The cratering record on Mercury and the origin of impacting objects

The cratering records of terrestrial planets are discussed with special consideration given to Mercury. The geologic units on Mercury most relevant to its cratering record are reviewed, and new observations are presented on the issues of equilibrium and saturation. The implications of the Mercurian cratering record for geologic processes are examined. Particular attention is given to the origin of objects responsible for the period of late heavy bombardment on terrestrial planets; it is suggested that the impactors were accretional remnants left over from the formation of the terrestrial planets and confined to the inner solar system. The cratering record in the outer solar system may have been produced largely by objects in planetocentric orbits.