Io is the closest of the moons to Jupiter. Io practically skims giant Jupiter's surface at a scant 261.000 miles away. The proximity of the tiny moon to its massive parent planet is bad news for Io. Jupiter's enormous gravity raises huge tides in the solid surface of Io, much like Earth's moon raises tides on Earth's mostly liquid surface. As a result, Io is being torn apart from the inside out.
Tidal forces heat up the planet's interior and melt the core of the long-suffering satellite.
The liquid core of the moon spews forth from giant volcanoes, which completely resurface the planet in a relatively brief ten million years or so. The volcanoes mostly belch forth hot sulfur, which quickly freezes into a moon-wide dusting of sulfur snow. Io's volcanic activity even affects Jupiter's second moon out, mysterious Europa. With a diameter of 1,925 miles, it is slightly smaller than Earth's moon. Europa's icy outer surface is smooth as a billiard ball would be if it were almost 2,000 wide.