Jupiter, the solar system's largest planet, is seen here next to Io, its closest Galilean moon. On the cloud tops of Jupiter near the left edge of the picture can be seen a dark circular spot which is caused by the shadow of Jupiter's largest moon Ganymede. Jupiter's cloud tops show light bands and dark belts. The clouds are primarily composed of hydrogen and helium, but their intense colors are probably caused by very small amounts of heavier elements such as sulfur or organic (carbon-containing) compounds.