The sixth major planet of the solar system in order from the Sun. Saturn is one of the four ╘gas giants', second in size only to Jupiter. Its equatorial diameter is 9.4 times the Earth's and its mass 95 times greater. However, its average density is only 0.7 times that of water. Hydrogen and helium make up the bulk of the mass. There is a central core, ten or fifteen times the mass of the Earth, made of rock or a mixture of rock and ice. In the high-pressure region surrounding the core, the hydrogen takes on the form of a metal. The outer half of the planet consists of a deep atmosphere; the visible features of the planet are cloud bands at the top of this atmosphere.
The cloud patterns on Saturn do not normally show much colour contrast. However, in late September 1990, a large white spot developed, expanding over a period of weeks to encircle much of the planet's equatorial region. This apparent eruption of material from the lower atmosphere followed a pattern of such occurrences over a 30-year cycle, corresponding to the orbital period. Similar spots were seen in 1876, 1903, 1933 and 1960, close to Saturnian mid-summer in the northern hemisphere.
Computer processing of images obtained by Voyager 1 and Voyager 2 during their encounters in 1980 and 1981 reveal complex circulation currents, similar to those observed on Jupiter. Saturn rotates rapidly, spinning once every 10 hours 32 minutes on average, though the rate varies with latitude. The resulting flattening at the poles is significant; the polar and equatorial diameters differ by 11 per cent.
The most striking feature is the spectacular ring system. The rings lie in the planet's equatorial plane, which is tilted at an angle of 27í to its orbit round the Sun. They are easily visible in a small telescope. As the relative positions of the Earth and Saturn change, the rings are presented at differing angles, sometimes appearing open, at other times edge-on so that they disappear from view. The rings have the appearance of a series of zones of differing brightness, separated by dark divisions. The most marked divisions are Cassini's and Encke's. The Voyager images of the rings showed that they consist of many thousands of narrow concentric ringlets, resulting in a grooved appearance. They are only one kilometre thick and are made up of a huge number of separate rocks and particles, perhaps ranging in size from a hundred metres down to a micrometre.
Before 1980, ten satellites of Saturn were known. More have been discovered since, some telescopically in 1980 when the ring system was edge-on (thus removing the glare) and some by the Voyager 1 and 2 spacecraft in 1980 and 1981. Eighteen are now known for certain, and there are probably three others and possibly more, subject to confirmatory observations.
