This image of Io, taken in 1997 by NASA's Galileo spacecraft
in orbit around Jupiter, captures two sulfurous eruptions on the surface of the volcanic moon. At the image top, a bluish plume rises nearly 140 kilometers above the surface of a volcanic caldera known as Pillan Patera. In the image middle, the ring shaped Prometheus plume is seen rising about 75 kilometers above Io. Named for the Greek god who gave mortals fire, the Prometheus plume is visible in every image ever made of the region dating back to the Voyager flybys of 1979 - presenting the possibility that this plume has been continuously active for at least 18 years.