There are places on Io where molten lava has been observed actively welling up from beneath the surface of this otherwise frozen world. Lavas on Io were originally thought to be molten sulfur, but with caldera temperatures far exceeding 1,000º F--too hot for liquid sulfur--these eruptions are now thought to be primarily molten silicate rocks.

In this image a dormant volcanic caldera re-awakens with a surge of new lava that breaks up and melts a frozen crust. A gibbous Jupiter hangs over the horizon.*

How does a small satellite like Io, so far from the Sun, get so hot? The heat probably comes from tidal (gravitational) interactions between Io, Jupiter, and Io's sister satellites Europa and Ganymede. Like Earth's Moon, Io always keeps the same side facing its host planet. However, the gravitational pulls of Europa and Ganymede orbiting further out from Jupiter conspire to twist Io away from this orientation. This twisting stretches and bends Io by as much as 300 feet, generating an enormous amount of internal heat from friction. This heat is expressed as volcanic upwellings and geysers.

* Note that no such discrete cracking of a lava crust, as illustrated here, has yet been observed on Io's surface and is therefore presented solely as a theoretical possibility by the artist.