Older terrain cut by young (lighter) terrain.

THE INTERIOR OF GANYMEDE

The low density (1940 kg m-3) of Ganymede is attributed to a high ratio of water ice to rock. It was once thought that Ganymede would either have no internal layering, or be poorly differentiated, with most of the ice and rock mixed together.

A cross section through Ganymede would reveal below the 300 kilometres ice and rock crust, a mantle of purer ice of 400 to 800 kilometres deep. Below the mantle is a large silicate core approximately 1000 kilometres thick.

Cross-section through Ganymede.

This layered model of Ganymede was confirmed by gravity field measurements made by the Galileo spacecraft. Galileo's gravity data further suggests that within the rock core of Ganymede is a metallic inner core. A metallic inner core implies that Ganymede's history involved a degree of heating which may account for the nature of the moon's surface. A metallic core could also account for the weak magnetic field detected at Ganymede.