Saturn is the sixth planet from the Sun and the second largest in the Solar System. It is named for the Roman god of agriculture. The planet has been known since prehistoric times. Galileo was the first to observe it with a telescope, in 1610. 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. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings.

Facts and Figures

Exploration

The first spacecraft to visit Saturn was Pioneer 11, in 1979, followed by Voyager 1 and Voyager 2, in November 1980 and August 1981 respectively. Cassini arrived at the planet on July 1, 2004, and will orbit Saturn for at least four years.

Composition, Structure, and Magnetic Field

Saturn is about 75% hydrogen and 25% helium, with traces of water, methane, ammonia and "rock." Its interior is similar to that of Jupiter's, consisting of a rocky core, a liquid metallic hydrogen layer, and a molecular hydrogen layer. Traces of various ices are also present.

Voyager images show whirls and eddies of clouds occurring deep in Saturn's atmosphere. The temperatures of Saturn's cloud tops are close to -176° C (-285° F).

Based on the movements of Saturnian storm clouds, the period of rotation of the atmosphere near the equator is about 10 hours and 11 minutes. Radio emissions that have been detected coming from the body of the planet indicate that the body of Saturn and its magnetosphere rotate with a period of 10 hours and 39 minutes. The approximately 28.5-minute difference between these two times indicates that Saturnian equatorial winds have velocities close to 1,700 km/hr (1,060 mph).

Saturn's magnetosphere consists of a set of doughnut-shaped radiation belts in which electrons and atomic nuclei are trapped. The belts extend to more than 2 million kilometers (1.3 million miles) from the center of Saturn and even farther in the direction away from the sun. The solar wind and Saturn's rings and satellites supply the particles that are trapped in the radiation belts.

Saturn's Rings

Although they look continuous from the Earth, Saturn's 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. The particles appear to be composed primarily of water ice, but may also include rocky particles with icy coatings. The total amount of material in the rings is actually quite small when compared to the overall size of the ring structure. In fact, if the rings were compressed into a single body, it would be no more than about 100 kilometers across. And, even though the outermost ring is more than 250,000 kilometers in diameter, the entire ring system is less than one kilometer thick.

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. The Voyager pictures show four additional faint rings. The outermost F-ring is a complex structure made up of several smaller rings along which "knots" are visible. Scientists speculate that the knots may be either clumps of ring material or mini-moons. Voyager confirmed the existence of radial "spokes" within the rings which had first been reported by amateur astronomers. The nature of the spokes remains a mystery, but may have something to do with Saturn's magnetic field.

Although the origin of Saturn's rings is unknown, scientists generally agree that they are not stable and must be regenerated by ongoing processes, such as the breakup of larger satellites. Saturn may have had rings since its formation, but the current set of rings may be only a few hundred million years old.

Saturn's Satellites

To date, 34 Saturnian moons have been named. Their diameters range from 20 to 5,150 kilometers. They consist mostly of the lighter, icy substances that prevailed in the outer parts of the gas and dust nebula from which the Solar System was formed and where radiation from the Sun could not evaporate the frozen gases.

The five larger inner satellites -- Mimas, Enceladus, Tethys, Dione, and Rhea -- are roughly spherical in shape and composed mostly of water ice. Rocky material may constitute up to 40 percent of Dione's mass. The surfaces of the five are heavily cratered by meteorite impacts.

The outer satellites Hyperion and Iapetus also consist mainly of water ice . Phoebe, the farthest satellite, moves in a retrograde orbit that is highly inclined to Saturn's equator. Phoebe is probably a cometary body that was captured by Saturn's gravitational field.

Titan, Saturn's largest moon, orbits between the inner and outer satellites. Its diameter is 5,150 kilometers, larger even than the planet Mercury. Titan has a nitrogen atmosphere with traces of methane, ethane, acetylene, ethylene, hydrogen cyanide, and carbon monoxide and dioxide. The interior probably consists of equal amounts of rock and water ice.

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Galileo
Christiaan Huygens
Voyager Program
Jupiter
Rhea
Titan
Mercury
Sir William Herschel

Saturn Science Cards