Saturn

Saturn is the sixth planet from the Sun and the second largest:

Orbit: 1,429,4000 km (887,220,000 miles) or (9.54 AU) from Sun
Diameter: 120,536 km or 74,901 miles (equatorial)
Mass: 5.68 x 10²⁶ kg or 12.5244 x 10²⁶ lbs

Cassini on its way! The Cassini mission to explore Saturn and its moon Titan was launched successfully on Oct. 15, 1997. The first of its four, planned gravity assist maneuvers has been successfully completed. It will arrive at Saturn on July 1, 2004.

In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word "Saturday."

Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low-resolution image of Saturn, therefore, changes drastically. It was not until 1659 that Christiaan Huygens determined the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus and shortly thereafter around Jupiter and Neptune.

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

Saturn is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar diameters vary by almost 10 percent (74,901 miles vs. 67,563 miles). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not nearly as much as Saturn.

Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water. If you could find a container big enough and fill it with water, Saturn would float.

Like Jupiter, Saturn is about 75 percent hydrogen and 25 percent helium with traces of water, methane, ammonia and "rock," similar to the composition of the primordial Solar Nebula from which the solar system was formed.

Saturn's interior is similar to Jupiter's and consists of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

Saturn's interior is hot (12,273 F 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, as in Jupiter. But this may not be sufficient to explain Saturn's luminosity. Some additional mechanism may be at work, perhaps the "raining out" of helium deep in Saturn's interior.

The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the cloud tops are invisible from Earth, so it was not until the Voyager encounters that any detail of Saturn's atmospheric circulation could be studied. Saturn also 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. In 1994, another, smaller storm was observed.

Two prominent rings (A and B) and one faint ring (C) can be seen from Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Gap. The Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings of the other planets, are very bright.

Though they look continuous from Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few mile-sized objects are also likely.

Saturn's rings are very thin. Though they're 155,350 miles or more in diameter, they're no more than one-mile thick. Despite their impressive appearance, there's really very little material in the rings. If the rings were compressed into a single body, it would be no more than 62 miles across.

The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.

Voyager confirmed the existence of a puzzling feature in the rings called "spokes" which were first reported by amateur astronomers. Their nature remains a mystery but may have something to do with Saturn's magnetic field.

Saturn's outermost ring, the F-ring, is a complex structure made up of several smaller rings along which "knots" are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The strange braided appearance visible in the Voyager 1 images is not seen in the Voyager 2 images perhaps because Voyager 2 imaged regions where the component rings are roughly parallel.

There are complex tidal resonances between some of Saturn's moons and the ring system. Some of the moons, the so-called "shepherding satellites" (i.e. Atlas, Prometheus and Pandora), are clearly important in keeping the rings in place. Mimas seems to be responsible for the paucity of material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid belt. Pan is located inside the Encke Gap. The whole system is very complex and as yet, poorly understood.

The origin of the rings of Saturn (and the other gas planets) is unknown. Though they may have had rings since their formation, the ring systems are not stable. They must be regenerated by ongoing processes, probably the breakup of larger satellites.

Like the other gas planets, Saturn has a significant magnetic field.

When it is in the nighttime sky, Saturn is easily visible to the naked eye. Though it is not nearly as bright as Jupiter, it is easy to identify as a planet because it doesn't "twinkle" like the stars. The rings and the larger satellites are visible with a small astronomical telescope.

Saturn has 18 named satellites, more than any other planet. There may also be several small ones yet to be discovered.

Of those moons for which rotation rates are known, all but Phoebe and Hyperion rotate synchronously. The three pairs, Mimas-Tethys, Enceladus-Dione and Titan-Hyperion, interact gravitationally in such a way as to maintain stable relationships between their orbits. The period of Mimas' orbit is exactly half that of Tethys. They are thus said to be in a 1:2 resonance. Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance. In addition to the 18 named satellites, at least a dozen more have been reported and given provisional designations.

Saturn's Satellites