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Saturn: Moons and Rings
Moons
The discovery of Saturn’s moons has been going on for more than three and a half centuries. Five moons were observed in the seventeenth century, two in the eighteenth, two in the nineteenth, and nine in the twentieth. Until the last decade when Uranus eclipsed it, Saturn was believed to posses the greatest number of moons. Also in recent years, more than a dozen other satellites were “discovered” and provisionally named; it now looks, however, as if they do not exist.
Though Saturn no longer has the greatest number of moons, many astronomers believe it has the most interesting ones. For example, there are three pairs of planets with gravitationally-linked orbits: Mimas-Tethys, Enceladus-Dione and Titan-Hyperion. In the first pair, Mimas’s orbit is precisely one-half of Tethys’s (they are in 1:2 resonance). Enceladusis’s rotational period is half of Dione’s (another 1:2). Titan-Hyperion resonate 3:4. Of all the moons with known rotational rates, all rotate synchronously (at the same time) except for Phoebe and Hyperion.And, perhaps the most interesting moon of all, Titan, the second largest planetary satellite in the solar system, rotates around Saturn. A source of considerable mystery, Titan is wrapped in a thick layer of nearly impenetrable gas. This atmospheric shrouding is so thick that for many years it lead astronomers to believe that Titan had a greater diameter than Jupiter’s Ganymede, in reality the largest moon. As it is, Titan still has a diameter greater than Mercury’s and a density greater than Pluto’s. Titan is also one of few moons in the solar system with a significant atmosphere. (It joins the company of Jupiter’s Io, Neptune's Triton, and maybe Pluto's Charon). Saturn’s satellites, in order from the planet, are: Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helene, Rhea, Titan, Hyperion, Iapetus, and Phoebe.
| Name | Distance (km) | Radius (km) | Mass (kg) | Discoverer (Year) |
| Atlas | 138,000 | 14 | Unknown | Terrile (1980) |
| Calypso | 295,000 | 13 | Unknown | Pascu (1980) |
| Dione | 377,000 | 560 | 1.05 x 1021 | Cassini (1684) |
| Enceladus | 238,000 | 260 | 8.40 x 1019 | Herschel (1789) |
| Epimetheus | 151,000 | 57 | 5.60 x 1017 | Walker (1980) |
| Helene | 377,000 | 16 | Unknown | Laques (1980) |
| Hyperion | 1,481,000 | 143 | 1.77 x 1019 | Bond (1848) |
| Iapetus | 3,561,000 | 730 | 1.88 x 1021 | Cassini (1671) |
| Janus | 151,000 | 89 | 2.01 x 1018 | Dollfus (1966) |
| Mimas | 186,000 | 196 | 3.80 x 1019 | Herschel (1789) |
| Pan | 134,000 | 10 | Unknown | Showalter (1990) |
| Pandora | 142,000 | 46 | 2.20 x 1017 | Collins (1980) |
| Phoebe | 12,952,000 | 110 | 4.00 x 1018 | Pickering (1898) |
| Prometheus | 139,000 | 46 | 2.70 x 1017 | Collins (1980) |
| Rhea | 527,000 | 765 | 2.49 x 1021 | Cassini (1672) |
| Telesto | 295,000 | 15 | Unknown | Reitsema (1980) |
| Tethys | 295,000 | 530 | 7.55 x 1020 | Cassini (1684) |
| Titan | 1,222,000 | 2,575 | 1.35 x 1023 | Huygens (1655) |
Rings
Saturn’s rings are the planet’s most distinguishing feature. Majestic and intricate, they are the most substantial planetary ring system known to man. Three rings can be seen from Earth, the A and B rings and the faint C ring. Voyager 1 confirmed the presence of four other rings circling the planet. These rings are composed of countless small particles composed primarily of ice or ice-covered rock orbiting independently around Saturn. Most range in size from a centimeter to several meters; it is likely a few are kilometer-sized. Despite their large appearance (more than 250,000 kilometers in diameter), Saturn’s rings are incredibly 
thin—no more than 1.5 kilometers thick. If all the rings were compressed into a single body, it would have a radius no larger than 50 kilometer, smaller than eleven of Saturn’s moons.These rings are not uniform in size and density. Voyager 2 revealed the presence of “spokes,” regular agglomerations (or “clumps”) of ring material. Perhaps caused by the planet’s magnetic field, their nature continues to be a mystery. Besides spokes, the rings of Saturn also have “knots.” The outermost F-ring is a complicated braid of several smaller rings, with visible "knots" in them. There is speculation that the knots may be ring material enveloping small moons.
There is a complicated and poorly-understood system of resonance between the rings and the shepherding satellites (moons whose gravity preserves the ring system). Atlas, Pandora, and Prometheus appear to anchor the system. Mimas has some part in keeping a ring division clear. Pan is an inter-ring satellite. The mechanics of this relationship have not yet been thoroughly set out.