Io is the innermost of Jupiter’s four Galilean moons and the most volcanically active body in the Solar System. More than 400 active volcanoes erupt lava and sulfur-compound plumes up to 500 km above Io’s surface — so prolifically that Io’s entire surface is resurfaced every ~1 million years, erasing impact craters.

This incredible volcanism is powered by tidal heating. Io is locked in a 4:2:1 Laplace orbital resonance with Europa and Ganymede; its slightly eccentric orbit means Jupiter’s immense gravity flexes Io by up to 100 m in each orbit, dissipating enough heat to keep the interior molten.

Io’s surface is painted in striking yellows, oranges, reds, whites and blacks from sulfur and sulfur-dioxide frosts deposited by volcanic plumes. A thin, patchy SO₂ atmosphere collapses onto the surface as frost on Io’s night side. Io was first imaged up close by Voyager 1 in March 1979 (which discovered its active volcanism), and has since been studied by Galileo, Cassini, New Horizons, and Juno — Juno continues to make close flybys through the mid-2020s.