Jupiter

Jupiter is largely composed of hydrogen and helium, liquefied by pressure in its interior, and probably with a rocky core larger than the Earth.

Its main feature is the Great Red Spot, a cloud of rising gases, revolving anticlockwise, 14,000 km wide and some 30,000 km long.

Its visible surface consists of clouds of white ammonia crystals, drawn out into belts by the planet's high speed of rotation. Darker orange and brown clouds at lower levels may contain sulphur, as well as simple organic compounds.
Further down still, temperatures are warm, a result of heat left over from Jupiter's formation, and it is this heat that drives the turbulent weather patterns of the planet. The Great Red Spot was first observed 1664. Its top is higher than the surrounding clouds; its colour is thought to be due to red phosphorus. Its strong magnetic field gives rise to a large surrounding magnetic `shell', or magnetosphere, from which bursts of radio waves are detected.

The Southern Equatorial Belt in which the Great Red Spot occurs is subject to unexplained fluctuation. In 1989 it sustained a dramatic and sudden fading. The four largest moons, Io, Europa, Ganymede, and Callisto, are the Galilean satellites, discovered in 1610 by Galileo (Ganymede is the largest moon in the solar system). Three small moons were discovered in 1979 by the Voyager space probes, as was a faint ring of dust around Jupiter's equator, 55,000 km above the cloud tops.