Thinkquest Entry 20632

OUR SUN	MERCURY	VENUS	EARTH	MARS
JUPITER	SATURN	URANUS	NEPTUNE	PLUTO

SATURN

Saturn, the solar system's second largest planet, is 9.4 times bigger than the Earth and 95 times as massive. It orbits the Sun once every 29.5 years. Like Jupiter, Saturn is a gas planet but measurements indicate that on Saturn, helium is depleted relative to its abundance on Jupiter. The planet has a rapid rotation of 10 hours 39 minutes and is even more oblate than Jupiter.

Saturn is much blander in appearance than Jupiter. In a telescope it has a distinct orange-tan colour but only the faintest suggestion of belts and zones. A banded cloud structure generally similar to that of Jupiter does exist, but is almost completely veiled by a layer of ammonia haze in Saturn's upper atmosphere. Due to Saturn's greater distance from the Sun, temperatures in the upper atmosphere are some 30 degrees colder than on Jupiter and more of the ammonia in the atmosphere is frozen out.

Saturn's interior structure is generally similar to that of Jupiter. Due to the planet's smaller mass the transition to metallic hydrogen occurs at a relatively deeper level. Like Jupiter, Saturn has an internal source of heat. The internal flux is about twice that received from the Sun. This ratio is larger than in the case of Jupiter mostly because Saturn, being twice as far, receives only one fourth of the solar energy Jupiter does.

Saturn is unique in that it is girdled by a truly spectacular ring system. The system is divided into several components, three of which are readily visible from Earth. The most prominent is the middle ring, ring B, which is separated from the outer ring, ring A, by an apparent 4,000 km gap, called Cassini's division. Another less prominent break can be glimpsed near the outer edge of ring A. Inward of ring B lies the faint ring C, often referred to in the past as the crepe ring.

More than a hundred years ago Maxwell proved that in order to be stable the rings must consist of individual particles each in its own Keplerian orbit about the planet. By the end of the 19th century this conclusion has been verified spectroscopically. During the 1960s spectral measurements showed that the rings consist of dirty water ice, and in the 1970s it was proved from radar echoes that the moonlets are on average the size of snowballs, a few centimetres across.

Since the rings lie in Saturn's equatorial plane and the planet's axis is tilted at 29 degrees to the ecliptic, their aspect from the Earth changes as Saturn orbits the Sun. Twice every 29 years the rings are presented to us edge on, at which time they briefly disappear, proving that they are very thin. Estimates suggest a total thickness of not more than a few kilometres.