Io is the closest of the Gallilean Satellites. In contrast to most of the moons in the outer solar system, Io and Europa may be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. Recent data from Galileo indicates that Io has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km.

Io's surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter. They had expected to see impact craters like those on the other terrestrial bodies and Instead of craters, Voyager 1 found hundreds of volcanic calderas.

Some of the volcanoes are active! Striking photos of actual eruptions with plumes 300 km high were sent back by both Voyagers and by Galileo. This may have been the most important single discovery of the Voyager missions. It was the first real proof that the interiors of other "terrestrial" bodies are actually hot and active. The material erupting from Io's vents appears to be some form of sulfur or sulfur dioxide. The volcanic eruptions change rapidly. In just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly.

Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases. Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out.