Europa is Jupiter's fourth largest satellite and is slightly smaller than the Earth's Moon. Europa is believed to be composed of silicate rocks with a layer of water ice covering the entire surface.

This image is inspired by recent discoveries on Europa of regions that look very much like pack-ice on Earth's polar seas during spring thaws. The perspective is from an altitude of several thousand feet. The large crater in the foreground is about a half-mile in diameter.

It may be that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 30 miles deep. This subsurface "ocean" would be kept liquid by tidally generated heat. If Europa does indeed harbor a liquid water ocean, it would be the only place in the Solar System besides Earth where liquid water exists in significant quantities. It may also be the only place in the Solar System where life has evolved outside the Earth.