Orbiting Jupiter is the biggest satellite in the Solar System, Ganymede. With a diameter of 5,262 kilometers, it is about 8% bigger than the planet Mercury, and more than twice as big as little Pluto. A view from the desolate, icy surface of Ganymede would be impressive, with Jupiter and the other satellites resplendent in the sky. It was discovered by Galileo and the name comes from mythology; Ganymede was the cupbearer to the Greek gods and Zeus’ lover.
Jupiter's moon Ganymede. Click here for original source URL
Ganymede's low density indicates that it's made mostly of silicate material mixed with ice. The Galileo spacecraft detected a magnetic field, making Ganymede the only known satellite to have its own magnetic field. This means it also has a small metallic core, one that is perhaps still molten. The icy surface of the satellite has old regions that are darker and heavily cratered, and younger, brighter regions with many grooves and fissures. Watery material has apparently erupted to form fresh, brighter icy deposits in swaths across the planet. Many impacting bodies have blasted away the gray-brown surface soils to reveal brighter fresher, whiter ice below. The poles are also covered with ice, in the form of frost. Its internal ocean may have more water than the sum of all the Earth's oceans.
What can explain the bizarre features of Ganymede? Its orbit today is almost perfectly circular, so it doesn't change its distance from Jupiter by a large enough amount to cause tidal heating. But researchers believe it may have had minor orbit variations in the past that could have produced tidal heating. This may have caused subsurface melting and shifting of the ice "plates" on the surface, creating fractures and causing water to well up, and then freeze into fresh icy swaths. Ganymded even has a thin atmosphere of oxygen and ozone. The European space Agency is planning a mission to the Jovian system to launch in 2022, with a plan to go into orbit around Ganymede.