Callisto is almost the same size as Mercury and is the third largest moon in the Solar System. It has a diameter of over 4,800km and takes about 16.7 Earth days to orbit Jupiter (which is also the same amount of time it takes to spin on its axis).

It is believed that under the icy crust is an ocean of salt water. They think this is because of the behaviour of Jupiter's magnetic field around the planet. They also think that there must be an antifreeze such as ammonia mixed in for the water to remain liquid.

Callisto has a very thin atmosphere of Carbon Dioxide. This is possibly being replenished by the sublimation of Carbon Dioxide ice that is on the surface.

Its surface is made up of nearly equal amounts of rock and ice. It is littered with craters. It is believed that the surface is now fully saturated in craters i.e. any new ones would erase older craters. The largest crater is Valhalla crater that has multiple rings that stretch to over 3800km in diameter!