At one-third the size of earth, Io is the most volcanically active body in the solar system. The nearest to Jupiter of the four large moons, Io's surface is riddled with volcanoes which erupt frequently, spewing lava, dust and gases onto its landscape.

Named after the river nymph who's beauty attracted Zeus, Io is strongly influenced by Jupiter's incredible gravitational grip, which causes Io to expand and contract due to the force of tides created by Io's orbit.

Io is believed to have a metallic core. Its surface is mostly flat plains, but mountains as high as 9 km have been observed by scientists.