AS WE KNOW TODAY....
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|Mean radius||1,426,725,400 km|
(29.423519 Julian years)
|Synodic period||378.1 days|
|Avg. Orbital Speed||9.6724 km/s|
|Number of satellites||30|
|Equatorial diameter||120,536 km|
|Surface area||4.38×1010 km2|
|Mean density||0.69 g/cm3|
or 0.914 gee
|10h 13m 59s|
|10h 39m 25s|
|Escape Speed||35.49 km/s|
|Avg. Cloudtop temp.||93K|
|Atmospheric pressure||140 kPa|
|Table of contents |
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 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. Leo Allatius, a theologian at the time, suggested that the ring may be the foreskin of Jesus Christ, ascended into the heavens.
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
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 will soon treat us with a view we have not seen in 25 years - the backside of the rings.
Compare images from Cassini this March, to a view from the Pioneer 11 spacecraft:
Pioneer 11 spacecraft: September 1, 1979
The thickest parts of the ring are almost invisible on the backlit view.
Cassini spacecraft: March 27, 2004
Notice both the shadow of Saturn on the ring, and the shadow of the ring onto the planet.
There will be plenty of marvelous images coming soon, showing the dance of light on Saturn, the rings, the moons, and their shadows.
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, now on its way, will arrive in 2004 to study Saturn and its moon Titan. The Cassini-Huygens spacecraft is expected to arrive at Saturn on July 1, 2004; on that day, the spacecraft will execute a complicated maneuver called SOI (Saturn Orbit Insertion). Additional details on the mission can be found http://saturn.jpl.nasa.gov. As Cassini approaches Saturn, the Program will release the "latest" images every Friday at http://ciclops.lpl.arizona.edu/
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
|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|
- Change of seasons on Saturn (http://hubblesite.org/newscenter/newsdesk/archive/releases/2001/15/image/a)