Europa is the smallest of Jupiter’s four planet-sized moons, yet it is only slightly smaller than Earth’s Moon. Its bright surface (roughly five times as reflective as the Moon), infrared water ice absorption bands, and the near absence of impact craters (only about five have been identified to date) indicate that the surface is ice rich and very young, perhaps only 30 million years old. Europa is covered by a water-ice shell no more than 150 kilometers thick. Calculations suggest that there could be liquid water at the base of this icy layer, leading to speculation that a primitive life form could have evolved in this dark, watery world. The thickness of the surface ice and the possible presence of liquid water have intrigued planetary scientists since the late 1970s. Europa was the most poorly observed of the Galilean satellites when Voyager passed through the Jupiter system in 1979, and scientists eagerly anticipate Galileo’s first images of this fractured icy world.

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