An ocean in Callisto?

When the Voyager spacecraft arrived at the Jupiter system in 1979, they revealed the remarkable richness of the Galilean satellites: the volcanism on Io; the young, fractured, icy surface of Europa; and the diverse terrains of Ganymede. Callisto was the one Galilean satellite that did the dull, expected thing. It was covered with ancient impact structures and showed no sign of interesting internal processes. Callisto was the "ugly duckling" and received much less attention than its exciting siblings. Since then, results from Galileo have forced us to rethink our ideas of Callisto-surely one of the biggest surprises of this highly successful mission. It seems that Callisto has an internal water ocean, and the moon's structure may be telling us something important about how the Galilean satellites formed.