Ganymede (Credit:NASA)

Ganymede is the largest of Jupiter's moons and the largest in the entire Solar System.

It is even larger than the planet Mercury, but only about half the mass. This means it must be much less dense. That said, it is still twice the mass of Earth's Moon.

The surface appears complicated with moutains, valleys, flat plains and craters. It also has many light and dark regions. The darker regions have more craters and so are probably older. The very light spots are likely to be recent craters.

Ganymede's surface is made up of a mixture of silicate rock and water ice. The recent Galileo spacecraft found evidence for a saltwater ocean nearly 200 km below its surface, sandwiched between layers of ice. There is a small chance that simple forms of life may exist in that ocean.