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]