Sat­urn's moons and rings

The Solar System in miniature: Saturn's moons

After Jupiter, Saturn is the planet in the Solar System with the most moons. Saturn has 293 known moons. It is no wonder that, by analogy with the Solar System, we also speak of the Saturnian system: a world of unparalleled complexity, comprising a central 'star' and its satellites – not to mention the ring system encircling the planet on its equatorial plane. As an artificial satellite, the Cassini spacecraft orbited Saturn until September 2017. During its mission, the orbiter repeatedly flew past the moons at varying distances and orbital inclinations, gathering scientific data from every conceivable perspective.

Much like the neighbouring gas giant Jupiter – the largest planet in the Solar System – Saturn's moons are extremely diverse in their composition and the age of their surfaces. With the exception of Titan, however, Saturn's moons are mostly significantly smaller than Jupiter's four large moons: Ganymede (the largest moon in the Solar System), Callisto, Io and Europa – which are known as the 'Galilean' moons after their discoverer.

Like the Galilean moons (with the exception of Io), Saturn's moons consist predominantly of ice, mixed with varying amounts of rock and, in some cases, organic substances – that is, carbon compounds. In addition to photographic documentation of surface geology using the camera system on board Cassini, the main objective of the Cassini-Huygens mission was to investigate the chemical and mineralogical composition of the moons – using, amongst other instruments, the spectrometers on board Cassini and, in the case of Titan, also on the Huygens lander.

Titan is the second-largest moon in the Solar System (with a diameter of 5150 kilometres, it is larger than the planet Mercury and the dwarf planet Pluto) and possesses a unique feature: a dense atmosphere of nitrogen and methane. Not least because of this, Titan has become one of the most important targets for exploring the outer Solar System.

The European atmospheric and landing probe Huygens was carried aboard Cassini on its seven-year journey to Saturn specifically for the purpose of exploring this enigmatic moon. On 14 January 2005, after a three-hour flight, the Huygens probe landed by parachute on Titan's surface – composed of frozen methane and with a temperature of minus 180 degrees Celsius – and took important measurements and photographs.

Saturn's first moons were discovered in the 17th century by the Italian-French astronomer Giovanni Cassini (1625–1712) and his Dutch contemporary Christiaan Huygens (1629–1695). It was ultimately NASA's two Pioneer space probes, and later especially Voyager 1 and 2, that caused a sensation in 1980 and 1981 with the first close-up images of these unknown worlds. One response was the most ambitious project to date for exploring the outer Solar System: the US-European Cassini-Huygens dual probe.

Classifying Saturn's moons is no easy task. Ranking them by size lists Titan as by far the largest satellite, followed by four moons of significantly smaller diameters, ranging from 1530 to 1060 kilometres: Rhea, Iapetus, Dione and Tethys. Finally, there are three moons with diameters between 270 and 500 kilometres: Enceladus, Mimas and Hyperion. Collectively, these moons are referred to as Saturn's 'major' moons.

Phoebe, Janus, Epimetheus, Prometheus and Pandora, the largest 'minor' moons, are no longer spherical in shape and have diameters ranging from approximately 80 kilometres (Pandora) to 180 kilometres (Janus). Starting with the 40-kilometre-wide Siarnaq, there are a dozen moons barely more than ten kilometres in diameter. Finally, there are numerous irregularly shaped moons that are smaller than ten kilometres in size.

A dozen of these tiny moons were discovered using what was at the time the world's largest reflecting telescope on Mauna Kea, Hawaii. Eleven of these bodies orbit Saturn in the opposite direction to its own rotation, fuelling the theory that they did not form alongside the planet, but were instead captured by its gravity from the comet reservoir of the Kuiper Belt, which lies between the orbits of Uranus, Neptune and Pluto. Following analyses using the Cassini VIMS spectrometer, a similar origin is also assumed for the moon Phoebe, which is over 200 kilometres in diameter. The theory that many of these irregular, small bodies did not originate in the Saturnian system but come from other regions of the Solar System is also supported by the orbital inclinations of these moons – some of which deviate significantly from the equatorial plane.