Jupiter's moon Io has active volcanoes. The Voyager spacecraft caught several erupting when they passed the energetic moon in 1979. In the above picture, several of Io's volcanoes are visible and one is seen actually erupting. Debris from this explosive event can be seen on the upper left of the photo, just beyond Io's edge. Io's volcanism is thought to be caused by the large tidal distortions raised by Jupiter, Europa, and Ganymede. These tides stretch Io, cause internal friction, and thus heat the interior. The hot interior then expands and forces its way out through volcanoes. Currently, the spacecraft Galileo is orbiting Jupiter and photographing Jupiter's Galilean moons.