Europa. Animation comparing the amount of water on Jupiter's moon Europa with that on Earth. The volume of water on Europa is thought to be more than twice the volume of that on Earth. Europa is the smallest of Jupiter's Galilean moons, although it is the sixth-largest moon in the Solar System, and at 3100 kilometres in diameter it is just smaller than our Moon. It was mapped by NASA's Voyager and Galileo spacecraft, revealing a surface of smooth water ice, cross-crossed by darker lines. This is thought to cover an ocean of liquid saltwater up to 100 kilometres deep, kept liquid by tidal heating due to its proximity to the immense Jupiter. It has been hypothesised that this ocean could contain life, and as such it is an attractive candidate for space exploration missions.