Voyage to Saturn - Wavelength Virtual Astronomical Tours

Planet Profile - Saturn

Mass: 5.69 x 10^26 kg
Diameter: 120660 km
Mean density: 690 kg/m^3
Escape velocity: 35600 m/sec
Average distance from Sun: 9.539 AU
Rotation period: 10.2 hours
Revolution period: 29.46 years
Obliquity: 26.7^°
Orbit inclination: 2.49^°
Orbit eccentricity: 0.056^°
Mean temperature: 88^° K (1 bar level)
Visual geometric albedo: 0.46
Atmospheric components: 97% hydrogen, 3% helium, .05% methane
Rings: There are four main ring groups and three more faint, narrow ring groups separated by gaps called divisions. Rings are 270,000 km in diameter, but only a few hundred meters thick.

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Saturn's Satellites

Saturn has 18 named satellites. 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, six good candidates and at least a dozen more have been reported and given provisional designations but it is now likely that only the six recently discovered ones are real.

Saturn's Ring System

Two prominent rings (A and B) and one faint ring (C) can be seen from the 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 Division but this is a misnomer since it was very likely never seen by Encke. The Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings of the other planets, are very bright, with an albedo of 0.2 - 0.6.

Though they look continuous from the 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 kilometer-sized objects are also likely.

Saturn's rings are extraordinarily thin: though they're 250,000 km or more in diameter they're no more than 1.5 kilometers 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 100 km 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 puzzling radial inhomogeneities 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 Division. The whole system is very complex and as yet poorly understood.

Saturn The Ringed Planet

Saturn is the sixth planet from the Sun and the second largest in the solar system. In Roman mythology, Saturn is the god of agriculture. Saturn is the root of the English word "Saturday".

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 the 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 correctly inferred 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 when viewed through a small telescope; 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 the least dense of the planets; its specific gravity of 0.7 is less than that of water. Like Jupiter, Saturn is about 75% hydrogen and 25% 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 consisting 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 12000 K at the core and Saturn 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 (red spot at center of image at left) and other features common on Jupiter. This unique red oval cloud feature is visible in Saturn's southern hemisphere. The difference in color between the red oval and surrounding bluish clouds indicates that material within the oval contains a substance that absorbs more blue and violet light than the bluish clouds. The oval feature did not change in appearance for several months before this photo was taken on November 6, 1980 at a distance of 8,500,000 km (6,300,000 miles).

In 1990, HST observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed.

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