Europa

Europa is the smallest of Jupiter's Galilean moons, at 1941 miles (3140 km) in diameter; it is slightly smaller than our own Moon. Europa completes one orbit around Jupiter in 3.55 days, at an average distance of 416,000 miles (670,000 km). Europa participates in a three-way, 1:2:4 orbital resonance with Jupiter's other inner Galilean moons (Io and Ganymede). In other words, for each orbit of Ganymede, Europa orbits Jupiter twice, and Io orbits Jupiter four times. Like the other Galilean moons, Europa's rotation is tidally locked with Jupiter, so Europa always shows the same face toward the giant planet.

Europa has the smoothest surface of any moon in the solar system. It is mostly made of water ice, and displays long streaks and cracks which run for up to several thousand kilometers across its surface. Like Io, Europa displays very few impact craters, so something must be erasing them. The Galileo orbiter photographed evidence that Europa's icy crust is in motion, like the sea ice floating on the Earth's polar oceans. Europa is too dense to be entirely composed of ice, however, and underneath the ice is a thick layer of silicate rock, and (probably) an iron core.

The same tidal forces which power Io's volcanoes also exist inside Europa, although to a lesser degree. These forces heat up Europa's interior enough to melt the ice. Europa's icy crust may be covering an ocean of liquid water up to 100 kilometers deep. Europa is thought to have twice as much liquid water as the Earth! For this reason, there is a possibility that life may exist on Europa, in the darkness under the icy crust, much as life exists in the Earth's undersea volcanic vents. Spacecraft missions to Europa are being planned to investigate this possibility.

The Galileo orbiter was deliberately flown into Jupiter after completing all of its mission objectives in 2003 to avoid any possibility of colliding with Europa and contaminating its surface.