Interior of Ganymede. Computer artwork of the interior of Ganymede, a moon of the planet Jupiter. This moon, the largest in the Solar System, has a small metal core (silver) surrounded by a layer of rock (beige). Around this is a layer of water ice (blue). The surface, also water ice, is heavily marked by fault lines. There is also evidence of volcanic activity, as brighter areas on the surface appear to be new, clean ice covering the darker, older layers. Ganymede also has a relatively strong magnetic field, caused by convection currents in the molten parts of its metal core.