Io, Europa, Ganymede & Callisto: Physical Mechanisms

Io orbits closest of the four and is dominated by silicate rock and sulfur-rich volcanic deposits. Europa has an icy shell over a salty ocean and rocky interior. Ganymede has differentiated rock, ice, and metal plus an ocean layer, while Callisto is less fully differentiated and has an old ice-rock 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.

The Laplace resonance links Io, Europa, and Ganymede and maintains small orbital eccentricities that prevent tidal deformation from dying away. Frictional flexing turns orbital energy into heat, extreme at Io and still important for Europa. Callisto, outside the strongest resonance, experiences much less tidal heating.