Saturn (planet) 

Physical characteristics

Saturn's shape is visibly flattened at the poles and bulging at the equator (an oblate[?] spheroid); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so. Saturn is also the least dense of the Solar System's planets with an average specific density of 0.69, significantly less than water. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water.

Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 K at the core, and it radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism[?] (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.

Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but Saturn's bands are much fainter and they're also much wider near the equator. Saturn's cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made. Saturn exhibits long-lived ovals and other features common on Jupiter; in 1990 the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters and in 1994 another, smaller storm was observed.

The exploration of Saturn

Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. The Cassini, now on its way, will arrive in 2004.

Saturn's rings

Saturn 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 isn't 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 Christiaan Huygens, using a telescope much more powerful than the ones available to Galileo in his time. In 1675 Giovanni 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 isn't widely accepted today, since Saturn's rings are thought to be unstable over periods of millions of years and therefore of relatively recent origin.

Saturn's moons

Saturn 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 natural satellites

Name	Diameter (km)	Mass (kg)	Mean orbital radius (km)	Orbital period
Pan	20	Unknown	133,583	0.575 days
Atlas	30 (40 x 20)	Unknown	137,670	0.6019 days
Prometheus	91 (145 x 85 x 62)	2.70×1017	139,350	0.6130 days
Pandora	84 (114 x 84 x 62)	2.20×1017	141,700	0.6285 days
Epimetheus	115 (144 x 108 x 98)	5.60×1017	151,422	0.6942 days	Co-orbital
Janus	178 (196 x 192 x 150)	2.01×1018	151,472	0.6945 days
Mimas	392	3.80×1019	185,520	0.942422 days
Enceladus	498	7.30×1019	238,020	1.370218 days
Tethys	1060	6.22×1020	294,660	1.887802 days	Co-orbital
Telesto	29 (34 x 28 x 36)	Unknown	294,660	1.887802 days
Calypso	26 (34 x 22 x 22)	Unknown	294,660	1.887802 days
Dione	1120	1.05×1021	377,400	2.736915 days	Co-orbital
Helene	33 (36 x 32 x 30)	Unknown	377,400	2.736915 days
Rhea	1530	2.49×1021	527,040	4.5175 days
Titan	5150	1.35×1023	1,221,830	15.94542 days
Hyperion	286 (410 x 260 x 220)	1.77×1019	1,481,100	21.27661 days
Iapetus	1460	1.88×1021	3,561,300	79.33018 days
Phoebe	220	4.00×1018	12,952,000	550.48 days

Front Row Seats

Saturn and its rings are best seen when the planet is at or near opposition (the configuration of a planet when it is a an elongation of 180° and thus appears opposite the Sun in the sky.)

Saturn's Opposition Periods 2001-2005

Date of Opposition	Distance to Earth (AU)	Angular diameter[?]
December 3, 2001	8.08	20.6 arcsec
December 17, 2002	8.05	20.7 arcsec
December 31, 2003	8.05	20.7 arcsec
January 13, 2005	8.08	20.6 arcsec

Solar system:
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets

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