Io is a moon of Jupiter, part of the Solar System in the universe. Jupiter is a planet associated with the Sun, our star, and this space encyclopedia describes the moon’s identity and place in its planetary system.

Io is the most volcanically active world known. Tidal heating driven by Jupiter and its neighbouring moons powers enormous volcanic eruptions and reshapes the surface.

Rocky volcanic body; silicate lava is observed at the surface. A complete elemental inventory is not supplied in this summary.

Discovery and designation
Io was discovered in 1610. JPL credits Galileo. The name and discovery entry identify this particular satellite of Jupiter; similarly named asteroids are separate objects.

Volcanic activity on this Jovian moon is so abundant, it resurfaces the face of Io with new deposits faster than comets and asteroids can mark it with large craters.

Jupiter's moon Io is the most volcanically active world in the solar system, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high.

Io is caught in a tug-of-war between Jupiter's massive gravity and the smaller but precisely timed pulls from two neighboring moons that orbit farther from Jupiter—Europa and Ganymede. Io’s volcanoes are at times so powerful that they are seen with large telescopes on Earth, and Io even has lakes of molten silicate lava on its surface. Only slightly larger than Earth’s Moon, Io is about one-quarter the diameter of Earth itself.

Most of what we know about Io comes from the Galileo spacecraft, which made a detailed study of the Jupiter system.

It was Voyager 1 that spotted the first signs of the little moon’s powerful volcanism in 1979. Since 2003, three spacecraft – Ulysses, Cassini, New Horizons, and Juno – have made additional close observations of Io. Juno is currently orbiting Jupiter, but its prime mission is focused on the giant planet, not the moons.

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Images of Io gathered by NASA spacecraft.