Ganymede, 3,275 mi (5,270 km) in diameter, is Jupiter's largest moon, and it is also
the largest satellite in the solar system. Ganymede is about one and one-half times
the size of our Moon. It is heavily cratered and probably has the greatest variety of
geologic process recorded on its surface. Ganymede is half water and half rock,
resulting in a density about two-thirds that of Europa, an ice-coated satellite.
The first close-up photos of Ganymede, taken by the Galileo spacecraft during its
June 1996 flyby, revealed a surface pockmarked with ancient craters and a
landscape wrinkled and torn by the same forces that make mountains and move
continents on Earth. Galileo's findings also indicated that Ganymede is enveloped in
its own magnetic field, possibly created by a molten iron core or even a thin layer of
electricity-conducting salty water underneath its icy crust.
Ganymede is the first known moon with its own magnetosphere.