Europa consists of silicate rock covered with a shell of water ice. Europa's ice may cover an ocean that has been kept liquid by tidal heating. If so, Europa would join Earth as the only places in the solar system with water oceans. If scientists were to confirm that Europa has a liquid ocean, they would want to study the ocean as a possible past or present home of life. Enceladus, one of Saturn's satellites, has a varied surface.
Small craters cover some regions, and plains with ridges extend over other areas.
The plains formed when material containing water erupted across the satellite's surface and then froze. Because it is small and far from the sun, scientists expected to find that it had cooled and become geologically inactive soon after it formed. Its continued activity may result from tidal heating and the chemistry of its interior. For example, the ice may contain a significant amount of ammonia, which greatly lowers the melting point of water ice. Ordinary water ice on Enceladus's cold surface would be as hard and strong as rock.