Jupiter. Southern hemisphere of Jupiter, imaged by NASA's Cassini spacecraft. Jupiter's moon Io (brown, upper right) and its shadow are also seen. Jupiter is the largest planet in the solar system. Its outer layer is composed of the gases hydrogen and helium, with some water, carbon dioxide, methane, and other simple molecules. High-speed winds in the outer layer are confined to wide bands which encircle the planet: light bands are known as zones, whilst dark bands are known as belts. The Great Red Spot (centre left) is a high- pressure region which has been visible from Earth for over 300 years. This image was taken by Cassini as it passed Jupiter on its way to Saturn.