Thinkquest Entry 20632

OUR SUN	MERCURY	VENUS	EARTH	MARS
JUPITER	SATURN	URANUS	NEPTUNE	PLUTO

JUPITER

Jupiter, the solar system's largest planet, is 11 times bigger than the Earth and 318 times as massive. It orbits the Sun once every 11.9 years. Its low mean density and other information indicates that the planet has a solar-like composition and consists mostly of hydrogen and helium. The planet's rapid rotation of 9 hours 55 minutes leads to a conspicuous polar flattening.

Jupiter is a striking telescopic object with a banded cloud structure of alternating reddish belts and whitish zones. The zones are believed to be regions of updraft where observers see the tops of cirrus-like ammonia clouds. The belts are probably downdrafts, and the reddish clouds may contain ammonium hydrosulphate, although other colouring materials including red phosphorus, sulphur, and various organic polymers have been suggested to explain Jupiter's vivid colours. Temperatures near the tops of the ammonia cirrus are about -145 degrees but increase rapidly inward. It is believed that below the visible clouds there are additional chemically segregated cloud layers, including a layer of water clouds some 60 km below the visible cloud tops.

Closely associated with the vivid colour patterns are remarkable cloud motions. Large differences in cloud velocities are observed between adjacent zones and belts. Such large shears at the boundaries lead to turbulence and to the production of striking cloud patterns. Other characteristic cloud features are the large ovals. The most famous of these, the Great Red Spot first detected by Cassini more than 300 years ago, is bigger than Earth. Superficially, it resembles a large hurricane-like storm system and rotates slowly counterclockwise once every 6 days. The Great Red Spot is unique for its size and red colour. Other smaller ovals exist, but for some unexplained reason, all are white.

Internally, Jupiter is very different from an Earth-like planet. Proceeding inward from cloud tops, the pressure and temperature increase but the material remains fluid. Jupiter does not have a solid surface. The outer 25 percent of the planet is a fluid shell of molecular hydrogen and helium. Near the centre of the planet, it is believed that a fluid core of rock-forming elements exist and the temperature is between 20,000 and 30,000 degrees.

Internal temperatures within Jupiter are, and have always been, too cold to permit nuclear burning, but the planet does have a vast store of internal energy that it is still radiating into space. This heat energy derives from the gravitational energy that was released during the formation of the planet some 4.6 billion years ago.

Even today, Jupiter's internal heat source contributes about the same amount of energy as the planet absorbs from the Sun. This contribution was very much larger in the beginning. In fact it is believed that for the first million years of its history, Jupiter was more than 10,000 times as luminous as it is today and that it produced a strong temperature gradient in its neighbourhood.