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Saturn (planet), sixth planet in order of distance from the sun, and the second largest in the solar system. Saturn's most distinctive feature is its ring system, which was first seen in 1610 by Italian scientist Galileo, using one of the first telescopes. He did not understand that the rings were separate from the body of the planet, so he described them as handles (ansae).The Dutch astronomer Christiaan Huygens was the first to describe the rings correctly. In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote a series of letters in code, which when properly arranged formed a Latin sentence that read in translation, “It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic.” The rings are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G, and E rings. These rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet.

As seen from earth, Saturn appears as a yellowish object—one of the brightest in the night sky. Observed through a telescope, the A and B rings are easily visible, whereas only under optimal conditions can the D and E rings be seen. Sensitive earth-based telescopes have detected nine satellites, and in the haze of Saturn's gaseous envelope, pale belts and zones parallel to the equator can be distinguished.

Three United States spacecraft have enormously increased knowledge of the Saturnian system. The Pioneer 11 (see Pioneer) probe flew by in September 1979, followed by Voyager 1 in November 1980 and Voyager 2 (see Voyager) in August 1981. These spacecraft carried cameras and instruments for analyzing the intensities and polarizations of radiation in the visible, ultraviolet, infrared, and radio portions of the electromagnetic spectrum (see Electromagnetic Radiation). The spacecraft were also equipped with instruments for studying magnetic fields and for detecting charged particles and interplanetary grains.

The National Aeronautics and Space Administration (NASA) launched an orbiter called the Cassini spacecraft toward Saturn in October 1997. It should reach Saturn in 2004, when it will begin studying Saturn and its moons, launching a probe (the Huygens Probe) into the atmosphere of Saturn's moon Titan.

The mean density of Saturn is eight times less than that of Earth because the planet consists mainly of hydrogen. The enormous weight of Saturn's atmosphere causes the atmospheric pressure to increase rapidly toward the interior, where the hydrogen gas condenses into a liquid. Closer to the center of the planet, the liquid hydrogen is compressed into metallic hydrogen, which is an electrical conductor. Electrical currents in this metallic hydrogen are responsible for the planet's magnetic field. At the center of Saturn, heavy elements have probably settled into a small rocky core with a temperature close to 15,000° C (27,000° F). Both Jupiter and Saturn are still settling gravitationally, following their original accretion from the gas and dust nebula from which the solar system was formed more than 4.7 billion years ago. This contraction generates heat, causing Saturn to radiate into space three times as much heat as it receives from the sun.

Characteristics of Saturn

Saturn's atmospheric constituents are, in order by mass, hydrogen (88 percent) and helium (11 percent); and traces of methane, ammonia, ammonia crystals, and such other gases as ethane, acetylene, and phosphine comprise the remainder. Voyager images showed whirls and eddies of clouds occurring deep in a haze that is much thicker than that of Jupiter because of Saturn's lower temperature. The temperatures of Saturn's cloud tops are close to -176° C (-285° F), about 27° C (49° F) lower than such locations on Jupiter.

Based on the movements of Saturnian storm clouds, the period of rotation of the atmosphere near the equator is about 10 hr 11 min. 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 hr 39 min 25 sec. The approximately 28.5-min difference between these two times indicates that Saturnian equatorial winds have velocities close to 1700 km/h (1060 mph).

In 1988, from studies of Voyager photos, scientists determined an odd atmospheric feature around Saturn's north pole. What may be a standing-wave pattern (see Wave Motion), repeated six times around the planet, makes cloud bands some distance from the pole appear to form a huge, permanent hexagon.

Saturn's magnetic field is substantially weaker than that of Jupiter, and Saturn's magnetosphere is about one-third the size of Jupiter's. 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 km (1.3 million mi) from the center of Saturn and even farther in the direction away from the sun, although the size of the magnetosphere fluctuates, depending on the intensity of the solar wind (the flow of charged particles from the sun). The solar wind and Saturn's rings and satellites supply the particles that are trapped in the radiation belts. The rotation period of 10 hr 39 min 25 sec for Saturn's interior was measured by Voyager 1 while passing through the magnetosphere, which rotates in synchrony with the interior of Saturn. The magnetosphere interacts with the ionosphere, the topmost layer of Saturn's atmosphere, causing auroral emissions of ultraviolet radiation.

Surrounding the Saturnian satellite Titan and Titan's orbit, and extending to the orbit of Saturn's moon Rhea, is an enormous doughnut-shaped cloud of neutral hydrogen atoms. A disk of plasma, composed of hydrogen and possibly oxygen ions, extends from outside the orbit of the moon Tethys almost to the orbit of Titan. The plasma rotates in nearly perfect synchrony with Saturn's magnetic field.

The visible rings stretch out to a distance of 136,200 km (84,650 mi) from Saturn's center, but in many regions they may be only 5 m (16.4 ft) thick. They are thought to consist of aggregates of rock, frozen gases, and water ice ranging in size from less than 0.0005 cm (0.0002 in) in diameter to about 10 m (33 ft) in diameter—from dust to boulder size. An instrument aboard Voyager 2 counted more than 100,000 ringlets in the Saturnian system.

The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini. Voyager's television showed five new faint rings within Cassini's division. The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that have ripples of varying density. The gravitational interaction between rings and satellites, which causes these density waves, is still not completely understood. The B ring appears bright when viewed from the side illuminated by the sun, but dark on the other side because it is dense enough to block most of the sunlight. Voyager images have also revealed radial, rotating spokelike patterns in the B ring.

Saturn has 18 confirmed moons and as many as 14 proposed new, unconfirmed moons. In the past many proposed new moons have turned out to be just dense spots in Saturn's rings, but the Cassini spacecraft should be able to definitively catalog Saturn's moons. The diameters of Saturn's satellites range from 20 to 5150 km (12 to 3200 mi). 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 distant 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. Enceladus has a smoother surface than the others, the least cratered area on its surface being less than a few hundred million years old. (Possibly Enceladus is still undergoing tectonic activity; see Plate Tectonics.) Astronomers suspect that Enceladus supplies particles to the E ring, which neighbors Enceladus's orbit. Mimas, far from being smooth, displays an impact crater the diameter of which is one-third of the diameter of the satellite itself. Tethys also bears a large crater and a valley 100 km (62 mi) in width that stretches more than 2000 km (1200 mi) across the surface. Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces. Some scientists conjecture these were caused either by ice ejected from craters by meteorites, or by fresh ice that has migrated from the interior.

Several small satellites have been discovered immediately outside the A ring and close to the F and G rings. Possibly four so-called Trojan satellites of Tethys and one of Dione have also been discovered. Trojan satellites occur in regions of stability that lead or follow a body in its orbit around a massive central body, in this case, Saturn. See Solar System.

The outer satellites Hyperion and Iapetus also consist mainly of water ice. Iapetus has a very dark region in contrast to most of its surface, which is bright. This dark region and the rotation of the satellite are the cause of the variations of brightness that were noticed by Cassini in 1671. Phoebe, the farthest satellite, moves in a retrograde orbit (in the opposite direction of the orbits of the other satellites) that is at a sharp angle to Saturn's equator. Phoebe is probably a cometary body captured by Saturn's gravitational field.

Between the inner and outer satellites orbits Titan, Saturn's largest moon. Scientist believe its diameter is 5150 km (3200 mi), larger even than the planet Mercury. The exact diameter of Titan, exclusive of its atmosphere, is not known, because a dense orange haze hides the surface. The thickness of Titan's atmosphere is probably about 300 km (about 186 mi). Titan has a nitrogen atmosphere with traces of methane, ethane, acetylene, ethylene, hydrogen cyanide, and carbon monoxide and dioxide. On the surface, the temperature is about -182° C (-296° F), and methane or ethane may be present in the forms of rain, snow, ice, and vapor. The interior of Titan probably consists of equal amounts of rock and water ice. No magnetic fields have been detected. The southern hemisphere is slightly brighter, and the only detail visible is a dark ring in the northern polar region. Much more will be known about Titan after the visit of the Cassini spacecraft and the Huygens Probe in 2004.