Saturn is a planet known for its majestic, gleaming rings. Saturn also ranks as the second largest planet in the solar system. Only Jupiter is larger. Saturn is the sixth planet from the sun. It was the farthest planet from Earth known to ancient observers. Saturn gets its name from the ancient Roman god of agriculture.
Saturn can be seen from Earth with the unaided eye, but its rings cannot. Viewed through a small telescope, the planet appears as a hazy yellow globe with few visible features. The rings appear as a thin, flat disk.
Saturn’s rings consist of ice particles that travel around the planet. It has seven main rings, which vary greatly in width. The outermost ring, called the E ring, may measure more than 180,000 miles (300,000 kilometers) across. But the rings are so thin that they cannot be seen with Earth-based telescopes when their edge is in direct line with Earth. The rings vary greatly in thickness, but most are probably under 100 feet (30 meters) thick.
In addition to the rings, Saturn has several large, fascinating moons and dozens of smaller satellites. Saturn’s largest moon, Titan, has an atmosphere thicker than that of Earth. Another moon, Enceladus, is geologically active, erupting water ice and organic molecules into space. The planet, together with its moons, rings, and magnetosphere, is sometimes called the Saturn system. The magnetosphere is a region of space dominated by Saturn’s magnetic influence.
The exploration of Saturn by spacecraft began with the flyby of Pioneer 11 in 1979. The twin Voyager 1 and 2 space probes passed through the Saturn system in 1980 and 1981. The space mission Cassini, launched in 1997, arrived in orbit around Saturn in 2004. Dropped by Cassini in late 2004, the Huygens probe touched down on Titan’s surface in 2005.
Characteristics
Astronomers have determined the basic characteristics of Saturn through observations by telescope and spacecraft. Saturn’s mass (amount of matter), density, and gravitational pull all suggest that the planet is a giant ball of hydrogen and helium with no solid surface. Print "Characteristics" subsection
Size. Saturn's diameter at its equator is about 74,900 miles (120,500 kilometers), more than nine times that of Earth and about 85 percent that of Jupiter. Saturn has more than 760 times Earth’s volume. Print "Size" subsection
Mass and density. Despite its huge size, Saturn has only 95 times Earth’s mass. Thus, its density (amount of matter divided by volume) is only about one-eighth that of Earth and just over half that of Jupiter. Saturn’s density is less than that of liquid water. Saturn has a lower density than any other planet in the solar system.
Saturn still has more mass, however, than any other planet in the solar system except Jupiter. Jupiter has more than three times Saturn’s mass. The difference in mass results in the large difference in density between the two planets. Hydrogen and helium are highly compressible—that is, they are easily squeezed to a greater density. The greater mass of Jupiter squeezes the hydrogen and helium much more than does that of Saturn, leading to Jupiter’s larger density.
Due to Saturn’s low density, the gravitational pull of Saturn is similar to Earth’s. A 100-pound object on Earth would weigh about 107 pounds on Saturn. Print "Mass and density" subsection
Orbit. Saturn orbits the sun at an average distance of about 890,750,000 miles (1,433,530,000 kilometers), compared with about 92,960,000 miles (149,600,000 kilometers) for Earth. Saturn takes about 29 1⁄2 Earth years to complete one orbit.
Saturn’s orbit is elliptical (oval-shaped). The planet’s distance from the sun varies from about 941,070,000 miles (1,514,500,000 kilometers) at its farthest point to about 840,440,000 miles (1,352,550,000 kilometers) at its closest point. At its closest approach to Earth, Saturn is about 742,800,000 miles (1,195,500,000 kilometers) away. Print "Orbit" subsection
Rotation. As Saturn travels around the sun, it spins on its axis, an imaginary line connecting its poles. Saturn's axis is tilted about 27 degrees from upright, in relation to its orbit. This tilt is a bit more than Earth’s tilt of 23 degrees, leading to more extreme seasonal variations in sunlight received by the planet.
Saturn rotates faster than any other planet in the solar system except Jupiter. Saturn’s atmosphere obscures a direct measurement of its rotation. But indirect measurements indicate that the planet spins around once in about 10 hours 33 minutes. This time is the length of a day on Saturn. The rapid rotation causes Saturn to bulge at its equator and to flatten at its poles. For this reason, the diameter of Saturn is about 7,337 miles (11,808 kilometers) larger at the equator than between the poles. Print "Rotation" subsection
Temperature. Saturn's tilt causes the sun to heat the planet's northern and southern halves unequally, resulting in temperature changes and seasons. The actual temperature also varies with altitude. At an elevation where the atmospheric pressure is one bar, Saturn averages –218 °F (–139 °C). Print "Temperature" subsection
Structure
Scientists have constructed a model of Saturn’s structure based on the behavior of hydrogen and helium at high pressures. The evidence suggests that the atmosphere hides a liquid interior and a solid core. Print "Structure" subsection
Atmosphere. A dense layer of clouds covers Saturn. Photographs of the planet show bands of varied colors on the cloud tops, called belts and zones. This banded appearance seems to be caused by differences in temperature and altitude among rising and falling gases.
The general haziness of Saturn’s atmosphere makes observing its features difficult. At infrared wavelengths—wavelengths longer than visible light—the haze disappears and the deeper atmosphere is visible. Infrared images taken by the Cassini orbiter show a highly unstable atmosphere with many rotating features. These features somewhat resemble storm systems on Earth. Winds on Saturn have been clocked at up to 1,100 miles (1,770 kilometers) per hour, much faster than on Jupiter.
Saturn receives about 1 percent of the amount of sunlight that Earth does. However, it gives off about two times as much energy as it receives. Some of this energy may be heat created as Saturn contracts under the influence of gravity. Another source of heat may be caused by helium, deep below the surface of Saturn, forming rain drops and falling farther into the center of Saturn.
Like all of Saturn, the atmosphere consists primarily of hydrogen and helium. The atmosphere also contains traces of water, ammonia, methane, and other compounds. Print "Atmosphere" subsection
Interior. Descending through Saturn’s atmosphere, the pressure of gases grows steadily. Hydrogen and helium take on a more viscous (thick) form, eventually merging with the planet’s liquid interior. Saturn’s interior may have a layer of highly compressed, liquid metallic hydrogen. Such hydrogen has taken on some metallic properties under extreme pressures and temperatures. Print "Interior" subsection
Core. Saturn’s spin, shape, and gravitational pull suggest that the planet has a hot, solid inner core. The core probably consists of carbon, iron, oxygen, silicon, and other elements heavier than hydrogen and helium. Print "Core" subsection
Rings
Saturn’s seven main rings surround the planet at its equator but do not touch it. They consist mainly of pieces of ice, ranging from dust-sized grains to chunks more than 10 feet (3 meters) in diameter. Many of the rings feature thin bands of varying brightness, called ringlets. Gaps of several thousand miles or kilometers separate most of the major rings. The gaps contain fewer particles than the rings. Some of the gaps have ringlets. In addition to the major rings, the ring system includes numerous diffuse (thinly spread) ringlets and small moons. Gravitational interactions between Saturn’s inner satellites and the ring particles play a major role in maintaining the structure of the rings, ringlets, and gaps.
Astronomers have assigned letters to the major rings, based roughly on the order of the rings’ discovery. From the closest to Saturn to the farthest away, they are the D, C, B, A, F, G, and E rings. The A, B, C, and D rings each consist of thousands of ringlets.
The A and B rings are the brightest. A dark gap called the Cassini Division separates the two. The gravitational influence of Saturn’s moon Mimas keeps the Cassini Division relatively free of particles. Mimas, which lies inside the E ring, orbits once around Saturn for every two orbits of the particles in the Cassini Division. This simple 2 to 1 ratio results in what is called a 2:1 orbital resonance between the ring particles and the moon. The resonance produces specific gravitational interactions between the moon and the particles, forcing particles out of the Cassini Division. The outer edge of the A ring is maintained by a 7:6 orbital resonance between the ring particles there and the moons Janus and Epimetheus. Within the A ring is a gap, called the Encke gap. It is caused by the presence of the small moon Pan. Pan orbits within the ring, clearing ring particles from its path. The fainter C and D rings lie inside the B ring.
The fairly bright F ring lies just outside the A ring. The F ring consists of a narrow band of ringlets that lie between the orbits of the satellites Prometheus and Pandora. Astronomers sometimes refer to these satellites as shepherd moons, because the moons’ gravitational pulls confine or “herd” the particles of the F ring into a narrow band. At certain points in Prometheus’s eccentric (oblong) orbit, the moon actually enters the ring. Where the moon enters, its gravitational pull creates “kinks” in the ring, giving the F ring a braided appearance.
The faint E and G rings are much thicker than the inner rings, and their particles are spread more thinly. The E ring consists mainly of microscopic particles. Scientists have determined that some of the ring's particles come from geysers on the moon Enceladus. The G ring is thinner than the E ring and contains larger particles. Astronomers think these particles may be dust spreading out from a ring arc (curved region of debris) held in its orbit by the gravity of Mimas.
Beginning with the Voyager missions, astronomers noticed dark markings reaching out from the center of the B ring, called spokes. The spokes appear to result from fine dust being raised above the main ring by electrical and magnetic forces. However, the details of their formation remain poorly understood. Print "Rings" subsection
Satellites
Astronomers have identified at least 61 satellites of Saturn, and many more may await discovery. The largest 8 moons were the first discovered, and they retain the greatest interest.
The largest of Saturn's satellites, Titan, has a diameter of about 3,200 miles (5,150 kilometers)—larger than that of the planet Mercury. Titan is the only satellite in the solar system with a significant atmosphere. In fact, Titan’s atmosphere is 4 times denser at the surface than is Earth’s atmosphere. Titan’s atmosphere consists largely of the gas nitrogen. The second most abundant gas is methane. Methane high in Titan’s atmosphere is broken apart by sunlight and by charged particles from Saturn’s magnetosphere. The fragments recombine to form a variety of carbon- and nitrogen-bearing molecules called hydrocarbons and nitriles. These molecules are of interest to scientists because they serve as some of the basic building blocks of life.
The Huygens probe descended to Titan by parachute in 2005, after the Cassini orbiter released it in late 2004. As the probe fell, it observed channels in the slopes of an icy hillside near its landing site. These channels appear to have been carved by a flow of liquid, most likely methane. Methane, typically a gas on Earth, is stable as a liquid on Titan’s surface. Another stable liquid on Titan is ethane. Both ethane and methane have been observed to form clouds in Titan’s atmosphere. The Cassini orbiter has observed large areas that appear to be lakes or seas. One of these lakes has been observed to contain ethane. Methane may be there as well, but it is difficult to detect.
Enceladus is another of Saturn’s moons of particular interest to scientists. Powerful plumes of water ice and various gases erupt explosively from its south polar region. Water ice and water vapor are the primary materials in the plume. However, Cassini measured other substances, such as carbon monoxide, methane, and other organic (carbon-based) molecules. The plumes are a source of particles for the E ring. Cassini also found sodium in the ice particles of the E ring, which suggests that the plumes might originate in a region with liquid water. The presence of liquid water and organic molecules would suggest a possibility of life on Enceladus.
Mimas’s distinguishing feature is the single impact crater that dominates its surface. The impact crater, named Herschel after the moon’s discoverer, was formed by an impact so large it almost broke the moon into pieces.
Tethys also has an unusually large single impact crater. This moon shares its orbit around Saturn with two smaller moons, Telesto and Calypso. Telesto orbits ahead of Tethys, and Calypso orbits behind it, both at an equal distance. The combined gravitational pulls of Tethys and Saturn hold Telesto and Calypso at these special positions, known as Lagrange points.
Dione also shares its orbit with two smaller moons, Helene and Polydeuces. Helene orbits ahead of Dione, and Polydeuces orbits behind Dione, both near the Lagrange points of Dione and Saturn.
Iapetus also features a number of giant impact craters, several of which are similar in scale to the Herschel crater. This moon features uniquely contrasting bright and dark sides. The bright side is similar in color to dirty snow. The dark side is close to black. The dark side of the moon faces forward in its orbit. Some scientists think the accumulation of debris, possibly from another moon, darkened it. Other scientists think the dark material may come from the interior of Iapetus. Iapetus also has a unique shape, with a pronounced ridge that runs at least 800 miles (1,300 kilometers) along its equator.
Rhea is similar in appearance to several of the other large moons. It has a dull, mostly uniform, gray surface with a few white streaks. Cassini detected what might be thin rings around Rhea. If confirmed, these rings would make Rhea the only known satellite with a ring system among all the planets.
Hyperion is shaped like a squat cylinder rather than a sphere. Unlike Saturn's other satellites, Hyperion's axis does not point toward the planet.
Most of Saturn’s other moons are much smaller. Many have irregular shapes and elongated orbits. Print "Satellites" subsection
History of study
In 1610, the Italian astronomer Galileo Galilei discovered Saturn's rings using one of the first telescopes. Galileo could faintly see bulges of the rings on each side of the planet, but he could not distinguish their true nature. Instead, he concluded that the bulges were two large satellites. In 1656, using a more powerful telescope, the Dutch astronomer Christiaan Huygens described a "thin, flat" ring around Saturn. Huygens thought the ring was a solid sheet of some unknown material. In 1675, the Italian-born French astronomer Giovanni Domenico Cassini discovered that the single ring was actually two, now known as the A and B rings. Cassini correctly guessed that the rings were made up of swarms of satellites. Later observations resulted in the discovery of the other rings. Beginning in the 1970’s, observations by spacecraft revealed the complex structure of the rings and the influences that shape them.
In 1973, the United States launched the space probe Pioneer 11 to study both Saturn and Jupiter. Pioneer 11 flew within 13,000 miles (20,900 kilometers) of Saturn on Sept. 1, 1979. The data and photographs sent back to Earth led to the discovery of the F ring and suggested the existence of the G ring. Pioneer 11 also found that the planet has a magnetic field (region of influence exerted by a magnetic object). The probe also discovered belts of radiation inside the planet's magnetosphere. The belts consist of high-energy electrons and protons. They are comparable to Earth's Van Allen belts.
In 1977, the United States launched two space probes, Voyager 1 and Voyager 2, to study Saturn and other planets. Voyager 1 flew within 77,000 miles (124,000 kilometers) of Saturn on Nov. 12, 1980. On Aug. 26, 1981, Voyager 2 flew within 63,000 miles (101,000 kilometers) of the planet. The Voyager probes confirmed the existence of Saturn's G ring. They also found that the planet's rings are made up of ringlets. In addition, the probes sent back data and photographs that led to the discovery or proved the existence of nine satellites. The Voyager probes determined that the atmosphere of Titan consists chiefly of nitrogen.
In 1997, the United States launched the Cassini probe to study Saturn, its rings, and satellites. The probe began orbiting Saturn in 2004. Images taken by Cassini showed waves of varying density in some of Saturn’s rings. These waves are caused by the gravitational pull of the inner satellites. The probe also discovered new details of Saturn’s weather and a great variety of geologic features on Saturn’s largest moons.
In 2006, scientists studying images taken by Cassini found small propeller-shaped gaps in the A ring, apparently created by bodies about 130 to 390 feet (40 to 120 meters) in diameter. The presence of these “moonlets,” which are too large to have accumulated from particles, supports the theory that the rings formed from a moon-sized body that broke apart. Cassini also discovered the locations of the active geyser systems of Enceladus.
In 2007, Cassini scientists provided firm proof for the presence of a vast hexagonal (six-sided) wind system in the atmosphere at Saturn's north pole. The Voyager space probes had first imaged the feature in the early 1980's. The hexagon measures nearly 15,000 miles (25,000 kilometers) across. On Earth, winds blow in a circle around the poles. Scientists have never observed a hexagonal wind system on any planet other than Saturn, and they still do not know its cause. Print "History of study" subsection
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