Surface units on Callisto: a pre-Galileo view

Abstract

The densely cratered surface of Callisto provides a clue to the geological conditions in the early Jovian system. Based on map-projected and contrast enhanced images, photogeologic maps in a scale of 1:10,000,000 from the Valfoedr and Igaluk regions of Callisto have been compild. The latter region will be a primary target of high resolution imaging during the Galileo flybys. At least three units were mapped: heavily cratered material, dark cratered material, and smooth material. Smooth, less densely cratered material indicates locally restricted zones of resurfacing. three types of craters may be distiguished: fresh crater material, degraded crater material and heavily degraded cratered material. Crater size-frequency measurements have been carried out on this units. As craterdensity has not reached a saturation level at least down to a diameter of about 8 km, crater retention ages of all units may be determined. Independence of crater retention ages from the distance to the apex point of orbital motion as well as crater scaling relationships indicate low velocity impacts derived from primarily planetocentric orbits for the major part of the projectiles. Additional analyses of lineament systems show that the Callistoan surface has not undergone any major surface modification since the time of early heavy bombardment.