Jupiter's moon Io is the most volcanically active world in our solar system, with over 400 active volcanoes.

Io's extreme volcanism was first predicted by Peale, Cassen, and Reynolds in 1979, just weeks before NASA's Voyager 1 spacecraft captured the first images of active eruptions. This geological activity is driven by a process called tidal heating. Io is locked in an orbital resonance with the moons Europa and Ganymede, meaning for every one orbit Ganymede completes, Europa completes two, and Io completes four.This resonance keeps Io's orbit slightly elliptical rather than perfectly circular. As Io moves closer to and farther from Jupiter, the planet's massive gravity causes the moon's surface to bulge up and down by as much as 100 meters. This constant physical deformation creates immense internal friction and heat, which melts the silicate crust into a subsurface magma ocean.Data from the Galileo spacecraft and the more recent Juno mission confirm that Io's volcanic output is roughly 100 times greater than that of all Earth's volcanoes combined. Some eruptions, like those at Tvashtar Paterae, blast sulfur dioxide gas and dust into space at speeds of 3,540 kilometers per hour. This material eventually escapes Io's gravity and forms a giant plasma torus around Jupiter, influencing the planet's powerful magnetosphere and aurorae.