Saturn, in astronomy: The Satellite System

Saturn has 82 confirmed natural satellites, many of which have not been named. Five of the discovered satellites have not been found since they were first identified (2004, 2007) and are considered lost. Because the increasing number of satellites makes it difficult to continue to name them after Greek Titans, as the first moons to be discovered were, a scheme was adopted for the outer satellites. These are now named after the giants of other cultures: Inuit, Norse, and Gallic. The satellites may be divided into nine groups for convenience. In the order of their distance from Saturn, the groups are shepherd (satellites whose orbit is within or just beyond Saturn's ring system), co-orbital (two satellites that share the same orbit and trade positions within it on a regular basis), inner large (large satellites within the E ring), Alkyonide (small satellites within the inner large group), Trojan (satellites that are co-orbital at Lagrangian points), outer large (large satellites beyond the E ring), and Inuit, Norse, and Gallic (each a group of outer satellites that have similar orbits). There are also ring moonlets orbiting in the B and A rings.

The five confirmed shepherd satellites are Pan, Daphnis, Atlas, Prometheus, and Pandora, are named. The co-orbital group comprises Epimetheus and Janus, but the shepherds Prometheus and Pandora also share an orbit. The inner large group comprises four satellites, Mimas, Enceladus, Tethys, and Dione; the three Alkyonides (Methone, Anthe, and Pallene) have orbits between Mimas and Enceladus. The Trojan group, also found within the inner large group, comprises four satellites, Telesto, Calypso, Helene, and Polydeuces. The outer large group comprises four satellites, Rhea, Titan, Hyperion, and Iapetus. The Inuit group comprises seven satellites, five of which are named: Kiviuq, Ijiraq, Paaliaq, Siarnaq, and Tarqeq. Of the 46 satellites comprising the Norse group, only 21 are named: Phoebe, Skathi, Skoll, Greip, Hyrrokkin, Jamsaxa, Mundilfari, Bergelmir, Narvi, Suttungr, Hati, Farbauti, Thrymr, Aegir, Bestia, Fenrir, Surtur, Kari, Ymir, Loge, and Fornjot. The Gallic group consists of four satellites, Albiorix, Bebhionn, Erriapus, and Tarvos; one additional satellite may be a member of this group.

Almost all of Saturn's inner moons form a regular system of satellites; that is, their orbits are nearly circular and lie in the equatorial plane of the planet; almost all of the outer moons' orbits are inclined. Except for Hyperion, which has a chaotic orbit, and Phoebe, all the satellites are believed to have synchronous orbits; that is, their orbital and rotational periods are the same, so that they always keep the same face turned toward Saturn. The largest satellite, Titan, is 3,200 mi (5,150 km) in diameter and has the size and cold temperatures necessary to retain an atmosphere; it is the only natural satellite in the solar system with a substantial atmosphere.

Saturn has six major icy satellites that can be easily seen through earth-based telescopes. The most prominent feature of heavily cratered Mimas, the innermost of the six, is a large impact crater about one third the diameter of the satellite. Certain broad regions of Enceladus are uncratered, indicating geological, mostly likely cryovolcanic, activity that has resurfaced the satellite within the last 100 million years. Tethys also has a very large impact crater, as well as an extensive series of valleys and troughs that stretches three quarters of the way around the satellite. Both Dione and Rhea have bright, heavily cratered leading hemispheres and darker trailing hemispheres with wispy streaks that are thought to be produced by deposits of ice inside surface troughs or cracks. Iapetus, the outermost of the large icy satellites, has a dark leading hemisphere and a bright trailing hemisphere.

The remaining satellites are smaller. The two largest of these, the dark-surfaced Phoebe and the irregularly shaped Hyperion, orbit far from the planet; the Norse group of satellites orbit with retrograde motion, i.e., opposite to that of the planet's rotation. The smallest satellites, less than c.6 mi (10 km) in diameter, include Daphnis, the Alkyonides, Polydeuces, some of the Inuit and Gallic groups, and nearly all of the Norse group.

Sections in this article:

• Introduction
• The Satellite System
• The Ring System
• Astronomical and Physical Characteristics of Saturn

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