Europa. Animation illustrating aspects of Jupiter's moon Europa. Europa's surface is a smooth layer of water ice, criss-crossed by darker lines. This is thought to cover an ocean of liquid water, kept liquid by tidal heating due to its proximity to the immense Jupiter. Europa has a variable magnetic field, which would also be explained by a hidden saltwater ocean. It has been hypothesised that this ocean could contain life, and as such it is an attractive candidate for space exploration missions. 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.