Ganymede is the largest moon in the solar system and has many craters. We can use the crater counts on solid worlds to estimate the age of the surface. The more craters, the longer the surface was exposed to battering from space & the older it must be. About a quarter of the surface of Ganymede appears to be as old and littered with craters as Callisto’s; the rest has only recently formed, as can be seen from the sparse coverage of the impact craters and the relative fresh-ness of these craters. Judging by the number of craters, this fresherr terrain on Ganymede is a little younger than the Lunar-Maria or martian volcanic plains. maybe 2 to 3 billion years old.
The difference between Ganymede and Callisto is not just superficial. Ganymede is a differentiated world, like the terrestrial planets. Measurements of its gravitational field show us that the rock sank to formed a core the size of our moon, on which a mantle and an ice crust “float” above. In addition, the Galileo spacecraft discovered that Ganymede possessed a magnetic field, the sure signature of a partially melted interior.
Liquid water is most likely trapped in its interior. Hence, Ganymede is not a dead world, but a place of intermittent geological activity powered by an internal heat source. Some surface features could be as young as the surface of Venus (a few hundred million years).
The young terrain formed by tectonic & volcanic forces. In some places, crust appear to have cracks, and many craters are filled with water from interior. The compression of the crust created extensive mountain ranges that formed long ridges with parallel valleys a few kilometers apart. In some areas, older impact craters split and split. There is even evidence of large-scale crustal movements similar to Earth’s plate tectonics.
Why is Ganymede so different from Callisto? Possibly the small size difference and internal warming between the two led to this divergence in their evolution, but it is more likely that Jupiter’s gravity is responsible for Ganymede’s continued geological activity. Ganymede is close enough to Jupiter that the giant planet’s tidal forces could have heated its interior episodically & it caused great convulsions in its crust.
A tidal force is the result of the uneven gravitational pull on two sides of a body. In a complex type of modern dance, big moons of Jupiter get caught in the variable gravity of both giant planet & each other. This causes gravitational bending or kneading at their centers, which can heat them up, an effect known as tidal heating. As we move towards Europe and Io, we will see that the role of jovian tides becomes more and more important for the moons near Jupiter.