Saturn was the outermost of the six planets known to ancient astronomers (the other five being Mercury, Venus, Earth, Mars, and Jupiter). The Creeks named it after the god Cronus, whose Roman equivalent was Saturn. It remained the most distant planet known until William Herschel discovered Uranus in 1781. Saturn is not much smaller than Jupiter, although it appears to be much fainter because it orbits nearly twice as far away from the sun, at a distance of 888,200,000 miles (1,429 million kilometers). Saturn appears to the naked eye as a very slow-moving, pale yellow star—even at favorable oppositions, its magnitude seldom exceeds + 0.7. It takes nearly 30 earthyears to complete its orbit.
Seen through a telescope, Saturn is the most beautiful of all the planets. It is circled by a large, shining, flat ring, which larger telescopes show to be made up of three separate concentric rings. Through telescopes, up to 10 satellites can be seen orbiting the planet, including the giant Titan, which is nearly half the size of the earth.
These telescopic views do not, however, do full justice to this jewel of a planet. When viewed close-up from space (as they were by the American Voyager space probes in 1980 and 1981), the three main rings can be seen each to consist of hundreds of separate ringlets. It is also now known that many more moons orbit the planet; by 1982, at least 17 had been detected.
The appearance of the planet
Seen from the earth, Saturn’s rings appear to change during the time of its orbit. This apparent variation is due to the inclination of the planet’s axis (about 27°) with respect to the plane of its orbit. In 1980, the rings were almost edge-on to our line of sight and they virtually disappeared from view; they did so again in 1995. In 2003, however, they will present their most open aspect and be seen at their best from earth.
Telescopes also reveal that Saturn is primarily a gaseous body that is rotating rapidly. This movement can be inferred from the distinctly oblate appearance of the disk—it bulges outward at the equator and is noticeably flattened at the poles. The diameter is 8,000 miles (13,000 kilometers) greater at the equator than at the poles. From its mass and volume, its average relative density has been calculated to be only about 0.69, which means that it could float in water. It is thus by far the least dense of all the planets.
Saturn’s cloud belts
Like Jupiter, Saturn has a disk that is crossed by alternate light and dark bands parallel to the equator; they are known respectively as zones (light) and belts (dark). These bands are formed by thick clouds in the deep atmosphere that are forced to travel in parallel bands by the rapid rotation of the planet. The belts and zones of Saturn are, however, less distinct than those on Jupiter because of the presence of a thick layer of haze above the tops of the clouds.
The winds that drive the cloud belts reach extremely high speeds of up to 1,100 miles (1,800 kilometers) per hour. Away from the equator, in both hemispheres, the winds decrease in speed symmetrically, and in the middle latitudes, they start to deviate from their main easterly course. At the boundaries where the wind streams change direction, the atmosphere experiences extreme turbulence, and violent storms prevail. These disturbances appear as dark and light oval markings similar to, but less distinctive than, the Great Red Spot and white ovals that appear on Jupiter’s disk.
The winds and storms on Saturn are generated mainly by the planet’s internal heat rather than by heat received from the sun, as happens on Earth. (Saturn receives only about one per cent of the solar heat that Earth does.) Its internal heat is believed to originate in the gravitational separation of, or interaction between, the hydrogen and helium that make up the planet.
At the center of Saturn, there is thought to be a rocky core that may be about twice the size of Earth. Surrounding the core is a layer of highly compressed “metallic” hydrogen, and above that, a layer of ordinary liquid hydrogen mixed with some liquid helium. The outermost layer comprises a very deep atmosphere of gaseous hydrogen and helium.
Temperatures at Saturn’s cloud tops are about —288° F. ( — 178° C). They would be even lower were it not for the heat generated from inside the planet, which causes it to release nearly twice as much heat as it receives from the sun.
Aurorae and radiation belts
The rotation of a massive iron core could account for Saturn’s powerful magnetic field, whose existence was confirmed only in 1979. The strength of this field is about 1,000 times greater than that of the earth. It extends for several million miles into the space around the planet to form a huge, magnetic “bubble,” or magnetosphere, shaped like a teardrop with the broad end facing the sun. The exact size and shape of the magnetosphere varies according to the strength of the solar wind. Unlike Earth, Jupiter, and the sun, in which the magnetic and geographic poles are some 10° apart, Saturn’s magnetic poles nearly coincide with its geographic poles.
The presence of a magnetic field around Saturn gives rise to phenomena similar to those that occur on Earth—aurorae in the polar regions. The magnetosphere is also responsible for the emission of radio waves and for concentrating charged particles into a torus—a ring girdling the planet—analogous to the Van Allen radiation belts that surround the earth.
Saturn’s inner radiation belt is composed mainly of hydrogen ions, oxygen ions, and electrons that probably originate in the splitting up of water from the surfaces of two of Saturn’s satellites, Dione and Tethys. A major source of particles in the outer belt is the atmosphere of Titan—the largest of Saturn’s satellites—which often comes within the planet’s magnetosphere. Titan, a moon, is itself surrounded by a torus of neutral hydrogen.
The ring system
When Galileo first focused his telescope on Saturn in 1610, he reported that the planet looked as though it had “ears,” and considered them to be close-orbiting moons, it was Christiaan Huygens, 45 years later, who provided the correct explanation—that Saturn has a ring around it. Over the years, the “ring” was resolved into three rings, A, B, and C. Then came the discovery, or confirmation, by the American space probes Pioneer 11 (1979), Voyager 1 (1980), and Voyager 2 (1981), of D, E, F, and G rings.
The outermost of the classic rings, Ring A, extends to a distance of some 47,000 miles (76,000 kilometers) from Saturn’s cloud tops. Within this ring is a narrow gap, known as the Encke Division. This gap is difficult to see even with high-powered instruments, but may be glimpsed at the extremities (ansae) of the rings. A much broader gap, the Cassini Division, separates the inner edge of Ring A from the outer edge of Ring B. It is visible in telescopes and measures some 2,000 miles (3,200 kilometers) across. Viewed close-up, these gaps” are not entirely empty, but contain a number of much fainter rings.
Ring B is noticeably brighter than Ring A and about twice as broad. Inside it is the much fainter Ring C, the Crepe or Dusky Ring. Inside Ring C is the very diffuse Ring D, which probably extends right down to Saturn’s atmosphere. On the outskirts of Ring A is the narrow Ring F and, even farther out, the faint and narrow Ring G. This ring is itself surrounded by an even fainter, very broad ring, Ring E, which spans the orbits of the moons Enceladus and Tethys.
Space photographs reveal that the classic rings are each made up of hundreds of separate ringlets, which appear to consist of orbiting particles of various sizes. The diffuse Rings E and F seem to be made up mainly of microscopic dust particles, whereas the particles in Ring A may be up to 10 feet (16 meters) or more across. The composition of the particles is unknown, but they are thought to consist of ice and ice-covered rock. The thicknesses of the rings also vary; they appear to be less than 10 miles (16 kilometers) thick.
The structures observed within the ringlet systems are in general transitory, although large-scale features, such as the Encke and Cassini Divisions, appear to be permanent. Transient features, such as the spokes in Ring B, or white spots, may be caused by traveling density waves triggered off by the passage of Saturn’s moons. The eccentric, or braided, ringlets observed near clear gaps in the rings may be caused by the presence of undetected moonlets. In a similar way, the braiding of the narrow Ring F appears to be due to the presence of two tiny “shepherd” moons, one on each side.
Saturn has at least 18 satellites (of which 9 are very large) that have been observed quite attentively, and named. Titan is the largest of the satellites and was also the first to be discovered, in 1665. It was once believed to be the largest satellite in the solar system, but since the Voyager probes, it has been found to be slightly smaller than Jupiter’s largest satellite, Ganymede. Enceladus, although one of the smaller satellites, is the brightest and is one of the most reflective bodies in the solar system with an albedo of almost 100 per cent. Tethys is similar to the planet Jupiter in that it has two bodies on its orbital plane, one located 60° ahead, and the other the same distance behind it. This configuration is known as the Lagrangian arrangement. Some minor satellites are “shepherd” satellites and others are coorbital—their orbits are almost identical, but they periodically overtake each other and interchange their orbits; Epimetheus and Janus are examples of this co-orbital relationship.