The planet Saturn, along with its rings and moons, forms one of the most impressive visual sights in the heavens. Prior to the arrival of Cassini-Huygens most of our knowledge of this world has come from the Voyager fly-bys in the 1980's and, more recently, from ground based observatories.
Saturn the planet
Saturn's atmosphere is made up almost entirely of hydrogen and helium - the same basic constituents as the Sun - with a hydrogen abundance of at least 93%. This scenario has given rise to the fact that the planet has a density lower than water.
Visually the atmosphere is more turbulent than Uranus and Neptune, but still considerably less than Jupiter's dynamic upper atmosphere. The whole of the atmosphere is subject to high speed winds. Near the equator the wind speed approaches 500ms-1 and blows in an easterly direction. At higher latitudes the winds are reduced in speed and appear to exist in at least three different zones in both the northern and southern hemisphere.
Beneath the atmosphere a liquid and possible solid core exists. Even at modest depths of several thousand kilometres below the cloud tops the pressures are sufficient for hydrogen to exist in liquid form. For Saturn, along with Jupiter, it is believed that pressure may reach a point whereby liquid metallic hydrogen exists. At greater depths many scientists believe a solid rocky core exists.
Saturn as a planet is defined by its ring structure. Although the other gas gaints also have ring structures, only on Saturn are they so rich and complex. The rings are split into at least seven groupings labelled A to G. The two primary rings A and B are separated by the Cassini division - a feature that was first obseved in 1675. The main rings lie from 75 000 to 136 775 km out from the centre of Saturn. The faint G ring, by contrast, is 170 to 175 000 km from the centre and the E ring stretches out to 483 000 km. All the rings are incredibly thin - maybe only a few tens of metres thick, and certainly no more than a couple of hundred.
Saturn has a largest number of known moons - 31 at the last count. Many of these moons are tiny - just a few tens of kilometres in size - and exist within the rings. Titan, however, is an exception. At just over 5000 km in diameter it is larger than Mercury and only fractionally smaller than Ganymede, the largest moon in the Solar System. Titan is also unique amongst moons in that it has its own atmosphere that may be similar in nature to the Earth's atmosphere several billion years ago.
Saturn is known to possess a strong magnetic field. Observations with the Hubble Space Telescope have revealed aurorae in the polar regions. The associated magnetosphere has been measured out to at least 20 Saturn radii.