I o can be classified as one of the most unusual moons in our solar system. It was discovered by Simon Marious and Galileo Galilei in 1610. Active volcanism on Io was the greatest unexpected discovery at Jupiter. It was the first time active volcanoes had been seen on another body in the solar system. The Voyagers observed the eruption of nine volcanoes on Io altogether. There is also evidence that other eruptions occurred between Voyager encounters. Plumes from the volcanoes extend to more than 300 kilometers (190 miles) above the surface, with material being ejected at speeds up to a kilometer (.6 miles) per second.

I o's volcanoes are apparently due to heating of the satellite by tidal pumping. Io is perturbed in its orbit by Europa and Ganymede, two other large satellites nearby, then pulled back again into its regular orbit by Jupiter. This tug-of-war results in tidal bulging as great as 100 meters (330 feet) on Io's surface.

T he temperature on Io's surface is about -143° C (-230° F); however, a large hot spot associated with a volcanic feature measured about 17° C (60° F). Scientists believe the hot spot may be a lava lake, although the temperature indicates the surface is not molten. This feature is reminiscent of lava lakes on Earth.

I o is composed primarily of rocky material with very little iron. Io is located within an intense radiation belt of electrons and ions trapped in Jupiter's magnetic field. As the magnetosphere rotates with Jupiter, it sweeps past Io and strips away about 1,000 kilograms (1 ton) of material per second. The material forms a torus, a doughnut shaped cloud of ions that glow in the ultraviolet. The torus's heavy ions migrate outward, and their pressure inflates the Jovian magnetosphere to more than twice its expected size. Some of the more energetic sulphur and oxygen ions fall along the magnetic field into the planet's atmosphere, resulting in auroras.