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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

Saturn

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

THE RINGS OF SATURN
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

Bibliography

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 <http://www.jpl.nasa.gov/cassini/english/teachers/guides/educatorguide>.

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. <http://saturn.jpl.nasa.gov/cassini/english/teachers/factsheets/casini_msn.pdf>.

Saturnian Satellite Fact Sheet. National Space Science Data Center.<http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html>.

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Saturn

Saturn

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

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.

http://lpl.arizona.edu/nineplanets/nineplanets/saturn.html; http://wr.usgs.gov

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Saturn

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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Saturn

Saturn in Roman mythology, an ancient god (Latin Saturnus may come from Etruscan), originally regarded as a god of agriculture, but in classical times identified with the Greek Cronus, deposed by his son Zeus (Jupiter). His festival in December, Saturnalia, eventually became one of the elements in the traditional celebrations of Christmas.

Saturn was the name given to the most remote of the seven planets known to ancient astronomy (now known to be the sixth planet from the sun in the solar system). In astrology, on account of its remoteness and slowness of motion, Saturn was supposed to cause coldness, sluggishness, and gloominess of temperament in those born under its influence, and in general to have a baleful effect on human affairs.

Saturn is also the name of a series of American space rockets, of which the very large Saturn V was used as the launch vehicle for the Apollo missions of 1969–72.

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Saturn

Saturn

Saturn, the Roman god of agriculture, was identified with the Greek god Cronus. In Roman mythology, Saturn fled Greece and settled in Italy after losing a battle with Jupiter*. Saturn became the king of Latium (the area of central Italy that includes Rome) and ruled over a golden age of peace and prosperity. During this time, he taught the people how to plant and tend crops and how to lead civilized lives.

His festival was the Saturnalia, a celebration beginning on December 17 and ending December 25. During Saturnalia, businesses closed, people exchanged presents, and slaves were given the freedom to do and say what they wished. Christians later honored the last day of Saturnalia as the date of the birth of Jesus. Saturn also gave his name to the day of the week known as Saturday.

See also Cronus.

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Saturn

Saturnbaton, batten, fatten, flatten, harmattan, Manhattan, Mountbatten, paten, patten, pattern, platen, Saturn, slattern •Shackleton • Appleton •Hampton, Northampton, Rockhampton, Southampton, Wolverhampton •Canton, lantern, Scranton •Langton, plankton •Clapton •Aston, pastern •Gladstone •Caxton, Paxton •capstan • Ashton • phytoplankton •Akhenaten, Akhetaten, Aten, Barton, carton, Dumbarton, hearten, Parton, smarten, spartan, tartan •Grafton •Carlton, Charlton •Charleston • kindergarten •Aldermaston •Breton, jetton, Sowetan, threaten, Tibetan •lectern •Elton, melton, Skelton •Denton, Fenton, Kenton, Lenten, Trenton •Repton •Avestan, Midwestern, northwestern, Preston, southwestern, western •sexton •Clayton, Deighton, Leighton, Paton, phaeton, Satan, straighten, straiten •Paignton • Maidstone •beaten, Beaton, Beeton, Cretan, Keaton, neaten, Nuneaton, overeaten, sweeten, uneaten, wheaten •chieftain •eastern, northeastern, southeastern •browbeaten • weatherbeaten •bitten, bittern, Britain, Briton, Britten, handwritten, hardbitten, kitten, Lytton, mitten, smitten, underwritten, witan, written •Clifton •Milton, Shilton, Stilton, Wilton •Middleton • singleton • simpleton •Clinton, Linton, Minton, Quinton, Winton •cistern, Liston, piston, Wystan •brimstone • Winston • Kingston •Addington • Eddington •Workington •Arlington, Darlington •skeleton •Ellington, wellington •exoskeleton •cosmopolitan, megalopolitan, metropolitan, Neapolitan •Burlington • Hamilton • badminton •lamington • Germiston • Penistone •Bonington • Orpington • Samaritan •Carrington, Harrington •sacristan • Festschriften •Sherrington • typewritten •Warrington • puritan • Fredericton •Lexington • Occitan • Washington •Whittington • Huntington •Galveston • Livingstone •Kensington •Blyton, brighten, Brighton, Crichton, enlighten, frighten, heighten, lighten, righten, tighten, titan, triton, whiten •begotten, cotton, forgotten, ill-gotten, misbegotten, rotten •Compton, Crompton •wanton • Longton •Boston, postern •boughten, chorten, foreshorten, Laughton, Morton, Naughton, Orton, quartan, quartern, shorten, tauten, torten, Wharton •Alton, Dalton, Galton, saltern, Walton •Taunton • Allston • Launceston •croton, Dakotan, Minnesotan, oaten, verboten •Bolton, Doulton, molten •Folkestone • Royston •Luton, newton, rambutan, Teuton •Houston • Fulton •button, glutton, Hutton, mutton •sultan •doubleton, subaltern •fronton • Augustan • Dunstan •tungsten • quieten • Pinkerton •charlatan • Wollaston • Palmerston •Edmonton • automaton • Sheraton •Geraldton • Chatterton • Betterton •Chesterton • Athelstan •burton, curtain, uncertain •Hurston

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Saturn

Sat·urn / ˈsatərn/ 1. Roman Mythol. an ancient god, regarded as a god of agriculture. 2. Astron. the sixth planet from the sun in the solar system, circled by a system of broad, flat rings. 3. a series of American space rockets, of which the very large Saturn V was used as the launch vehicle for the Apollo missions of 1969–72.

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Saturn

Saturn Italic god of agriculture OE.; (astron.) one of the primary planets XIV (in OE. Sæternes steorra); †(alch.) lead. — L. Sāturnus poss. of Etruscan orig.
So Saturnalia festival of Saturn marked by unrestrained revelry XVI (transf. XVIII). — L., sb. use of n. pl. of Sāturnālis; see -AL1. Saturnian ancient Roman metre. XVI. saturnine (-INE1) born under Saturn, (hence) of cold and gloomy temperament. XV, — F. saturnin — medL. *sāturnīnus.

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Saturn

Saturn

Basic characteristics

Saturns atmosphere

Saturnian storms

Saturns rings

Saturns icy moons

Resources

Saturn, sixth planet from the sun, is the most remote of the planets that were known to ancient astronomers. Saturn is a gas giant with no solid surface; it is 9.45 times wider than the Earth and 95 times more massive. It is circled by hundreds of rings consisting of small, ice-covered particles and is also host to 56 confirmed natural satellites (as of 2006), including Titan, largest moon in the solar system and the only one with an extensive atmosphere.

Basic characteristics

Saturn orbits the sun at a mean distance of 9.539 astronomical units (AU, where 1 AU is the average distance between Earth and the sun). Its slightly eccentric (noncircular) orbit, however, allows the planet to be far from the sun as 10.069 AU and as close as 9.008 AU. Saturn takes 29.46 Earth years to complete one orbit around the sun. Saturn has an equatorial diameter of 74,855 mi (120,540 km), making it the second-largest planet in the solar system (Jupiter is about the same size as Saturn but is 3.35 times more massive). Saturn spins on its axis 2.25 times more rapidly than Earth, that is, every 10 hours 14 minutes. This rapid rotation causes it to be 8,073 mi (13,000 km) wider at the equator than it is from pole to pole.

Saturn has an average density of 0.69 g/cm3, less than that of water (1.0 g/cm3) and lowest of all the planets in the solar system. This low density indicates that the planet must be composed mainly of hydrogen and helium (the most abundant elements in the universe). Theoretical models suggest that Saturn has a rocky inner core that accounts for only about 26% of its mass. This central core is surrounded by a thick layer of liquid metallic hydrogen, a form of hydrogen that occurs only under extreme pressure. This mantle is surrounded by an atmosphere composed mostly of molecular hydrogen and helium that is liquid at its base and gradually becomes less dense at higher altitudes, finally becoming a gaseous atmosphere at the highest levels.

When the two Voyager spacecraft flew past Saturn in 1980 and 1981, they confirmed that Saturn has a magnetic field. Like Jupiters magnetic field, Saturns is probably produced in the planets metallic-hydrogen mantle. The magnetic field at Saturns cloud tops, however, is about one tenth that observed on Jupiter. Indeed, Saturns equatorial magnetic field is only about two-thirds as strong as Earths.

Careful measurements of Saturns energy budget (balance of energy absorbed versus energy radiated) show that the planet radiates 1.5 to 2.5 times more energy into space than it receives from the sun. This radiated energy indicates that the planet must have an internal heat source. Scientists accept that Saturn draws its extra energy from two sources: (1) heat left over from the planets formation approximately 4.5 billion years ago, still radiating out into space, and (2) the raining out of atmospheric helium. Just as water condenses in terrestrial clouds to produce rain, droplets of liquid helium form in Saturns atmosphere. As these droplets fall through Saturns atmosphere they acquire kinetic energy. This energy is absorbed into deeper layers where the droplets meet resistance and slow their fall, and the temperature in those regions increases. This thermal energy is eventually circulated by convection back up through the higher layers of the atmosphere and radiated into space. The helium raining out of Saturns upper layers is left over from the planets formation; in about two billion more years all of Saturns helium will have sunk deep into the planet, at which time heating by helium condensation will cease.

Support for the helium-condensation model was obtained during the Voyager encounters, when it was found that the abundance of helium in Saturns atmosphere was much lower than that observed in Jupiters. Upper-atmospheric depletion of helium has not yet occurred on Jupiter because its atmosphere has only recently become cool enough to permit helium condensation; on Saturn, in contrast, helium has been raining out for about two billion years, settling half the available helium toward the core.

Saturns atmosphere

The intensity of sunlight at Saturns orbit is about one hundredth that at Earths orbit and about one fourth that the orbit of Jupiter. Consequently, and in spite of its internal heat sources, Saturns surface is cold. When compared at levels with corresponding pressures, Saturns atmosphere is some 270°F (150°C) cooler than Earths and about 90°F (50°C) cooler than Jupiters.

The atmospheres of both Saturn and Jupiter feature distinctive banded structures, horizontal zones of wind flowing in opposite directions at high speeds (e.g., 1,100 mph [1,800 km/hr] at Saturns equator). Jupiter and Saturns zonal jets, as these winds are termed, are, according to one theory, the surface manifestations of gigantic, counter-rotating cylindrical shells of fluid in these planets interiors. Such cylinders

Table 1. Saturian Satellites Larger than 200 KM in Diameter2. (Thomson Gale.)
Saturian satellites larger than 200 km in diameter(2)
NameDiameter (km)Density (kg/m3)AlbedoMean distance (10,000 km)Orbital period (day)
(2) Distances are given in units of 1,000 km. The albedo is a measure of the amount of sunlight reflected by the satellite. An albedo of zero corresponds to no reflection, while an albedo of unity corresponds to complete reflection.
Phoebe2200.0512,960550.46
Hyperion2550.31,4812l.276
Mimas3901,2000.81870.942
Enceladus5001,1001.02381.370
Tethys1,0601,2000.82951.888
Dione1,1201,4000.63782.737
Iapetus1,4601,2000.080.43,56179.331
Rhea1,5301,3000.65264.517
Titan5,5501,8800.21,22115.945

form because fluids in a rotating body tend to align their motions with the bodys axis of rotation (in this case, the planets); where the edges of these cylinders contact the approximately spherical outer surface of the planet, matching zonal jets are produced in the northern and southern hemispheres.

Saturnian storms

The outermost regions of Saturns hydrogen-helium atmosphere support ammonia, ammonium hydrosulfide, and water clouds. Saturns storm features are not as pronounced or as long-lived as those observed on Jupiter, and Saturn has no weather feature as long-lived as Jupiters Great Red Spot. However, isolated spots and cloud features are occasionally distinguishable from Earth. English astronomer William Herschel (17381822), for example, reported seeing small spots on Saturns disk in 1780. Since that time, however, very few other features have been reported. The most dramatic recurring feature observed on Saturn is its Great White Spot. This feature was first observed by American astronomer Asaph Hall (18291907) on December 7, 1876, and six subsequent displays have been recorded. A Great White Spot was observed by the Hubble Space Telescope in 1990; smaller spots are observed in most Saturnian summer seasons.

All the white spots observed on Saturn are thought to be giant storm systems. When they first appear, the Great White Spotsthe largest of these stormsare circular in form and some 12,420 mi (20,000 km) in diameter. Atmospheric winds gradually stretch and distort the spots into wispy bands, which can often be seen for several months. All of the Great White Spots have been observed in Saturns northern hemisphere, with a recurrence interval equal to one Saturnian year (29.51 years). That the storms tend to repeat every Saturnian year suggests that they are a seasonal effect, with the storms being produced whenever Saturns northern hemisphere is at maximum tilt toward the sun. It is likely that storms also occur in Saturns southern hemisphere when it is tilted toward the sun, but the angle for viewing such events from Earth is not favorable. It is assumed that the Great White Spots are produced by an up-welling of warm gas. Indeed, they have been likened to atmospheric belches. In this manner, the spots are similar to the cumulonimbus thunderheads observed in terrestrial storm systems. The prominent white color of the Saturnian storms is due to the freezing-out of ammonia ice crystals. These crystals form as the warm gas pushes outward into the frigid outer layers of the planets atmosphere.

Saturns rings

When Italian astronomer Galileo Galilei (1564 1642) first pointed his telescope towards Saturn in 1610, he saw two features protruding from the planets disk. These puzzling side-lobes were in reality Saturns ring feature, though Galileos telescope was too small to resolve their shape and extent. When these side-lobes started vanishing, as the rings began gradually assuming a position edgewise to Earth, Galileo was not able to explain the nature of his observations. Dutch astronomer Christiaan Huygens (16291695)

was the first scientist to suggest, in 1659, that Saturn was surrounded by a flattened ring.

Soon after Huygens had suggested that a ring existed around Saturn, the Italian-born French astronomer Jean-Dominique Cassini (16251712) discovered, in 1675, that there were in fact several distinct rings about the planet. Several divisions in Saturns rings are now recognized, the dark band between the A and B rings being known as the Cassini Division. The A ring is further subdivided by a dark band termed the Encke Division after German astronomer Johann F. Encke (17911865), who first observed this feature in 1838.

Saturns rings are best seen when the planet is near opposition, that is, at its closest approach to Earth. At this time, the rings are seen at the greatest angle (Figure 1). The rings are aligned with Saturns equator, thus tilted at 26.7 degrees to the ecliptic (i.e., the plane of the planets orbits about the sun). During the course of one Saturnian year the rings, as seen from Earth, are alternately viewed from above and below. Twice each Saturnian year (i.e., once every 15 years) the rings are seen edge-on, an event termed a ringplane crossing. That the rings nearly disappear from when seen edge-on indicates that they must be thin. Recent measurements suggest that the rings are no more than 1.24 mi (2 km) thick.

That the rings of Saturn cannot be solid was first proved mathematically by Scottish physicist James Clerk Maxwell (18311879) in 1857. Maxwell showed that a solid planetary disk would literally tear itself apart, and concluded that the rings must be composed of orbiting particles. Subsequent observations have confirmed Maxwells deductions, and it is now known that the rings are made of pieces of ice and ice-coated rock varying in size from dust particles to chunks on the order of 10 yards across.

Images obtained by the Voyager and Pioneer space probes have shown that Saturns rings are really composed of numerous ringlets. The apparently empty regions between ringlets are thought to be caused both by gravitational resonance with Saturnian moons (which boost particles out of those regions by delivering periodic pushes or pulls) and by a mechanism called shepherding. Just as gaps (Kirkwood gaps) have been produced in the asteroid belt through

gravitational resonance with Jupiter, so gaps have been formed in Saturns rings due to resonance with its major satellites. The Cassini Division, for example, is the result of a 2-to-1 resonance with the moon Mimas. Some of the narrower rings, on the other hand, are believed to be maintained as distinct objects by shepherding satellites. Orbiting nearby on either side of a ring, the shepherding satellites prevent the ring particles from dispersing into higher or lower orbits. The faint F ring, 62 mi (100 km) wide, for example, is maintained by two small satellites, Prometheus (62 mi [100 km] in diameter) and Pandora (56 mi [90 km] in diameter). Indeed, the F ring, which was discovered by the Pioneer 11 space probe in 1979, shows some remarkably complex structures, with the ring being made of several interlaced and (apparently) braided particle strands.

In 1995 and 1996, ring-plane crossings occurred three times with respect to Earth, as well as once with respect to the sun. This provided a unique opportunity for observing the rings and satellites of Saturn, since glare from the rings is greatly reduced during ring plane crossing, enabling astronomers to observe faint objects. On the ring-plane crossing of May 22, 1995, the Hubble Space Telescope discovered four new Saturn moons, including a third shepherd satellite which may account for the braiding of the F ring. The Hubble also discovered that the orbit of Prometheus had shifted, perhaps due to a collision with the F ring. During the August 10, 1995, ring plane crossing, the Hubble detected clouds of debris near the outer edge of the ring system. These may be the remains of small satellites shattered in collisions. These clouds may be the source of material for Saturns rings.

Saturns icy moons

Saturn has many satellites. The known Saturnian moons range in size from a few tens of kilometers up to several thousand kilometers in diameter (Table 1). In all, as of October 2006, 56 Saturnian moons have been discovered (and confirmed) and 35 have received officially sanctioned names from the International Astronomical Union. Titan, Saturns largest moon and the first to be discovered, was first observed by Huygens in 1655. The satellites discovered by Cassini were Iapetus (1671), Rhea (1672), Dione (1684), and Tethys (1684). Herschel discovered Mimas and Enceladus in 1789. The latest of Saturns moons to be named was the 3.7 to 5.0 mi (6 to 8 km) in diameter Daphnis, which was discovered by the Cassini Imaging Science Team on May 6, 2005, from images taken by the Cassini space probe. It is located within the Keeler Gap within the A ring.

The densities derived for the larger Saturnian moons are all about 1 g/cm3; consequently their interiors must be composed mainly of ice. All of the larger Saturnian satellites except Phoebe were photographed during the Voyager flybys, and while the images obtained showed, as expected, extensive impact cratering, they also revealed many unexpected features indicating that several of the satellites had undergone extensive surface modification. This observation supports an ice composition for these satellites, as iceeven the extremely cold, extremely rigid ice of the moons of the outer solar systemis easier to melt or deform than rock.

The Voyager images showed that Rhea and Mimas have old, heavily cratered surfaces, just as one would

expect for small, geologically inactive bodies. Images of Mimas revealed a remarkably large impact crater, subsequently named Herschel, that was nearly one-third the size of the satellite itself. If the body that struck Mimas to produce Herschel had been slightly larger it probably would have shattered the moon to pieces.

In contrast to Rhea and Mimas, the surfaces of Dione and Tethys, while still heavily cratered, show evidence for substantial resurfacing and internal activity. Both moons were found to support smooth, planar regions suggesting that icy material has oozed from the interior to the surface. The Saturnian moon that shows the greatest evidence for resurfacing and internal activity is Enceladus. The surface of this moon is covered by a patchwork of smooth, icy surfaces, so shiny that they reflect nearly 100% of the light that strikes them. Even the most heavily cratered regions on Enceladus show fewer craters than the other Saturnian satellites. Enceladus also shows many surface cracks and ridges. Planetary geologists believe that the smooth regions on the surface of Enceladus may be no older than 100 million years. Since bodies as small as Enceladus, which is some 310 mi (500 km) in diameter, should have cooled off very rapidly after their formation, it is still unclear how such recent resurfacing could have taken place. Orbital resonance with Dione may supply the heat needed to keep the interior of Enceladus liquid.

Voyager images of Iapetus revealed a remarkable brightness difference between the moons leading and trailing hemispheres. Iapetus, just like the other Saturnian moons, circles Saturn in a synchronous fashion, that is, it keeps the same hemisphere directed toward Saturn at all times (just as the moon orbits Earth). The images recorded by Voyager showed that the leading hemisphere, the one that points in the direction in which Iapetus is moving about Saturn, is much darker than the trailing hemisphere. Indeed, while the trailing hemisphere reflects about 40% of the light that falls on it, the leading hemisphere reflects only about 8%. The leading hemisphere is so dark, in fact, that no impact craters are visible. The most probable explanation for the dark coloration on Iapetus is that the moon has swept up a thick frontal layer of dark, dusty material as it orbits around Saturn.

Titan is the most remarkable of Saturns moons. With a diameter in excess of 3,100 mi (5,000 km), Titan is larger than the planet Mercury. The suggestion that Titan might have an atmosphere appears to have been first made by the Spanish astronomer Jose Comas Sola (1868-1937), who noted in 1903 that the central regions of the moons disk were brighter than its limb (outer portions of its disk). Convincing spectroscopic evidence for the existence of a Titanian atmosphere was obtained in 1944 by American astronomer Gerard P. Kuiper (1905-1973).

Initial Earth-based observations revealed that Titan had an atmosphere containing methane and ethane. The Voyager 1 space probe, however, showed that Titans atmosphere is mostly nitrogen, with traces of propane, acetylene, and ethylene. The atmospheric pressure at Titans surface is nearly that at sea-level on Earth.

Titans aerosol-hazy atmosphere is estimated to be about 250 mi (400 km) thick, with the main body of the satellite being about 3,200 mi (5,150 km) in diameter. The escape velocity from Titan is a mere 1.5 mi/sec (2.5 km/sec), which should have made escape of atmospheric gasses easy; the most likely reason that Titan has maintained its atmosphere is that the Saturnian system itself originally formed at a low temperature. Titans present-day surface temperature is about 200°F (90K).

Titans atmosphere is a distinctive dull orange color. Telescopic measurements at optical wavelengths have not been able to probe the surface of Titan; the atmospheric haze that surrounds the moon is too thick. However, bservations made at infrared wavelengths have been able to observe surface features, and Mark Lemmon and co-workers at the University of Arizona reported in early 1995 that Titan, as might well be expected, is in synchronous rotation about Saturn. The worlds largest telescope, the Keck telescope on Mauna Kea in Hawaii, detected dark areas on Titan in 1996 that scientists consider may be liquid seas of hydrocarbons formed in Titans atmosphere by the action solar radiation and rained onto the surface.

One of the many interesting features revealed by the Voyager space probes was that Titans atmosphere exhibits a distinct hemispherical asymmetry at visual wavelengths. The asymmetry observed on Titan is different from that seen on Iapetus, in the sense that the division on Titan is between the north and south hemispheres, rather than the leading and trailing hemispheres. When the Voyager probes imaged Titan, the northern hemisphere was slightly darker than the southern hemisphere. Follow-up observations of Titan made with the Hubble Space Telescope found that the hemispherical color asymmetry had switched during the ten years since the Voyager encounters, with the southern hemisphere being the darker one in 1990. Scientists believe that the color variation and hemisphere switching is a seasonal heating effect driven by periodic changes in Saturns distance from the sun.

KEY TERMS

Albedo The fraction of sunlight that a surface reflects. An albedo of zero indicates complete absorption, while an albedo of unity indicates total reflection.

Oblateness A measure of polar to equatorial flattening. A sphere has zero oblateness.

Shepherding satellite A satellite that restricts the motion of ringlet particles, preventing them from dispersing.

The spacecraft Cassini, launched in 1997, traveled seven years (and 2.175 billion mi [3.5 billion km]) to reach Saturn on June 30, 2004. Cassini began sending back images and data to Earth a few days later. It will spend until 2008 probing the Saturnian system. Cassini, the first spacecraft to visit Saturn since Voyager 2 and the first spacecraft ever to take up orbit around Saturn, will observe Saturns atmosphere, magnetic field, rings, and moons.

Cassini released its Huygens Probe on December 25, 2004. Huygens carried six science instruments onboard in order to study the dynamics of Titans atmosphere and information on its surface. During its descent, its camera took more than 750 images. Huygens five other instruments took samples of the atmosphere to determine its composition and structure. The probe landed on Titan on January 14, 2005.

Cassini contains a number of specialized cameras and other instruments. The Magnetosphere Imaging Instrument (MIMI), for example, is allowing the first-ever imaging of a planets magnetic field. MIMI is obtaining images of the plasma and radiation surrounding Saturn and enveloping its moons and is observing the glow of Titans exosphere (highest layer of atmosphere) caused by bombardment of high-speed protons trapped in Saturns magnetic field.

The instruments onboard Cassini and Huygens are continuing to provide data and images of the Saturnian system. After the primary mission to study Saturn and its moons is finished, Cassini may fly closer to Saturn and pass inside the G ring while evading the regions known to contain a high density of potentially damaging ring particles. The spacecraft may also be sent into orbit around Titan to make a closer study.

See also Keplers laws; Planetary ring systems.

Resources

BOOKS

Lorenz, Ralph, and Jacqueline Mitton. Lifting Titans Veil: Exploring the Giant Moon of Saturn. Cambridge: Cambridge University Press, 2002.

Morton, Oliver. Mapping Mars. New York: Picador, 2002. Sobel, Dava. The Planets. New York: Viking, 2005.

PERIODICALS

Gladman, Brett, et al. Discovery of 12 Satellites of Saturn Exhibiting Orbital Clustering. Nature 412 (July 12, 2001): 163166.

Hamilton, Douglas P. Saturn Saturated With Satellites. Nature 412 (July 12, 2001): 132133.

OTHER

Jet Propulsion Laboratory, California Institute of Technology. Cassini-Huygens: Mission to Saturn and Titan October 25, 2006, <http://saturn.jpl.nasa.gov/index.cfm> (October 25, 2006).

Jet Propulsion Laboratory, National Aeronautics and Space Administration. Welcome to the Planets: Saturn. <http://pds.jpl.nasa.gov/planets/choices/saturn1.htm> (accessed October 25, 2006).

SpaceKids, National Aeronautics and Space Administration. Tour the Solar System and Beyond. <http://spacekids.hq.nasa.gov/osskids/animate/mac.html> (accessed October 14, 2006).

TeachNet-lab.org. A Tour of the Planets. <http://www.teachnet-lab.org/miami/2001/salidoi2/a_tour_of_the_planets.htm> (accessed October 14, 2006).

Martin Beech

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Saturn

Saturn

Saturn, sixth planet from the Sun , is the most remote of the planets that were known to premodern astronomers. Saturn is a gas giant with no solid surface; it is 9.45 times wider than Earth and 95 times more massive. It is circled by hundreds of rings consisting of small, ice-covered particles and is also host to at least 30 moons, including Titan, largest moon in the solar system and the only one with an extensive atmosphere.


Basic characteristics

Saturn orbits the Sun at a mean distance of 9.539 astronomical units (AU, where 1 AU is the average distance between the Earth and the Sun). Its slightly eccentric (noncircular) orbit , however, allows the planet to be far from the Sun as 10.069 AU and as close as 9.008 AU. Saturn takes 29.46 Earth years to complete one orbit around the Sun. Saturn has an equatorial diameter of 74,855 mi (120,540 km), making it the second-largest planet in the solar system (Jupiter is about the same size as Saturn but is 3.35 times more massive). Saturn spins on its axis 2.25 times more rapidly than the Earth, that is, every 10 hours 14 minutes. This rapid rotation causes it to be 8,073 mi (13,000 km) wider at the equator than it is from pole to pole.

Saturn has an average density of 0.69 g/cm3, less than that of water (1.0 g/cm3) and lowest of all the planets in the solar system. This low density indicates that the planet must be composed mainly of hydrogen and helium (the most abundant elements in the Universe). Theoretical models suggest that Saturn has a rocky inner core that accounts for only about 26% of its mass . This central core is surrounded by a thick layer of liquid metallic hydrogen, a form of hydrogen that occurs only under extreme pressure . This mantle is surrounded by an atmosphere composed mostly of molecular hydrogen and helium that is liquid at its base and gradually becomes less dense at higher altitudes, finally becoming a gaseous atmosphere at the highest levels.

When the two Voyager spacecraft flew past Saturn in 1980 and 1981, they confirmed that Saturn has a magnetic

TABLE 1. SATURIAN SATELLITES LARGER THAN 200 KM IN DIAMETER 2
Name Diameter (km) Density (kg/m 3) Albedo Mean distance (10000 km) Orbital period (day)
2 Distances are given in units of 1000 km. The albedo is a measure of the amount of sunlight reflected by the satellite. An albedo of zero corresponds to no reflection, while an albedo of unity corresponds to complete reflection.
Phoebe2200.0512,960550.46
Hyperion2550.3148121.276
Mimas39012000.81870.942
Enceladus50011001.02381.370
Tethys106012000.82951.888
Dione112014000.63782.737
Iapetus146012000.08 - 0.4356179.331
Rhea153013000.65264.517
Titan555018800.2122115.945

field. Like Jupiter's magnetic field, Saturn's is probably produced in the planet's metallic-hydrogen mantle. The magnetic field at Saturn's cloud tops, however, is about one tenth that observed on Jupiter. Indeed, Saturn's equatorial magnetic field is only about twothirds as strong as Earth's.

Careful measurements of Saturn's energy budget (balance of energy absorbed versus energy radiated) show that the planet radiates 1.5–2.5 times more energy into space than it receives from the Sun. This radiated energy indicates that the planet must have an internal heat source. Scientists accept that Saturn draws its extra energy from two sources: (1) heat left over from the planet's formation approximately 4.5 billion years ago, still radiating out into space, and (2) the "raining out" of atmospheric helium. Just as water condenses in terrestrial clouds to produce rain, droplets of liquid helium form in Saturn's atmosphere. As these droplets fall through Saturn's atmosphere they acquire kinetic energy. This energy is absorbed into deeper layers where the droplets meet resistance and slow their fall, and the temperature in those regions increases. This thermal energy is eventually circulated by convection back up through the higher layers of the atmosphere and radiated into space. The helium raining out of Saturn's upper layers is left over from the planet's formation; in about two billion more years all of Saturn's helium will have sunk deep into the planet, at which time heating by helium condensation will cease.

Support for the helium-condensation model was obtained during the Voyager encounters, when it was found that the abundance of helium in Saturn's atmosphere was much lower than that observed in Jupiter's. Upper-atmospheric depletion of helium has not yet occurred on Jupiter because its atmosphere has only recently become cool enough to permit helium condensation; on Saturn, in contrast, helium has been raining out for about two billion years, settling half the available helium toward the core.


Saturn's atmosphere

The intensity of sunlight at Saturn's orbit is about one hundredth that at the Earth's orbit and about one fourth that the orbit of Jupiter. Consequently, and in spite of its internal heat sources, Saturn's surface is cold. When compared at levels with corresponding pressures, Saturn's atmosphere is some 270°F (150°C) cooler than Earth's and about 90°F (50°C) cooler than Jupiter's.

The atmospheres of both Saturn and Jupiter feature distinctive banded structures, horizontal zones of wind flowing in opposite directions at high speeds (e.g., 1,100 miles/hr [1,800 km/hr] at Saturn's equator). Jupiter and Saturn's zonal jets, as these winds are termed, are, according to one theory, the surface manifestations of gigantic, counter-rotating cylindrical shells of fluid in these planets' interiors. Such cylinders form because fluids in a rotating body tend to align their motions with the body's axis of rotation (in this case, the planet's); where the edges of these cylinders contact the approximately spherical outer surface of the planet, matching zonal jets are produced in the northern and southern hemispheres.


Saturnian storms

The outermost regions of Saturn's hydrogen-helium atmosphere support ammonia , ammonium hydrosulfide, and water clouds. Saturn's storm features are not as pronounced or as long-lived as those observed on Jupiter, and Saturn has no weather feature as long-lived as Jupiter's Great Red Spot. However, isolated spots and cloud features are occasionally distinguishable from Earth. English astronomer William Herschel (1738–1822), for example, reported seeing small spots on Saturn's disk in 1780. Since that time, however, very few other features have been reported. The most dramatic recurring feature observed on Saturn is its Great White Spot. This feature was first observed by American astronomer Asaph Hall (1829–1907) on December 7, 1876, and six subsequent displays have been recorded. A Great White Spot was observed by the Hubble Space Telescope in 1990; smaller spots are observed in most Saturnian summer seasons .

All the white spots observed on Saturn are thought to be giant storm systems. When they first appear, the Great White Spots—the largest of these storms—are circular in form and some 12,420 mi (20,000 km) in diameter. Atmospheric winds gradually stretch and distort the spots into wispy bands, which can often be seen for several months. All of the Great White Spots have been observed in Saturn's northern hemisphere, with a recurrence interval equal to one Saturnian year (29.51 years). That the storms tend to repeat every Saturnian year suggests that they are a seasonal effect, with the storms being produced whenever Saturn's northern hemisphere is at maximum tilt toward the Sun. It is likely that storms also occur in Saturn's southern hemisphere when it is tilted toward the Sun, but the angle for viewing such events from the Earth is not favorable. It is assumed that the Great White Spots are produced by an up-welling of warm gas. Indeed, they have been likened to atmospheric "belches." In this manner the spots are similar to the cumulonimbus thunderheads observed in terrestrial storm systems. The prominent white color of the Saturnian storms is due to the freezing-out of ammonia ice crystals. These crystals form as the warm gas pushes outward into the frigid outer layers of the planet's atmosphere.


Saturn's rings

When Italian astronomer Galileo Galilei (1564–1642) first pointed his telescope towards Saturn in 1610, he saw two features protruding from the planet's disk. These puzzling side-lobes were in reality Saturn's ring feature, though Galileo's telescope was too small to resolve their shape and extent. When these side-lobes started vanishing, as the rings began gradually assuming a position edgewise to the Earth, Galileo was not able to explain the nature of his observations. Dutch astronomer Christiaan Huygens (1629–1695) was the first scientist to suggest, in 1659, that Saturn was surrounded by a flattened ring.

Soon after Huygens had suggested that a ring existed around Saturn, the Italian-born French astronomer Jean-Dominique Cassini (1625–1712) discovered, in 1675, that there were in fact several distinct rings about the planet. Several divisions in Saturn's rings are now recognized, the dark band between the A and B rings being known as the Cassini Division. The A ring is further subdivided by a dark band termed the Encke Division after German astronomer Johann F. Encke (1791–1865), who first observed this feature in 1838.

Saturn's rings are best seen when the planet is near opposition, that is, at its closest approach to the Earth. At this time the rings are seen at the greatest angle. The rings are aligned with Saturn's equator, thus tilted at 26.7 degrees to the ecliptic (i.e., the plane of the planets' orbits about the Sun). During the course of one Saturnian year the rings, as seen from Earth, are alternately viewed from above and below. Twice each Saturnian year (i.e., once every 15 years) the rings are seen edge-on, an event termed a ring-plane crossing. That the rings nearly disappear from when seen edge-on indicates that they must be thin. Recent measurements suggest that the rings are no more than 1.24 mi (2 km) thick.

That the rings of Saturn cannot be solid was first proved mathematically by Scottish physicist James Clerk Maxwell (1831–1879) in 1857. Maxwell showed that a solid planetary disk would literally tear itself apart, and concluded that the rings must be composed of orbiting particles. Subsequent observations have confirmed Maxwell's deductions, and it is now known that the rings are made of pieces of ice and ice-coated rock varying in size from dust particles to chunks on the order of 10 yards across.

Images obtained by the Voyager and Pioneer space probes have shown that Saturn's rings are really composed of numerous ringlets. The apparently empty regions between ringlets are thought to be caused both by gravitational resonance with Saturnian moons (which boost particles out of those regions by delivering periodic pushes or pulls) and by a mechanism called shepherding. Just as gaps (Kirkwood gaps) have been produced in the asteroid belt through gravitational resonance with Jupiter, so gaps have been formed in Saturn's rings due to resonance with its major satellites. The Cassini Division, for example, is the result of a 2-to-1 resonance with the moon Mimas. Some of the narrower rings, on the other hand, are believed to be maintained as distinct objects by shepherding satellites. Orbiting nearby on either side of a ring, the shepherding satellites prevent the ring particles from dispersing into higher or lower orbits. The faint F ring, 62 mi (100 km) wide, for example, is maintained by two small satellites, Prometheus (62 mi [100 km] in diameter) and Pandora (56 mi [90 km] in diameter). Indeed, the F ring, which was discovered by the Pioneer 11space probe in 1979, shows some remarkably complex structures, with the ring being made of several interlaced and (apparently) braided particle strands.

In 1995 and 1996, ring-plane crossings occurred three times with respect to Earth, as well as once with respect to the Sun. This provided a unique opportunity for observing the rings and satellites of Saturn, since glare from the rings is greatly reduced during ring plane crossing, enabling astronomers to observe faint objects. On the ring-plane crossing of May 22, 1995, the Hubble Space Telescope discovered four new Saturn moons, including a third shepherd satellite which may account for the braiding of the F ring. The Hubble also discovered that the orbit of Prometheus had shifted, perhaps due to a collision with the F ring. During the August 10, 1995, ring plane crossing, the Hubble detected clouds of debris near the outer edge of the ring system. These may be the remains of small satellites shattered in collisions. These clouds may be the source of material for Saturn's rings.


Saturn's icy moons

Saturn has many satellites. The known Saturnian moons range in size from a few tens of kilometers up to several thousand kilometers in diameter. In all, 30 Saturnian moons have been discovered and 18 have received officially sanctioned names from the International Astronomical Union. Titan, Saturn's largest moon and the first to be discovered, was first observed by Huygens in 1655. The satellites discovered by Cassini were Iapetus (1671), Rhea (1672), Dione (1684), and Tethys (1684). Herschel discovered Mimas and Enceladus in 1789. The latest of Saturn's moons to be named was the 12.4 mi-sized (20 km) Pan, discovered by U.S. astronomer M. Showalter in 1990.

The densities derived for the larger Saturnian moons are all about 1 g/cm3; consequently their interiors must be composed mainly of ice. All of the larger Saturnian satellites except Phoebe were photographed during the Voyager flybys, and while the images obtained showed, as expected, extensive impact cratering, they also revealed many unexpected features indicating that several of the satellites had undergone extensive surface modification. This observation supports an ice composition for these satellites, as ice—even the extremely cold, extremely rigid ice of the moons of the outer solar system—is easier to melt or deform than rock.

The Voyager images showed that Rhea and Mimas have old, heavily cratered surfaces, just as one would expect for small, geologically inactive bodies. Images of Mimas revealed a remarkably large impact crater , subsequently named Herschel, that was nearly one-third the size of the satellite itself. If the body that struck Mimas to produce Herschel had been slightly larger it probably would have shattered the moon to pieces.

In contrast to Rhea and Mimas, the surfaces of Dione and Tethys, while still heavily cratered, show evidence for substantial resurfacing and internal activity. Both moons were found to support smooth, planar regions suggesting that icy material has oozed from the interior to the surface. The Saturnian moon that shows the greatest evidence for resurfacing and internal activity is Enceladus. The surface of this moon is covered by a patchwork of smooth, icy surfaces, so shiny that they reflect nearly 100% of the light that strikes them. Even the most heavily cratered regions on Enceladus show fewer craters than the other Saturnian satellites. Enceladus also shows many surface cracks and ridges. Planetary geologists believe that the smooth regions on the surface of Enceladus may be no older than 100 million years. Since bodies as small as Enceladus, which is some 310 mi (500 km) in diameter, should have cooled off very rapidly after their formation, it is still unclear how such recent resurfacing could have taken place. Orbital resonance with Dione may supply the heat needed to keep the interior of Enceladus liquid.

Voyager images of Iapetus revealed a remarkable brightness difference between the moon's leading and trailing hemispheres. Iapetus, just like the other Saturnian moons, circles Saturn in a synchronous fashion, that is, it keeps the same hemisphere directed toward Saturn at all times (just as our Moon orbits the Earth). The images recorded by Voyager showed that the leading hemisphere, the one that points in the direction in which Iapetus is moving about Saturn, is much darker than the trailing hemisphere. Indeed, while the trailing hemisphere reflects about 40% of the light that falls on it, the leading hemisphere reflects only about eight percent . The leading hemisphere is so dark, in fact, that no impact craters are visible. The most probable explanation for the dark coloration on Iapetus is that the moon has swept up a thick frontal layer of dark, dusty material as it orbits around Saturn.

Titan is the most remarkable of Saturn's moons. With a diameter in excess of 3,100 mi (5,000 km), Titan is larger than the planet Mercury. The suggestion that Titan might have an atmosphere appears to have been first made by the Spanish astronomer Jose Comas Sola (1868–1937), who noted in 1903 that the central regions of the moon's disk were brighter than its limb (outer portions of its disk). Convincing spectroscopic evidence for the existence of a Titanian atmosphere was obtained in 1944 by American astronomer Gerard P. Kuiper (1905–1973).

Initial Earth-based observations revealed that Titan had an atmosphere containing methane and ethane. The Voyager 1 space probe, however, showed that Titan's atmosphere is mostly nitrogen , with traces of propane, acetylene, and ethylene. The atmospheric pressure at Titan's surface is nearly that at sea-level on Earth.

Titan's aerosol-hazy atmosphere is estimated to be about 250 mi (400 km) thick, with the main body of the satellite being about 3,200 mi (5,150 km) in diameter. The escape velocity from Titan is a mere 1.5 mi/sec (2.5 km/sec), which should have made escape of atmospheric gasses easy; the most likely reason that Titan has maintained its atmosphere is that the Saturnian system itself originally formed at a low temperature. Titan's present-day surface temperature is about -200°F (90K).

Titan's atmosphere is a distinctive dull orange color. Telescopic measurements at optical wavelengths have not been able to probe the surface of Titan; the atmospheric haze that surrounds the moon is too thick. Recently, however, observations made at infrared wavelengths have been able to observe surface features, and Mark Lemmon and co-workers at the University of Arizona reported in early 1995 that Titan, as might well be expected, is in synchronous rotation about Saturn. The world's largest telescope, the Keck telescope on Mauna Kea in Hawaii, detected dark areas on Titan in 1996 that scientists consider may be liquid seas of hydrocarbons formed in Titan's atmosphere by the action solar radiation and rained onto the surface.

One of the many interesting features revealed by the Voyager space probes was that Titan's atmosphere exhibits a distinct hemispherical asymmetry at visual wavelengths. The asymmetry observed on Titan is different from that seen on Iapetus, in the sense that the division on Titan is between the north and south hemispheres, rather than the leading and trailing hemispheres. When the Voyager probes imaged Titan, the northern hemisphere was slightly darker than the southern hemisphere. Follow-up observations of Titan made with the Hubble Space Telescope found that the hemispherical color asymmetry had switched during the ten years since the Voyager encounters, with the southern hemisphere being the darker one in 1990. Scientists believe that the color variation and hemisphere switching is a seasonal heating effect driven by periodic changes in Saturn's distance from the Sun.

The spacecraft Cassini, launched in 1997, will reach Saturn in 2004. It will go into orbit around Saturn and release a separate device, the Huygens probe, that will parachute through the atmosphere of Titan to its surface. If all goes well, Cassini/Huygens may resolve some of the outstanding mysteries about Saturn and Titan.

See also Kepler's laws; Planetary ring systems.


Resources

books

Lorenz, Ralph, and Jacqueline Mitton. Lifting Titan's Veil: Exploring the Giant Moon of Saturn. Cambridge: Cambridge University Press, 2002.

Morton, Oliver. Mapping Mars. New York: Picador, 2002.


periodicals

Gladman, Brett, et al. "Discovery of 12 Satellites of Saturn Exhibiting Orbital Clustering." Nature 412 (July 12, 2001): 163–166.

Hamilton, Douglas P. "Saturn Saturated With Satellites." Nature 412 (July 12, 2001): 132–133.

Nicholson, Philip, D. "Saturn's Rings Turn Edge On." Sky & Telescope (May 1995).

Rothery, David. "Icy Moons of the Solar System." New Scientist (28 March 1992).


other

Jet Propulsion Laboratory, California Institute of Technology. "Cassini-Huygens: Mission to Saturn and Titan" January 17, 2003 [cited January, 20, 2003]. <http://saturn.jpl. nasa.gov/index.cfm>.


Martin Beech

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Albedo

—The fraction of sunlight that a surface reflects. An albedo of zero indicates complete absorption, while an albedo of unity indicates total reflection.

Oblateness

—A measure of polar to equatorial flattening. A sphere has zero oblateness.

Shepherding satellite

—A satellite that restricts the motion of ringlet particles, preventing them from dispersing.

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