Martian Cratering

Abstract

New counts of Martian and lunar craters are used in a new analysis of the Mariner IV photographs. The difference in impact velocities on the two planets, neglected by other authors, is found to introduce an appreciable effect. The age of a Martian surface layer capable of retaining craters larger than 50-km diameter is found to be about 4 × 10 9 years, within a factor of about 2. The Martian craters have the diameter distribution expected from impacts of asteroidal fragments, within errors of measurement. Although the age quoted above apparently refers to the formation of the present surface layer (to a depth on the order of 3 km), the “crater retention ages” at smaller diameters are less, owing to an assumed erosive process. The crater retention time for a structure of about 1-km diameter is estimated to be on the order of 10 8 years, and it appears that an erosive process flattens large-scale relief at a rate of about 10 −4 cm/year. This indicates that fundamental conclusions about Mars' erosion history cannot be drawn from the ages of the large craters alone.