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"Animation of Jupiter's moon Europa passing in front of the planet. Europa is the second closest to Jupiter of its Galilean moons, orbiting some 671, 000 kilometres from the centre of the planet. It is the smallest of the 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. Its surface is a smooth layer of water ice, cross-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. It has been hypothesised that this ocean could contain life, and as such it is an attractive candidate for space exploration missions."