With rocky cores heated by tugs from Jupiter, these moons are thought to have potentially life-supporting seas beneath their icy crusts.

REUTERS/JPL/NASA/File
This 2002 composite shows the edge of Jupiter, with its Great Red Spot, and Jupiter's four largest moons, known as the Galilean satellites. From top to bottom, the moons shown here are Io, Europa, Ganymede, and Callisto.