer‐shaped craters (D0 > 80–90 km) cannot be solely attributed to rim‐slumping modification. Ejecta fallback and basement rebound also play a role in modifying impact crater cavities; however, the manner in which the volume of fallback ejecta and basement rebound material varies with increasing crater size is unknown. The discrepancy between cavity depths extrapolated from small craters and those obtained from the terrace restoration model suggests that impact excavation cavities become relatively shallower at larger diameters. However, this cannot be conclusively demonstrated until the effects of rebound and ejecta fallback are quantitatively accounted for.