Io. Hemisphere view of Io, one of the four Galilean moons of Jupiter. The strong orange colour comes from sulphur compounds which cover the surface. Io is the most active body in the solar system, and is dotted with volcanoes such as Pele (heart-shaped feature right of centre) and Loki (next to black horseshoe-shaped lava lake, upper left). Gravitational tidal forces pull at Io so much that enormous heat is generated in the interior. The heated rock rises through strata containing sulphur and sulphur dioxide, causing them to erupt like a geyser. White patches are probably solid sulphur dioxide frost. The data for this image were gathered by Voyager spacecraft.