It is probably best known for its famous planetary rings. They were first observed by Galileo Galilei in 1610 with his telescope, but he clearly did not know what to make of it. He wrote to the Grand Duke of Tuscany that "Saturn is not alone but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one (Saturn itself) is about three times the size of the lateral ones (actually the edges of the rings)." He also described Saturn has having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613 they reappeared again, further confusing Galileo. The riddle of the rings was not solved until 1655 by [Christian Huygens]?, using a telescope much more powerful than the ones available to Galileo in his time. In 1675? [Giovanni Domenico Cassini]? determined that Saturn's ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division.
The rings can be seen in quite modest modern telescopes or a good pair of binoculars?. They are composed of silica? rock, [iron oxide]?, and ice particles ranging in size from specks of dust to the size of a small automobile. There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by [Edward Roche]? in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces. A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material that Saturn formed out of. This theory is not widely accepted today, since Saturn's rings are thought to be unstable over periods of millions of years and therefore of relatively recent origin.
It has a large number of moons, 18 of which have names. The exact number of moons is uncertain, there being large numbers of objects of all sizes in orbit around Saturn. A recent survey starting in late 2000 found another 12 moons in orbits suggesting that they were the fragments of larger bodies captured by Saturn (Nature vol. 412, p.163-166)
Saturn's moons:
| Name | Orbital radius (km) | Diameter (km) | Mass (kg) |
|
|---|---|---|---|---|
| Pan | 133,583 | 20 | Unknown | |
| Atlas | 137,670 | 30 (40 x 20) | Unknown | |
| Prometheus | 139,350 | 91 (145 x 85 x 62) | 2.70×1017 | |
| Pandora | 141,700 | 84 (114 x 84 x 62) | 2.20×1017 | |
| Epimetheus | 151,422 | 115 (144 x 108 x 98) | 5.60×1017 | Co-orbital |
| Janus | 151,472 | 178 (196 x 192 x 150) | 2.01×1018 | |
| Mimas | 185,520 | 392 | 3.80×1019 | |
| Enceladus | 238,020 | 498 | 7.30×1019 | |
| Tethys | 294,660 | 1060 | 6.22×1020 | Co-orbital |
| Telesto | 294,660 | 29 (34 x 28 x 36) | Unknown | |
| Calypso | 294,660 | 26 (34 x 22 x 22) | Unknown | |
| Dione | 377,400 | 1120 | 1.05×1021 | Co-orbital |
| Helene | 377,400 | 33 (36 x 32 x 30) | Unknown | |
| Rhea | 527,040 | 1530 | 2.49×1021 | |
| Titan | 1,221,830 | 5150 | 1.35×1023 | |
| Hyperion | 1,481,100 | 286 (410 x 260 x 220) | 1.77×1019 | |
| Iapetus | 3,561,300 | 1460 | 1.88×1021 | |
| Phoebe | 12,952,000 | 220 | 4.00×1018 |
Solar system:
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets
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