Landform degradation on Mercury, the Moon, and Mars: Evidence from crater depth/diameter relationships

Abstract

Morphologic classification of craters and quantitative measurements of crater depth as a function of diameter are used to investigate the relative degradational histories of Mercury, the moon, and Mars. Martian craters exhibit considerable depth variation and are generally shallower than their lunar or mercurian counterparts. On Mercury and the moon, visually fresh and degraded craters on smooth plains show no significant depth degradation except that attributed to lava flooding or local inundation by ejecta from large impacts. More heavily cratered regions on both planets display a large range of both visual and depth degradation, suggesting that most landform modification occurred before the final phase of formation of the oldest smooth plains on both planets. Depth/diameter data presented here are discussed as they relate to two early history scenarios. One scenario based on cratering and the ballistic transport of material has been suggested for Mercury, the moon, and Mars by several authors. Owing to discrepancies between this ballistic scenario and observations of crater densities and morphologies, we suggest that landforms on all these bodies also record nonballistic degradation associated with the formation of intercrater plains. Whichever scenario is applied, early, intense, bombardment-associated degradation appears to be a common element in the histories of the terrestrial planets.