Jupiter’s moon known for its icy surface and potential subsurface ocean is Europa. Europa is one of the four Galilean moons, discovered by Galileo Galilei in 1610, and is slightly smaller than Earth’s Moon. Its bright surface is made largely of water ice and is crossed by reddish-brown cracks and ridges.
Scientists infer that a salty liquid-water ocean lies beneath Europa’s ice shell. Jupiter’s powerful gravity repeatedly flexes the moon as it orbits, producing tidal heating that can keep water liquid despite Europa’s frigid surface temperatures. Galileo spacecraft measurements also detected an induced magnetic field, consistent with a conductive subsurface ocean.
Europa’s surface has relatively few large impact craters, suggesting that it is geologically young and continually renewed. Its smoothness and ocean make it one of the Solar System’s leading places to investigate possible habitats beyond Earth. However, a potential ocean is not proof of life: scientists still do not know its exact depth, chemistry, or whether materials from the surface reach it easily. Ganymede and Callisto are also icy moons, but Europa is the classic answer to this clue.