Sat­urn's moons and rings

Like the gas giant Jupiter, the largest planet in the Solar System, the moons of Saturn vary enormously in their composition and the age of their surfaces. With the exception of Titan, however, Saturn's moons are much smaller than Jupiter's four large moons: Ganymede (the largest satellite in the Solar System), Callisto, Io and Europa, known collectively as the 'Galilean' moons after the astronomer who discovered them.

Like the Galilean moons, the moons of Saturn (except Io) are composed primarily of ice, mixed with varying proportions of rock and, in some cases, organic substances (carbon compounds). As well as photographically documenting the surface geology using the camera system on board the Cassini spacecraft, another principal objective of the Cassini-Huygens mission is to study the chemical and mineralogical composition of the planet's moons using the spectrometers onboard the Cassini probe (and in the case of Titan, the Huygens lander).

Titan's upper atmosphere in ultraviolet

Titan, the second largest moon in the Solar System (its diameter of 5150 kilometres makes it even larger than Mercury and Pluto), has one unique feature: its dense atmosphere of nitrogen and methane. This is one of the features that rank Titan amongst the most important targets of exploration in the outer Solar System.

The European atmospheric and lander probe Huygens was taken on a six-year journey to Saturn by Cassini with the express aim of exploring the mysterious moon. The Huygens probe touched down on Titan's icy-cold (-180º) surface of frozen methane on 14 January 2005 after a three-hour parachute descent, during which it took important measurements and pictures.

The first of Saturn's satellites were discovered in the 17th century by the Italian-French astronomer Giovanni Cassini (1625-1712) and his Dutch contemporary Christiaan Huygens (1629-1695). However, it was the two US Pioneer probes, and later Voyager I and II, that provided us in 1980 with the first close-up pictures of these unknown worlds, providing the stimulus for the most ambitious project ever launched in the exploration of the outer Solar System: the US-European probe Cassini-Huygens.

Classifying Saturn's moons is no easy task, but categorising them by size puts Titan at the top of the list by a considerable amount. There are four much smaller moons, with diameters ranging from 1060 to 1530 kilometres: Rhea, Iapetus, Dione and Tethys. Then there are three moons whose diameters range from almost 200 to 500 kilometres: Enceladus, Mimas and Hyperion. These moons are also known as the 'main' satellites.

Saturn's moon Enceladus above the planet's rings

Janus, Phoebe, Epimetheus, Prometheus and Pandora, the largest of the smaller satellites, are not at all spherical in shape, and each has a diameter of between 10 and 100 kilometres. Then there are a dozen satellites barely more than 10 kilometres in diameter, starting with Siarnaq, which measures 40 kilometres. Finally, there are numerous irregularly shaped satellites, with sizes of less than 10 kilometers.

A dozen of these minuscule moons were discovered using what was then the largest reflecting telescope in the world, on Mauna Kea in Hawaii. Eleven of these bodies orbit Saturn in the opposite direction to the planet's own direction of rotation, supporting the idea that they were not formed together with the planet in their current location in the Solar System, but originated in the 'comet reservoir' of the Kuiper Belt between the orbital paths of Uranus, Neptune and Pluto before being captured by Saturn's gravitational force. Analyses performed by the VIMS spectrometer on board Cassini suggest that the 200 kilometre diameter moon Phoebe may be of similar origin. The hypothesis that many of these small, irregular bodies do not originate within the Saturn system but in other regions of the Solar System is also supported by the fact that these moons often have orbital inclinations that deviate markedly from the plane of the equator.