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Saturn's moons and rings



Solar system writ small: Saturn's moons

Saturn has the most moons of any planet in the Solar System. By the end of 2005, 47 natural satellites were known to be in orbit around the ringed giant, Of these almost four-dozen moons, 14 satellites alone were discovered in 2004 and 2005; not all of these have been officially confirmed by the International Astronomical Union (IAU). This is expected to happen in 2006.

 The 21 largest satellites of Saturn
zum Bild The 21 largest satellites of Saturn

It's really no surprise that people talk about the Saturn system as being an analogy of the solar system. This is a world of unique complexity, made up of a central body and its satellites, not to mention the famous ring surrounding the planet at its equator. Saturn is also being orbited by a man-made satellite, the space probe Cassini, which will remain in orbit around the planet until at least 2008. The orbiter will perform repeated fly-bys of the planet's moons at various distances and at different orbital inclinations, gathering scientific data from every conceivable angle.

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's upper atmosphere in ultraviolet
zum Bild 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 American Pioneer probes, and later Voyagers 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 km to 1530 km: Rhea, Iapetus, Dione and Tethys. Then there are three moons whose diameters range from almost 200 km to 500 km: Enceladus, Mimas and Hyperion. These moons are also known as the 'main' satellites.

Saturn's moon Enceladus above the planet's rings

 Saturn's moon Enceladus over Saturn's rings
zum Bild Saturn's moon Enceladus over Saturn'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 km and 100 km. Then there are a dozen satellites barely more than 10 km in diameter, starting with Siarnaq, which measures 40 km. Finally, we currently know of another 22 very irregularly shaped satellites which measure less than 10 km, including S/2005 S 2, recently discovered by Cassini.

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 km 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.

Known moons of Saturn (as of 12/2005)

Name of
moon

Astronomical designation

Diameter (km)

Distance_to_ Saturn centre (km)

Discoverer

Titan 5150 1 221 900 1655
Christiaan Huygens
Rhea 1528 527 100 1672
Giovanni Cassini
Iapetus 1436 3 560 800 1671
Giovanni Cassini
Dione 1118 377 400 1684
Giovanni Cassini
Tethys 1060 294 700 1684
Giovanni Cassini
Enceladus 499 238 100 1789
Wm Herschel
Mimas 397 185 600 1789
Wm Herschel
Hyperion 266 1 464 100 1848
W. & G. Bond; W. Lassell
Janus S/1980 S 1 178 151 500 1980
Audouin Dollfus
Phoebe 120 12 944 300 1898
W. H. Pickering
D. L. Stewart
Epipmetheus S/1980 S 3 119 151 400 1980
R. Walker
Prometheus S/1980 S 27 100 139 400 1980
S. Collins
Pandora S/1980 S 26 84 141 700 1980
S. Collins
Siarnaq S/2000 S 3 40 18 160 000 2000
Gladman & Kavelaars
Atlas S/1980 S 28 32 137 700 1980
Richard Terrile
Helene S/1980 S 6 32 377 400 1980
P. Lacques & J. Lecacheux
Albiorix S/2000 S 11 32 16 404 000 2000
Matthew Holman
Telesto S/1980 S 13 24 294 700 1980
B. Smith et al
Paaliaq S/2000 S 2 22 15 199 000 2000
Brett Gladman
Pan S/1980 S 13 20 133 600 1981
Mark Showalter
Calypso S/1980 S 25 19 294 700 1980
D. Pascu et al
Ymir S/2000 S 1 18 23 096 000 2000
Brett Gladman
Kiviuq S/2000 S 5 16 11 365 000 2000
Brett Gladman
Tarvos S/2000 S 4 15 18 247 000 2000
Kavelaars & Gladman
Ijiraq S/2000 S 6 12 11 440 000 2000
Kavelaars & Gladman
Erriapo S/2000 S 10 10 17 616 000 2000
Kavelaars & Gladman
Skathi (Skadi) S/2000 S 8 8 15 647 000 2000
Kavelaars & Gladman
Daphnis S/2005 S 1 7 136 500 2005
Cassini ISS Team
Mundilfari S/2000 S 9 7 18 709 000 2000
Gladman & Kavelaars
Narvi S/2003 S 1 7 18 719 000 2003
Sheppard et al
Suttungr S/2000 S 12 7 19 463 000 2000
Gladman & Kavelaars
S/2004 S 18 7 19 650 000 2004
David Jewitt et al
Thrymr S/2000 S 7 7 20 382 000 2000
Gladman & Kavelaars
S/2004 S 11 6 16 950 000 2004
S. S. Shepard
S/2004 S 13 6 18 450 000 2004
David Jewitt et al
S/2004 S 15 6 18 750 000 2004
David Jewitt et al
S/2004 S 10 6 19 350 000 2004
David Jewitt et al
S/2004 S 7 6 19 800 000 2004
David Jewitt et al
S/2004 S 14 6 19 950 000 2004
David Jewitt et al
S/2004 S 8 6 22 200 000 2004
David Jewitt et al
S/2004 S 12 5 19 650 000 2004
David Jewitt et al
S/2004 S 9 5 19 800 000 2004
David Jewitt et al
Pallene S/2004 S 2 4 211 000 2004
Charnóz & Porco
Polydeuces S/2004 S 5 4 377 400 2004
Carolyn Porco
S/2004 S 17 4 18 600 000 2004
David Jewitt et al
S/2004 S 16 4 22 200 000 2004
David Jewitt et al
Methone S/2004 S 1 3 194 000 2004
Charnóz & Porco

Saturn's rings

Ring

Distance

Width

D 66 970 km 7540 km
C 74 510 km 17 490 km
B 92 000 km 25 580 km
A 122 170 km 14 610 km
F 140 180 km 50 km
G 170 180 km 500 to several thousand km
E 181 000 km 302 000 km

Distance is from Saturn's centre to closest edge of ring





Created: 25/07/2005 14:38:00
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