Io is melted by the flexing of its orbit, not by the Sun

Jupiter's innermost large moon is the most volcanic body in the solar system. The heat is tidal. The orbit is kept slightly squashed by the other moons, so Io is kneaded, continuously, as it goes around.

In the weeks before Voyager 1 arrived, Stanton Peale and colleagues calculated that the resonance among Io, Europa and Ganymede would hold Io's orbit just eccentric enough for Jupiter's tides to flex the moon, and that the flexing would melt it. Linda Morabito, navigating the spacecraft, then found the plumes in a picture taken for a different purpose: an umbrella of volcanic gas standing off the limb. The prediction and the photograph landed in the same season. Io's surface is resprayed with lava and sulphur so fast that impact craters do not survive. The power running through it is more, per square metre, than the most volcanic regions of Earth.

The Sun at Jupiter is a pale disc. It is not the engine. A moon in a perfect circle would flex once, settle, and cool. Io cannot settle, because Europa and Ganymede tug it back into an ellipse on every pass, and Jupiter does the kneading. The same resonance helps keep Europa's ocean liquid, which is a quieter consequence of the same clockwork. On Io the consequence is not quiet. The moon is being used as a brake, and the brake is incandescent.