The next moon, Europa (diameter of 3138 km; slightly smaller than our Moon), is more than 15 percent water which is probably in the form of a thick outer layer.

Images of Europa show a bright, uniform surface with few craters (geologically young). The surface is scored by numerous cracks which are often brownish in color (due to organic molecules?). The best pictures from the Galileo spacecraft, now exploring Jupiter and its moons, show "ice rafts" and other flows which indicate motion over a warmer, near melting, sub-surface.

The interior of Europa should be heated by the tidal interaction with Io and radioactive decay of core elements. This strongly suggests that an ocean of liquid water exists some 100 m to few km below the icy surface we see.

Life may exist on Europa. Early in the history of the Solar System, Jupiter was much hotter and Europa may have had liquid oceans on its surface. Even today, undersea vulcanism may provide the energy source to fuel life at the bottom of Europa's ocean (like Earth).

Planners are considering future missions to Europa in which a "cryobot" would melt its way through the ice and release a "hydrobot" to explore the ocean. A trial run is planned for Lake Vostok in Antarctica (a freshwater lake buried under 4 km of ice).

[Slides from the Galileo Mission]