Saturn is probably the best known, and most beautiful planet in the Solar System. While its possession of a ring system it not unique, it has a set of rings which are far more extensive and more easily seen than any other planet. It is this ring system that makes Saturn so beautiful.
Saturn is the second largest planet in the solar system with a diameter of 120,000 kilometres. It orbits the Sun every 30 years at a distance about ten times that of the Earth. The shape of the planet is a markedly oblate spheroid with a polar diameter some ten per cent smaller than that at the equator. Saturn is the least dense of all the planets its mean density being only 0.7 times that of water.
The visits by the Voyager spacecraft revised almost all that we know about Saturn, its rings and its satellites.
The Planetary Interior
Like Jupiter, Saturn is composed mainly of the light elements hydrogen and helium. At its centre there is believed to be a core of rocky material about the size of the Earth, but more dense. Around this is a metallic hydrogen shell some 30,000 kms deep. Above this is a region composed of liquid hydrogen and helium with a gaseous atmosphere some 1,000 kms deep in which are the cloud structures that look like the surface of the planet.
The Atmosphere
Saturn is composed of about 94 percent hydrogen and 6 percent helium. The clouds are composed of very small amounts of other chemical elements combined with hydrogen to give such compounds as ammonia, methane and phosphine. Because Saturn is colder than Jupiter the more colourful chemicals occur lower in its atmosphere and are not seen; this results in much less dramatic markings but they are similar to those seen on Jupiter, taking the form of bands with some smallish spots.
The Rings
Saturn's rings were first seen by Galileo but were identified as a ring system by Huygens in 1656. For many years Saturn was thought to be unique in having a ring system but we know now that all the major gaseous-planets have ring systems although none is so prominent as that of Saturn.
The rings are divided up into several distinct rings with gaps between them. The largest gap was discovered by Cassini in 1675 but we now know that there is a very complex structure to the ring system.
The rings are composed of many, many small particles up to about 10 metres across. These are thought to have originated in a satellite which collided with a minor planet and/or that they are made of matter which was present when the planets were formed. Saturn's rings are very reflective and could be composed of ices such as make up comets.
Satellites
Saturn has 8 satellites with diameters greater than 200 kms. Of these Titan is, by far, the largest; with a dimeter of 5,150 km it is the second largest satellite in the solar system. It is probably the only satellite which has an atmosphere; this atmosphere is denser than the Earth's but is composed almost entirely of methane.
Mimas has a diameter of 390 km. Its surface is very cratered and the Voyager pictures show one giant crater with a diameter almost equal to one third of that of the satellite.
Enceladus has a diameter of 500 km. It shows cratering and also complex geological structures indicating large crustal movements.
Tethys has a diameter of 1,050 km. It appears to be made of ice and is heavily cratered. There is a huge trench-like structure extending a quarter of the way around the satellite which is 100 km wide and 4 to 5 km deep.
Dione is 1,120 km in diameter. It shows many craters and large plains.
Rhea has a diameter of 1,530 km and is heavily cratered.
There are several small satellites some of which are believed to be responsible for `shepherding' some of the features seen in the structure of the rings.
Appearance
Saturn can easily be seen with the naked-eye. With good binoculars it can be seen to have a non-circular shape and the rings can be seen with a small telescope which will also show the largest satellite, Titan. About every 15 years the Earth passes through the plane of the rings which are then seen edge-on (or rather, not seen!) although their shadow onto the disk of Saturn may still be visible.
