Jupiter

Io is the most striking of the Jovian satellites and it is the most volcanically active body in the solar system. The Voyager spacecraft discovered active volcanoes on the surface, with plumes as high as 186 mi (300 km); eight were recorded by Voyager 1, and six of these were seen to be still active when Voyager 2 arrived several months afterward. Based on the colors of the surface of the moon (yellow, orange, red, and black), scientists believe that the surface consists mostly of sulfur and sulfur compounds.

Most of the surface of the satellite had been transformed by the time Galileo arrived, 17 years later. The color and contours of the surface in the southern hemisphere had changed significantly, giving evidence of nearly continuous volcanic activity. Io has the most rapid planetary resurfacing process in the solar system, and hence the youngest overall surface of any known planetary body. Volcanic features abound on Io, including lava flows, ash falls, and volcanic vents and caldera.

Only in a few, isolated spots is there a hint of older (perhaps not volcanic) crust on Io. These spots are called massifs and plateaus, and consist of highly fractured rocky crust. Impact craters are absent on Io, suggesting—again—that the crust is very young.

Current theories suggest that the volcanism on Io is caused by its proximity to Jupiter, and hence the strong gravitational forces, which continually squeeze and stretch the satellite. Because of this continual reprocessing of the surface, impact craters are not seen as on the other satellites. Io also has an important interaction with the Jovian magnetosphere.

It is hypothesized that Io has an internal structure consisting of a liquid iron-sulfur core, a mantle of silicate rock that may be partially molten, a lithosphere of brittle silicate rock, a "thiosphere" (sulfur layer) of liquid or plastic sulfur, and a crust of solid sulfur and sulfur compounds.