This image of Jupiter's volcanic moon Io was captured by NASA's Galileo spacecraft. In this image, wwo prominent features can be seen. The big red ring is known as Pele, and the dark horseshoe feature is called and Loki. Io's surface is so volcanically active that iit is constantly changing. Features that were seen by the Voyager spacecraft in 1979 were completely gone when Galileo arrived in 1995. The bright colors on Io are caused by sulfur and sulfur compounds.