(See the Europa Gallery)
Io's surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter. They had expected to see impact craters like those on the other terrestrial bodies and Instead of craters, Voyager 1 found hundreds of volcanic calderas.
Europa and Io are somewhat similar in bulk composition to the terrestrial planets: primarily composed of silicate rock. Unlike Io, however, Europa has a thin outer layer of ice. Recent data from Galileo indicate that Europa has a layered internal structure perhaps with a small metallic core.
But Europa's surface is not at all like anything in the inner solar system. It is exceedingly smooth. Only a few features are more than a few hundred meters high. The prominent markings seem to be only albedo features with very low relief.
The images of Europa's surface strongly resemble images of sea ice on Earth. It is possible that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat. If so, it would be the only place in the solar system besides Earth where liquid water exists in significant quantities.
Europa's most striking aspect is a series of dark streaks crisscrossing the entire globe. The larger ones are roughly 20 km across with diffuse outer edges and a central band of lighter material. The latest theory of their origin is that they are produced by a series of volcanic eruptions or geysers.
Europa has a weak magnetic field, about 1/4 of the strength of Ganymede's. And most interestingly, it varies periodically as it passes thru Jupiter's massive magnetic field. This is very strong evidence that there is a conducting material beneath Europa's surface, most likely a salty ocean.