Io, Europa, Ganymede & Callisto: Structures & Vocabulary

The four Galilean moons form a natural experiment in how distance from Jupiter, composition, tidal heating, and orbital history produce very different worlds. Io is intensely volcanic, Europa likely contains a global subsurface ocean, Ganymede is the Solar System's largest moon and has an intrinsic magnetic field, and Callisto preserves an ancient heavily cratered surface.

The Laplace resonance links the orbital periods of Io, Europa, and Ganymede, maintaining small orbital eccentricities that allow Jupiter's gravity to flex their interiors repeatedly. This tidal heating is extreme at Io, weaker but still important at Europa and Ganymede, and far smaller at Callisto.

Voyager, Galileo, Juno, and Earth-based observations reveal active volcanoes on Io, disrupted icy terrain on Europa, Ganymede's magnetic field, and Callisto's cratered surface. Induced magnetic signatures at Europa and Ganymede provide evidence for electrically conducting subsurface oceans.

Io is intensely volcanic; Europa has a young icy shell above a salty ocean; Ganymede is the largest moon and has its own intrinsic magnetic field; Callisto is more heavily cratered and less differentiated. Their increasing distance from Jupiter broadly corresponds to decreasing tidal processing.