The innermost of the four Galilean Moons, orbitingJupiter at a distance of some 422,000 kilometres. Io is only slightly larger than Earth'sMoon, with a sulphurous surface mottled with yellows and oranges. The gravitational forces of the massive planetJupiter, less than half a million kilometres away, exert immense tidal stresses on the moon, which help to account for the dozens of active volcanoes on Io's surface.
