On the comparative evolution of Ganymede and Callisto

Differences in the apparent ages of the surfaces of Ganymede and Callisto, as revealed by Voyager images, could be due to the persistence of tectonic activity on Ganymede beyond the time of early, heavy bombardment. The slightly greater radioactive content expected in Ganymede could prolong such activity by as much as 0.5 billion years beyond the cessation of endogenic surface activity on Callisto. Tidal dissipation could not have been important for Ganymede for more than 10 8 years, and it was never important for Callisto.