Credit: NASA

Callisto is the second largest moon of the planet Jupiter, and was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the whole Solar System. Just like Earth's own Moon it is tidally locked with Jupiter - meaning that it always shows the same side to the planet.

Although it's about the same diameter as the planet Mercury, Callisto is only about a third of its mass. This means it only has a density of around twice that of water.

The surface of Callisto is heavily cratered and thought to be the oldest in the Solar System, having never shown any signs of volcanoes or plate tectonics. It's surface is heavily cratered, caused by many asteroid impacts.

Callisto is composed of rock and ice but it is thought to have an ocean under the surface. It is surrounded by an extremely thin atmosphere composed of carbon dioxide and possibly, small traces of oxygen.