Topographic changes on airless bodies: Theoretical considerations

A theoretical model of the evolution of topographic features on airless bodies is based on the erosion caused by the impact of small meteorites and the deposition of material ejected from nearby impacts. Three differing conditions are postulated: (i) the rate of erosion equals the rate of sedimentation, as may be expected on large bodies like the Earth's moon and Mercury, (ii) the rate of erosion exceeds the rate of sedimentation, as may be expected in smaller bodies like the asteroids, where portions of the ejecta may reach escape velocity, and (iii) the rate of sedimentation exceeds the rate of erosion, as may be expected on bodies during planetary growth.Although quantitative conclusions cannot yet be reached, qualitative relationships appear to exist, capable of elucidating the mass balance of airless planetary bodies. Under conditions of erosion equalling sedimentation, a crater tends to evolve into a rim-less bowl, the center of which is below the level of the surrounding plain. Under conditions of high erosion, the tendency for a crater is to evolve into a circular low hill around a bowl, the center of which is at the same level as the surrounding plain.