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.

Scaling impact melting and crater dimensions: Implications for the lunar cratering record

“…The scale and style of the cavity-modification processes also contribute to the greater visibility of the melt deposits in larger craters. Wall failure in larger craters takes place on a much greater scale; slumping occurs in units that are considerably more coherent and slide into the interior of the crater en mane (e.g., Mackin, 1969;Grieve et al, 1977;Settle and Head, 1979;Melosh, 1989, Chap. 8).…”

Identification of ancient impact structures: Low-angle faults and related geological features of crater basements

“…The inclination of faults at the surface is normally 60 ° . These fault-surfaces may be equivalents to crater inward dipping headscarps which separate terrace-like features as observed in many extraterrestrial complex craters (Melosh 1977;Settle and Head 1979). The fault inclination reveals information about cohesion and frictional properties of these near surface rocks, which are assumed to be more intact than the underlying ones.…”

Lunar and Venusian radar bright rings

“…That is, the inner wall and the raised portion of the outer rim which are rougher on centimeter and meter scales than nearby areas. On the Moon, impact processes result in the net down slope movement of fine, comminuted material and also in the exposure of fresh material on steep slopes; mass wasting may play an important role (Head, 1975;Settle and Head, 1979). We suggest that there is an analogous set of Venusian processes that smooth level terrain while leaving slopes with a rough surface.…”

“…Settle (1980) has proposed that the thick Venusian atmosphere may entrain crater ejecta such that a significant amount of ejecta falls back in the crater cavity. Settle's studies indicate that for craters which sizes greater than 10 km, half of the primary ejecta falls back on the crater.…”

“…Settle's studies indicate that for craters which sizes greater than 10 km, half of the primary ejecta falls back on the crater. This would not produce a radar dark floor since much of this entrained ejecta would be meter-sized blocks (Settle, 1980). The low velocity of this secondary crater would not be sufficient to comminute these large blocks into a fine-grained regolith.…”