Which moon of Jupiter is the most volcanically active body in the solar system?

Io, which has hundreds of active volcanoes throwing plumes up to 500 kilometres above the surface, and resurfaces itself so fast that it has essentially no impact craters - they are buried faster than they accumulate. It is the most geologically active object anywhere in the Solar System.

The energy comes from being squeezed. Io orbits close to Jupiter and is locked in a resonance with Europa and Ganymede that keeps its orbit slightly elliptical, so Jupiter's enormous gravity flexes it as it moves - the solid surface rises and falls by up to a hundred metres, and the friction of that kneading melts the interior. It is a moon being heated by being bent.

It was predicted before it was seen, which is rare. Three researchers published a paper calculating that tidal heating should make Io volcanically active, and Voyager 1 photographed a plume days later. The surface is coated in sulphur compounds, which is why it looks like a mouldering pizza, and it sheds a tonne of material a second into space, forming a torus of charged particles around Jupiter that feeds the planet's auroras.