Io is a moon of Jupiter. It is a little bigger than our Moon, but it has hundreds of active volcanoes. Where does its heat come from?

Jupiter's strong gravity pulls on Io and squeezes it again and again. This makes Io hot inside. In 1979, scientists said Io might have volcanoes. Just days later, the Voyager 1 spacecraft took pictures of a volcano erupting there!

Io, one of Jupiter's moons, is the most volcanically active place in the solar system, with hundreds of active volcanoes. Small worlds usually cool down quickly, so why is Io so hot? The answer is tidal heating. Io orbits close to giant Jupiter, and its orbit is kept slightly oval by the neighboring moons Europa and Ganymede.

As Io moves closer to Jupiter and then farther away, its whole body is stretched and squeezed, and this creates heat. In 1979, three scientists published a paper predicting volcanoes on Io. Only days later, the Voyager 1 spacecraft passed Io, and an engineer spotted a volcanic plume in its images. It was the first active volcano found beyond Earth.

Jupiter's moon Io, only slightly larger than our own Moon, is the most volcanically active body in the solar system. Its energy source is not sunlight but tidal heating. Io is locked in an orbital resonance with Europa and Ganymede, completing four orbits for every two of Europa and one of Ganymede, which keeps its path slightly elliptical.

As its distance from Jupiter changes, Io is repeatedly flexed, and internal friction generates heat. This mechanism produced a famous scientific success. On March 2, 1979, three researchers predicted in Science that tidal heating could melt Io's interior and drive volcanism; three days later Voyager 1 made its closest approach, and a navigation engineer soon spotted an eruption plume in its images.

Recent studies have added detail. In 2024, measurements of sulfur isotopes in Io's atmosphere indicated that it has lost most of its original sulfur, implying volcanic activity for most or all of its history. The same year, gravity data from Juno's close flybys suggested that Io lacks a shallow global magma ocean, although both the isotope and gravity results depend on model assumptions.