- One of the four satellites of Jupiter discovered by Galileo in 1610. 2640km in diameter, it has a 1.07 million km, 7.2 day orbit. Ganymede is the largest of the Galilean satellites and of low density. Its surface is cratered and crossed by tectonic features like long, deep grooves. The appearance of Ganymede is thought to imply the presence of both ice and rocky material in the satellite’s interior, possibly with layers of dirty ice overlying a rocky core.