Interpreting the cratering record: Mercury to Ganymede and Callisto

The large-crater populations on the terrestrial planets differ markedly from those on the Galilean satellites. On the densely cratered terrains of Mercury, Mars, and the Moon, the crater populations > 8 km diameter bear fundamental similarities to one another. These similarities encompass both the shape of their crater curves (i.e., their size-frequency distribution functions) and their overall crater densities. We argue that these similarities reflect the attributes of the ancient impacting population and are not, as sometimes hypothesized, the result of crater saturation. The densely cratered terrains of Ganymede and Callisto, and the grooved terrains of Ganymede, have some similarities with each other but bear few common attributes with those on the terrestrial planets. Some of the differences between the crater populations on Ganymede and Callisto apparently result from differences in the physical states of their icy crusts during and soon after the period of late heavy bombardment. The emplacement of the grooved terrain clearly altered the young crater population on Ganymede, and some evidence exists that a similar alteration of the most ancient crater population also occurred. We are unable to reconcile the observed crater populations on Ganymede and Callisto (modified by any of the proposed mechanisms) with an initial population similar to that of the terrestrial plants, suggesting that a different reservoir of impacting bodies cratered surfaces in the inner and outer solar system. Devolving from this interpretation are constraints on the origins of both of the impacting populations and strong cautions about extrapolating time scales derived from the lunar cratering record to the Galilean satellites