Europa's surface. Artwork of the surface of Europa, smallest of the four main (Galilean) moons of Jupiter (appearing at upper left). Europa's surface is covered in ice which is cracked and piled in a similar way to ice floes on the Earth's polar oceans. These formations have led scientists to suspect that there is, or was, a body of liquid water under the ice cap. This water may harbour life forms. Unlike its moon, Jupiter is mainly made of hydrogen and helium gas. Europa has a diameter of 3,100 kilometres (km) and its mass is two-thirds that of our Moon. It orbits Jupiter every 3 days, 13 hours, 14 minutes at a distance of 671,000 km, about twice the Earth-Moon distance.