GANYMEDE

Ganymede is the largest moon in the Solar System and the only moon known to have its own intrinsic magnetic field. It is even larger than the planet Mercury, although its lower density means that it contains considerably less mass. Ganymede has a differentiated interior with a metallic iron-rich core, a rocky mantle and layers of water ice. Scientists have strong evidence that a large salty ocean exists deep beneath its icy surface. The moon's surface contains two very different types of terrain: ancient, dark regions covered with impact craters and younger, brighter regions crossed by enormous grooves.