Jupiter and Saturn have much larger diameters and are much more massive than the other planets in the solar system. Their densities, however, are lower than the terrestrial planets. This implies that Jupiter and Saturn are made up primarily of the light elements, hydrogen and helium.
The parts of Jupiter and Saturn that we can see are atmospheric clouds. For both planets, rotation period varies with latitude, producing a system of zonal winds similar to the Earth's jet stream.
Jupiter and Saturn have tropospheres in which pressure and temperature decrease steadily with altitude. The atmospheres of Jupiter and Saturn are made primarily of hydrogen and helium.
Jupiter shows many cloud features. One of these, the Great Red Spot, is an atmospheric storm that has persisted for centuries. Other features, such as white ovals, last for decades. Saturn also shows many cloud features, but not as many as Jupiter.
The clouds of Jupiter and Saturn are separated into several decks with clear regions in between. They consist primarily of ammonia, ammonium hydrosulfide, and water, all of which are colorless. The compounds that color the clouds brown, blue, red, and orange have not yet been identified.
The dark cloud features of Jupiter and Saturn are warmer and lie deeper in the atmosphere than the light cloud features. The Great Red Spot is an exception, however. It is one of the coldest, highest atmospheric features on the planet.
In the atmospheres of Jupiter and Saturn, hydrogen exists as a molecular gas. Beneath the atmospheres, however, as the pressure increases, hydrogen gradually turns into a liquid. Even deeper within each planet, hydrogen changes abruptly to a metallic liquid.
Both Jupiter and Saturn are too dense to be made entirely of hydrogen and helium. They must contain other, heavier materials, such as rock, ice, and metal. It is likely that the heavy materials form the cores of Jupiter and Saturn.
Both Jupiter and Saturn are self-luminous. For each planet, one-half or more of the energy emitted is derived from internal sources.
Just after it formed Jupiter was much larger than it is now. While it contracted to its present size, Jupiter converted large amounts of gravitational energy into heat. For a brief time during its early contraction, it was almost 1% as luminous as the Sun. Today Jupiter's luminosity is derived from energy stored from the time when it was contracting rapidly. Although Saturn also converted gravitational energy into heat as it formed, most of the planet's present internal energy has a different origin. Possibly it is due to the separation of hydrogen and helium within Saturn.
Jupiter and Saturn have large magnetic fields and enormous magnetospheres. In fact, Jupiter's magnetotail extends beyond the orbit of Saturn.
The rings of Jupiter and Saturn consist of many individually orbiting particles. Collisions among the particles have gradually brought all of the particle orbits into the same plane. Thus the rings are extremely thin compared with their diameters.
The rings of Saturn consist of many individual ringlets separated by gaps. Some of the gaps are caused by the gravitational influences of Saturn's satellites.
Spokes, dark radial features in Saturn's rings, may be the shadows of minute particles that have been lifted above the rings by electromagnetic forces.
Most of the material in planetary rings lies inside the Roche distance, the distance from the planet within which bodies are destroyed by tidal forces. Planetary rings were probably formed relatively recently by the destruction of a body, perhaps a satellite, which ventured inside the planet's Roche distance.
