Tidal bulges in Io's crust are as high as a 30-story building. As the moon revolves around Jupiter the bulge moves, flexes the crust, and heats Io's interior like a paper clip bent rapidly back and forth. This is the source of energy for volcanoes that spew lava almost constantly. The plumes which rise 300 km into space are so large they can be seen from Earth by the Hubble Space Telescope. As a result, the moon bulges on two sides like an American football.
Scientists believe that they can learn more about volcanism on Earth by studying Io.
To get a closer look, NASA has sent its Galileo probe on a daring series of Io flybys. Close-up pictures and other data that were stored on Galileo's onboard tape recorder during the flyby are being transmitted to Earth now. Next in line is Jupiter's second big moon, Europa. On the surface the two worlds couldn't be more different. Io is peppered with bubbling geysers and streams of steaming lava.