This intense activity is ultimately generated by gravitational tugs from Jupiter, with an assist from the nearby moons Europa and Ganymede.
Io completes two orbits for every one that Europa makes, and four for every one of Ganymede's laps. As a result of this regular timing, Europa and Ganymede have pulled the orbit of Io into an oval, with explosive consequences for the 2,260-mile-wide (3,640 km) moon.
As Io moves closer to Jupiter, the planet's powerful gravity pulls hard on the moon, deforming it. This force decreases as Io retreats, and the moon bounces back. This cycle of flexing creates friction in Io's interior, which in turn generates enormous amounts of volcano-driving tidal heat.
Common sense suggests that Io's volcanoes would be located above the spots with the most dramatic internal heating. But Hamilton and his colleagues found that the volcanoes are significantly farther to the east than expected.
They reached this surprising conclusion after studying data gathered by several ground-based telescopes and a number of spacecraft, including NASA's Voyager and Galileo probes, then comparing this information to a detailed geologic map of Io that scientists put together last year.