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Saturn (in astronomy)

Saturn, in astronomy, 6th planet from the sun.

Astronomical and Physical Characteristics of Saturn

Saturn's orbit lies between those of Jupiter and Uranus; its mean distance from the sun is c.886 million mi (1.43 billion km), almost twice that of Jupiter, and its period of revolution is about 291/2 years. Saturn appears in the sky as a yellow, starlike object of the first magnitude. When viewed through a telescope, it is seen as a golden sphere, crossed by a series of lightly colored bands parallel to the equator.

Saturn, like the other Jovian planets (Jupiter, Uranus, and Neptune), is covered with a thick atmosphere composed mainly of hydrogen and helium, with some methane and ammonia; its temperature is believed to be about -270°F (-168°C), suggesting that the ammonia is in the form of ice crystals that constitute the clouds. Like Jupiter's interior, Saturn's consists of a rocky core, a liquid metallic hydrogen layer, and a molecular hydrogen layer. Traces of various ices have also been detected. The wind blows at high speeds—reaching velocities of 1,100 mph (1,770 kph)—across Saturn. The strongest winds are found near the equator and blow mostly in an easterly direction. At higher latitudes, the velocity decreases uniformly and the winds counterflow east and west. Because no permanent markings on the planet are visible, the planet's exact period of rotation has not been determined. However, the period of each atmospheric band varies from 10 hr 14 min at the equator to about 10 hr 38 min at higher latitudes. This rapid rotation causes the largest polar flattening among the planets (over 10%). Saturn is the second largest planet in the solar system; its equatorial diameter is c.75,000 mi (120,000 km), and its volume is more than 700 times the volume of the earth. Its mass is about 95 times that of the earth, making Saturn the only planet in the solar system with a density less than that of water. Saturn has been encountered by four space probe missions: Pioneer 11 (1979), Voyager 1 (1980), Voyager 2 (1981), and Cassini and Huygens (2004). Among the discoveries made by the Voyager probes was a magnetosphere (a region of charged particles consisting primarily of electrons, protons, and heavy ions captured partly from the atmosphere of the satellite Titan) that encloses 13 of Saturn's satellites and its ring system. Huygens landed on Saturn's moon Titan in 2005 and returned photographs of its surface.

The Ring System

Saturn's most remarkable feature is the system of thin, concentric rings lying in the plane of its equator. Although first observed by Galileo in 1610, it was not until 1656 that the rings were correctly interpreted by Christiaan Huygens, who did not reveal his findings about their phases and changes in shape until his treatise Systema Saturnium was published in 1659. Saturn's rings were believed to be unique until 1977, when very faint rings were found around Uranus; shortly thereafter faint rings were also detected around Jupiter and Neptune.

Although the main ring system is almost 167,770 mi (270,000 km) in diameter, it is only some 330 ft (100 m) thick. From earth, this system appears to consist mainly of two bright outer rings, denoted A and B (lettered from the outermost), separated by a dark rift—discovered by the Italian-French astronomer Gian Domenico Cassini—known as Cassini's division, plus a third, faint inner crepe ring (denoted C). The Encke Division, or Encke Gap, which splits the A ring, is named after the German astronomer Johann Franz Encke, who discovered it in 1837. In 1859 the Scottish physicist James Clerk Maxwell showed that the main rings must consist of countless tiny particles each orbiting the planet in accordance with the laws of gravitation. In the 1980s pictures from the Voyager probes showed four additional rings. The exceedingly faint D ring lies closest to the planet. The faint F Ring is a narrow feature just outside the A Ring. Beyond that are two far fainter rings named G and E. In 2009 an enormous but faint ring consisting of tiny dust particles was discovered extending from 3.7 to 7.4 million mi (6 to 12 million km) away from Saturn. Lying at a 27° angle to the main rings, this ring has a retrograde orbit and is believed to have originated in material ejected from the moon Phoebe by small impacts. When edgewise to the earth Saturn's main rings appear as a nearly imperceptible ribbon of light across the planet; this occurs twice during the 291/2-year period of revolution. Twice during each orbit the rings reach a maximum inclination to the line of sight, once when they are visible from above and once when visible from below.

The Voyager 1 (1980) and 2 (1981) space probes revealed incredible new detail as they passed within 78,000 mi (126,000 km) and 63,000 mi (101,000 km) of Saturn, respectively. They recorded hundreds of tiny rings that are grouped into the seven major rings. The three brightest rings (A, B, and C) dissolved into more than 1,000 narrow ringlets, 100 of which are in the Cassini division. The outer F ring was found to contain braids, knots, and strands, possibly caused by nearby moons that shepherd it, that is, limit the extent of a planetary ring through gravitational forces. The main rings are believed to have been formed mainly from larger satellites that were shattered by the impact of comets and meteoroids; geyserlike eruptions on Enceladus contribute material to the E ring. The Cassini revealed that the rings consist mainly of water ice.

The Satellite System

Saturn has 61 confirmed natural satellites, 52 of which are named. Because the increasing number of satellites makes it difficult to continue to name them after Greek Titans, a scheme was adopted for the outer satellites. These are now named after the giants of other cultures: Inuit, Norse, and Gallic. The satellites may be divided into nine groups for convenience. In the order of their distance from Saturn, the groups are shepherd (satellites whose orbit is within or just beyond Saturn's ring system), co-orbital (two satellites that share the same orbit and trade positions within it on a regular basis), inner large (large satellites within the E ring), Alkyonide (small satellites within the inner large group), Trojan (satellites that are co-orbital at Lagrangian points), outer large (large satellites beyond the E ring), and Inuit, Norse, and Gallic (each a group of outer satellites that have similar orbits).

Five of the six confirmed shepherd satellites, Pan, Daphnis, Atlas, Prometheus, and Pandora, are named. The co-orbital group comprises Epimetheus and Janus, but the shepherds Prometheus and Pandora also share an orbit. The inner large group comprises four satellites, Mimas, Enceladus, Tethys, and Dione; the three Alkyonides (Methone, Anthe, and Pallene) have orbits between Mimas and Enceladus. The Trojan group, also found within the inner large group, comprises four satellites, Telesto, Calypso, Helene, and Polydeuces. The outer large group comprises four satellites, Rhea, Titan, Hyperion, and Iapetus. The Inuit group comprises five satellites, Kiviuq, Ijiraq, Paaliaq, Siarnaq, and Tarqeq. Of the 29 satellites comprising the Norse group, only 21 are named: Phoebe, Skathi, Skoll, Greip, Hyrrokkin, Jamsaxa, Mundilfari, Bergelmir, Narvi, Suttungr, Hati, Farbauti, Thrymr, Aegir, Bestia, Fenrir, Surtur, Kari, Ymir, Loge, and Fornjot. The Gallic group consists of four satellites, Albiorix, Bebhionn, Erriapus, and Tarvos.

Almost all of Saturn's inner moons form a regular system of satellites; that is, their orbits are nearly circular and lie in the equatorial plane of the planet; almost all of the outer moons' orbits are inclined. Except for Hyperion, which has a chaotic orbit, and Phoebe, all the satellites are believed to have synchronous orbits; that is, their orbital and rotational periods are the same, so that they always keep the same face turned toward Saturn. The largest satellite, Titan, is 3,200 mi (5,150 km) in diameter and has the size and cold temperatures necessary to retain an atmosphere; it is the only natural satellite in the solar system with a substantial atmosphere.

Saturn has six major icy satellites that can be easily seen through earth-based telescopes. The most prominent feature of heavily cratered Mimas, the innermost of the six, is a large impact crater about one third the diameter of the satellite. Certain broad regions of Enceladus are uncratered, indicating geological activity that has resurfaced the satellite within the last 100 million years. Tethys also has a very large impact crater, as well as an extensive series of valleys and troughs that stretches three quarters of the way around the satellite. Both Dione and Rhea have bright, heavily cratered leading hemispheres and darker trailing hemispheres with wispy streaks that are thought to be produced by deposits of ice inside surface troughs or cracks. Iapetus, the outermost of the large icy satellites, has a dark leading hemisphere and a bright trailing hemisphere.

The remaining satellites are smaller. The two largest of these, the dark-surfaced Phoebe and the irregularly shaped Hyperion, orbit far from the planet; the Norse group of satellites orbit with retrograde motion, i.e., opposite to that of the planet's rotation. The smallest satellites, less than c.6 mi (10 km) in diameter, include Daphnis, the Alkyonides, Polydeuces, some of the Inuit and Gallic groups, and nearly all of the Norse group.

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Saturn, the sixth planet from the Sun, revolves around the Sun in a slightly elliptical orbit at a mean distance of 1.4294 billion kilometers (888,188,000 miles) in 29.42 years. Perhaps best known for its rings, Saturn also has a large collection of moons orbiting around it.

Physical and Orbital Properties

One of four gas giant outer planets (along with Jupiter, Uranus, and Neptune), Saturn is the second most massive planet in the solar system. It has a mass equivalent to 95.159 times Earth's and possesses an atmosphere composed primarily of the gases hydrogen and helium (by mass, comprising approximately 78 percent and 22 percent of the atmosphere, respectively).

It is the trace elements and their compounds that give the planet its golden color and the faint banded structure of the cloud tops in its lower-most stratosphere . Methane, ethane, other carbon compounds, and ammonia are observed in the atmosphere. Winds can exceed 450 meters per second (1,000 miles per hour). There is no solid surface beneath the clouds. With depth, the atmosphere slowly thickens from gas to liquid. At very great depths, liquid hydrogen may be compressed enough to become metallic. Saturn has a molten core of heavy elements including nickel, iron, silicon, sulfur, and oxygen, which totals as much as three Earth-masses.

Saturn's magnetic field is much like the field of a simple bar magnet and similar to the planetary magnetic fields of Earth, Jupiter, Uranus, and Neptune. But its near-perfect alignment with the planet's rotation axis makes its origin mysterious. The magnetic field governs Saturn's huge, tadpole-shaped magnetosphere , the volume of space controlled by Saturn rather than by the interplanetary magnetic field.

Saturn is the second largest planet in the solar system. Its equatorial diameter is 120,660 kilometers (74,975 miles). Saturn rotates rapidly, having a day lasting only 10 hours and 39.9 minutes. The centrifugal force of this rapid rotation forces the planet to look slightly squashed: its polar diameter is 108,831 kilometers (67,624 miles). Saturn's axis of rotation is inclined to the plane of its orbit by 25.2 degrees, much like Earth's inclination of 23.4 degrees. Like Earth, Saturn has seasons and it constantly changes its presentation to Earth over its long orbit. Weather on Saturn is controlled not by its seasons or the Sun but by the flow of heat from inside the planet. This outward heat flow exceeds the heat received from the Sun by a factor of about three. Its origin is still being investigated.

The combination of Saturn's mass and volume leads to an average density unique in the solar system: at 0.70 grams per cubic centimeter it is less dense than water (1 gram per cubic centimeter). Because of the planet's large size, the force of gravity at Saturn's cloud tops is only 1.06 times Earth's. Nevertheless, to escape from Saturn, a rocket launched from its cloud tops would have to achieve a speed of 35.5 kilometers per second (22 miles per second), more than three times Earth's escape velocity of 11.2 kilometers per second (7 miles per second).

The Rings of Saturn

Italian mathematician and astronomer Galileo Galilei noted Saturn's odd telescopic appearance in 1610, but Dutch astronomer Christiaan Huygens, who had discovered Saturn's largest moon, Titan, in 1655, was the first to identify it as a ring in 1659. Huygens also demonstrated how the ring plane was tilted, explaining the odd behavior seen over the previous decades.

Italian-born French astronomer Giovanni Domenico Cassini noted a gap within Huygens's single ring in 1675. Now called the Cassini division, this gap separates the outer A ring from the inner B ring. The C ring, inside the others, was discovered in 1850. More than a century later, hints of the D ring were found (and then confirmed by the spacecraft Voyager 1 in 1980), and in 1966 the E ring was observed. The Pioneer 11 spacecraft discovered the F and G rings in 1979. In order outward from the planet, the rings are D, C, B, A, F, G, E. (See table below.)

While Saturn's main rings span a huge distance, they are less than 1 kilometer (0.6 mile) thick and their plane is slightly warped. Ring particles in the main rings range in size from a few tens of meters across down to the size of smoke particles, about 1 micrometer (10 -6 meter). The E ring is different, being composed of small particles that orbit within a much thicker volume.

The Satellite System of Saturn

Saturn's system of satellites (moons) is notable, ranging from inside the A ring to almost 13 million kilometers (about 8 million miles) from the planet. The classical nine largest moons were discovered between 1655 (Titan) and 1898 (Phoebe). With the rings nearly invisible during the ring plane crossing of 1966, two additional co-orbital (sharing an orbit) moons were discovered, situated between the F and G rings.

Observations in 1980-1981 by the Voyager spacecraft added more moons. Besides an A-ring shepherd moon (which limits the outer edge of the ring) and one in the A ring's Encke gap, small moons trapped in gravitationally

Ring Designation Distance from Saturn
km Rs
Saturn Radius, Rs 60,330 1.00
D (inner edge) 66,970 1.11
C (inner edge) 74,510 1.24
B (inner edge) 92,000 1.53
B (outer edge) 117,580 1.95
(Cassini Division)
A (inner edge) 122,170 2.03
A (ring gap center) 133,400 2.21
A (outer edge) 136,780 2.27
F (center) 140,180 2.32 (width 50 km)
G (center) 170,180 2.82 (width variable)
E (inner edge) ~181,000 ~3
E (outer edge) ~483,000 ~8

stable points (called Lagrangian points, L4 and L5) in the orbits of two of the larger moons were discovered. By 1990 Saturn's satellite count had reached eighteen.

State-of-the-art telescopes and techniques increased Saturn's moon count during the last half of 2000. Twelve additional, tiny outlying satellites were discovered, with additional ones awaiting confirmation. Saturn's total moon count thus reached thirty and was likely to increase further. Some of these small, distant, outer moons orbit Saturn backwards compared to its rotation direction, as Phoebe does, whereas others move in the same direction as the rotation but have orbits highly inclined to Saturn's equator.

Among the classical set of icy satellites, Enceladus and Iapetus are particularly noteworthy. Enceladus, with a diameter of only 498 kilometers (310 miles), is the most reflective solid body in the solar system. Surprisingly for a small, cold moon, the Voyager spacecraft showed that large areas of its surface have recently (over a small fraction of the age of the solar system) melted. Interestingly, the E ring has its maximum density at the same orbital distance as Enceladus.

Iapetus, second largest of the icy moons (and third overall, at 1,436 kilometers [892 miles]), has one hemisphere that reflects as well as snow, whereas its other hemisphere is blacker than asphalt.

In a class by itself is the giant moon Titan. Its diameter of 5,150 kilometers (3,200 miles) exceeds that of the planet Mercury. It has a nitrogen (plus methane) atmosphere, like Earth's (nitrogen plus oxygen), but with a surface pressure about 1.5 times Earth's air pressure at sea level. Titan may be a deep-frozen copy of what Earth was like shortly after its formation.

Beginning in 2004, the Cassini spacecraft and Huygens probe will explore Saturn and Titan. Our understanding of the fascinating and mysterious Saturnian system will increase enormously.

see also Cassini, Giovanni Domenico (volume 2); Exploration Programs (volume 2); Galilei, Galileo (volume 2); Huygens, Christiaan (volume 2); Jupiter (volume 2); NASA (volume 3); Robotic Exploration of Space (volume 2); Planetary Exploration, Future of (volume 2).

Stephen J. Edberg


Bishop, Roy, ed. Observer's Handbook, 2000. Toronto: Royal Astronomical Society of Canada, 1999.

Edberg, Stephen J., and Lori L. Paul, eds. Saturn Educators Guide. Washington, DC:NASA, 1999. Also available at <>.

Spilker, Linda J., ed. Passage to a Ringed World. Washington, DC: National Aeronautics and Space Administration, 1997.

Internet Resources

The Cassini Mission to Saturn (fact sheet). Pasadena, CA: Jet Propulsion Laboratory400-842, rev. 1, 1999. <>.

Saturnian Satellite Fact Sheet. National Space Science Data Center.<>.

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Saturn, the sixth planet from the Sun, is named for the Roman god of agriculture, who was based on the Greek god Cronus. The second largest planet in the solar system, it measures almost 75,000 miles (120,600 kilometers) in diameter at its equator. Despite its large size, Saturn is the least dense of all the planets. It is almost 30 percent less dense than water; placed in a large-enough body of water, Saturn would float.

Saturn completes one rotation on its axis very quickly, roughly 10.5 Earth hours. As a result of this spinning, the planet has been flattened at its poles. The measurement around its equator is 10 percent greater than the measurement around the planet from pole to pole. In contrast to the length of its day, Saturn has a very long year. Lying an average distance of 887 million miles (1.4 billion kilometers) from the Sun, Saturn takes 29.5 Earth years to complete one revolution.

Saturn consists primarily of gas. Its hazy yellow clouds are made of crystallized ammonia, swept into bands by fierce, easterly winds that have been clocked at up to a speed of 1,100 miles (1,770 kilometers) per hour at its equator. Winds near the poles, however, are much tamer. Covering Saturn's surface is a sea of liquid hydrogen and helium that gradually becomes a metallic form of hydrogen. This sea conducts strong electric currents that, in turn, generate the planet's powerful magnetic field. Saturn's core, which is several times the size of Earth, is made of rock and ice. The planet's atmosphere is composed of about 97 percent hydrogen, 3 percent helium, and trace amounts of methane and ammonia. Scientists estimate the surface temperature to be about 270°F (168°C).

About every 30 years, following Saturn's summer, a massive storm takes place on the planet. Known as the Great White Spot, it is visible for nearly a month, shining like a spotlight on the planet's face. The spot then begins to break up and stretch around the planet as a thick white strip. The storm is thought to be a result of the warming of the

atmosphere, which causes ammonia to bubble up, solidify, and then be whipped around by the planet's monstrous winds.

Saturn's rings

Saturn's most outstanding characteristic are its rings. The three other largest planets (Jupiter, Uranus, and Neptune) also have rings, but Saturn's are by far the most spectacular. For centuries, astronomers thought the rings were moons. In 1658, Dutch astronomer Christiaan Huygens first identified the structures around Saturn as a single ring. In later years, equipped with stronger and stronger telescopes, astronomers increased the number of rings they believed surrounded the planet.

In 1980 and 1981, the Voyager 1 and Voyager 2 space probes sent back the first detailed photos of Saturn and its spectacular rings. The probes revealed a system of over 1,000 ringlets encircling the planet at a distance of 50,000 miles (80,450 kilometers) from its surface.

The rings, which are estimated to be one mile (1.6 kilometers) thick, are divided into three main parts: the bright A and B rings and the dimmer C ring. The A and B rings are divided by a gap called the Cassini Division, named for it discoverer, seventeenth-century French astronomer Giovanni Domenico Cassini. The A ring itself contains a gap, called the Encke Division after German astronomer Johann Encke, who discovered it in 1837. The Encke Division contains no matter, but the Voyager missions found that the Cassini Division contains at least 100 tiny ringlets, each composed of countless particles. Voyager confirmed the existence of puzzling radial lines 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.

While scientists do not know the full composition of the rings, they do know that the rings contain dust and a large quantity of water. The water is frozen in various forms, such as snowflakes, snowballs, hailstones, and icebergs. The forms range in size from about 3 inches (7.6 centimeters) to 30 feet (9 meters) in diameter. Scientists are also not sure how the rings were formed. One theory states that they were once larger moons that were smashed to tiny pieces by comets or meteorites. Another theory holds that the rings are pre-moon matter, cosmic fragments that never quite formed a moon.

Saturn's moons

Saturn has 18 known moons that have received officially sanctioned names from the International Astronomical Union. In late 2000, astronomers detected up to twelve possible new moons orbiting the planet, some at a distance between 6.2 and 12.4 million miles (10 and 20 million kilometers). These have all been given provisional designations, but scientists believe only six out of the twelve may turn out to be real moons. All the known moons are composed of about 30 to 40 percent rock and 60 to 70 percent ice. All but two have nearly circular orbits and travel around Saturn in the same plane.

Christiaan Huygens discovered Saturn's first moon Titan, in 1655. It is the only moon in the solar system with a substantial atmosphere, which is composed mainly of nitrogen. Voyager 1 revealed that Titan may have seas of liquid methane bordered by organic tarlike matter. Titan's thick blanket of orange clouds, however, prevent a direct view of the surface.

Cassini mission to Saturn

The Cassini orbiter, which was launched in October 1997, will deliver much more information about Saturn and its moons. With a budget of $3.4 billion, it is the last of the National Aeronautics and Space Administration's (NASA) big-budget, big-mission planetary probes. Cassini, which weighs nearly 13,000 pounds (5,900 kilograms), carries 18 scientific instruments that will take a variety of measurements of Saturn's atmosphere, its moons, and the dust, rock, and ice that comprise its rings. After traveling some 2.2 billion miles (1 billion kilometers), the orbiter is scheduled to arrive at Saturn in mid-2004. It carries with it a probe, called Huygens, that was built by the European Space Agency. The probe will drop onto the surface of Titan for a detailed look at the moon's surface. If it survives the impact of its landing, Huygens will transmit data from the surface back to Cassini for up to 30 minutes. After releasing the probe, Cassini will orbit Saturn at least 30 times over a four-year period, gathering information and sending back more than 300,000 color images taken with an onboard camera.

[See also Solar system ]

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Saturn Sixth planet from the Sun and second-largest in the Solar System. Viewed through a telescope, it appears as a flattened golden yellow disk encircled by white rings. The rings are made up of particles ranging from dust to objects a few metres in size, all in individual orbits. The main rings are only a kilometre or so thick. Voyager space probes revealed the ring system to be made up of thousands of separate ringlets. Saturn has an internal heat source, which probably drives its weather systems. It is assumed to be composed predominantly of hydrogen, and to have an iron-silicate core about five times the Earth's mass, surrounded by an ice mantle of perhaps twenty Earth masses. The upper atmosphere contains 97% hydrogen and 3% helium, with traces of other gases.;

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Saturn The sixth planet in the solar system, distant 9.52 AU from the Sun. Its radius is 60 000km, density 704 kg/m3, mass 95 × Earth mass, volume 833 × Earth volume, and it has an equatorial inclination to the ecliptic of 29°. An outer zone of hydrogen and helium is underlain by a zone of metallic hydrogen, around an ice—silicate core. It has 17 known satellites and is famous for its ring system.

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