Surface of Io, animation. Io is the closest major moon of Jupiter to the planet, orbiting just 350, 000 kilometres from its cloud tops. The enormous gravitational field of nearby gas giant planet stretches and squeezes the moon as it orbits, heating it so much that it melts some of its internal rock. This has made Io the most geologically active body in the Solar System, with hundreds of active volcanoes known. These constantly spew lava onto the surface, and eject material hundreds of kilometres above the surface in tall jets.