Ganymede

Biggest moon in the solar system -- bigger than Mercury.

The Galileo probe detected a magnetic field, suggesting the interior has a small inner metallic core, surrounded by rocky outer core and a massive ice mantle.

Task: Look at the picture and tell me something about the how active Ganymede surface is.

Space probes have found grooved fractures (high-res Galileo image superimposed on earlier Voyager image):

Signs of tectonic-like activity? Plates of ice rather than rock...

Probes also photographed plenty of impact sites. What does this tell us?

Whatever heated Ganymede no longer is doing so. Perhaps early period of heating was triggered by orbital motions of the Galilean moons as they "locked into" their resonant orbits?