Saturn is the sixth planet from the Sun. It is a gas giant, the second-largest planet in the solar system after Jupiter. It was named after the Roman god Saturn.

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.

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 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 as 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. A contemporary theologian, Leo Allatius, suggested that the ring may be the foreskin of Jesus Christ, ascended into the heavens.

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 viewed using a quite modest modern telescope or with 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 Edouard 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.

The dark side of the rings

Compare images from Cassini in March 2004, to a view from the Pioneer 11 spacecraft:

: September 1, 1979: Backlit rings. The thickest parts of the ring are almost invisible on the backlit view.]]

: March 27, 2004 Frontlit rings. Notice both the shadow of Saturn on the ring, and the shadow of the ring onto the planet.]]

Saturn's rings have two sides, sunward and backside, that look very different, although from Earth we do not get to appreciate this because the Earth cannot view Saturn from a direction that is very far from the sun. The NASA Cassini spacecraft in 2004 was expected to show a view that has not been seen in 25 years–the backside of the rings.

It is expected that there be many striking images delivered from the Cassini mission showing the dance of light on Saturn, the rings, the moons, and their shadows.

The spokes of the rings

Until recently, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. However as soon as 'spokes' were found, there was an assumption that they are connected to electromagnetic interaction, as they rotate almost synchronously with the magnetosphere of Saturn.

Exploration of Saturn

Saturn was first visited by Pioneer 11 in 1979 and the following two years by Voyager 1 and Voyager 2. The Cassini-Huygens orbiter and probe has arrived to study Saturn and its moon Titan. The Cassini-Huygens spacecraft achieved orbit on July 1, 2004, after executing a complicated maneuver called SOI (Saturn Orbit Insertion). Additional details on the mission can be found http://saturn.jpl.nasa.gov. The Program releases the "latest" images every Friday at http://ciclops.lpl.arizona.edu/

Saturn's moons

Main article: Saturn's natural satellites

Saturn has a large number of moons, 30 of which have names; the precise figure is uncertain as there are many objects in orbit around the planet with a wide range of sizes. (Nature vol. 412, p.163–166). Especially noteworthy is Titan, the only moon in the solar system with a dense atmosphere.

Best viewing of Saturn

Saturn and its rings are best seen when the planet is at or near opposition (the configuration of a planet when it is at 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

External links

The Solar System

(For other objects and regions, see: List of solar system objects, Astronomical objects)

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