Europa. Animation illustrating aspects of Jupiter's moon Europa. 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, criss-crossed by darker lines. The lines are cracks in the moon's icy crust which have been filled by material coming up from beneath. These fractures can be more than 3000 kilometres long. The surface ice is thought to cover an ocean of liquid water containing salts, kept liquid by tidal heating due to its proximity to the immense Jupiter.