Io

Io is the innermost of Jupiter's Galilean moons, with a mean orbital radius of 262,000 miles (422,000 km) and an orbital period of 1.77 days. It is slightly larger than the Earth's Moon, with a diameter of 2270 miles (3660 km). All four Galilean moons are rotationally locked with Jupiter, forever showing the same face toward the giant planet.

Voyager 1 discovered that Io is the most volcanically active body in the solar system. At least one volcano is erupting somewhere on Io's surface at all times. Due to Io's low surface gravity and its lack of any substantial atmosphere, Io's volcanic plumes can rise hundreds of kilometers above the surface.

Not surprisingly, Io's surface shows no impact craters; instead, it is dominated by volcanic mountains, lava flows, and pools of liquid sulfur. Io is slowly "turning inside out", and vulcanism resurfaces the entire moon every few years.

Although Io's volcanoes were not actually observed until the Voyager 1 flyby, they were predicted by theorists several months before. Jupiter's inner three Galilean satellites (Io, Europa, and Ganymede) orbit Jupiter in a three-way, 1:2:4 orbital resonance. In other words, for each orbit of Ganymede, Europa orbits Jupiter twice, and Io orbits Jupiter four times. This resonance forces Io's orbit to be slightly non-circular, and as a result its distance from Jupiter varies slightly over the course of each orbit.

Because of this, the tidal forces exerted by Jupiter's gravity also vary - but because Jupiter is so much larger than the Earth's moon, the varying tides raised by Jupiter are hundreds of times greater than those raised by the Moon's gravity on the Earth's oceans. The solid surface of Io rises and falls by as much as 100 meters (300 ft) due to these enormous tidal forces! They are the engine that powers Io's tremendous vulcanism.