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 heartshaped feature right of centre and Loki next to black horseshoeshaped 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