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[Middle English Saturnus, from Old English, from Latin Sāturnus, of Etruscan origin.]
The second-largest planet in the solar system and the sixth in order of distance to the Sun. The outermost planet known prior to 1781, Saturn is surrounded by a beautiful system of rings. Saturn is also the only planet that has a satellite (Titan) with a dense atmosphere. This distant planetary system has been visited by four NASA spacecraft, including Cassini, which went into orbit around the planet in July 2004.
Saturn makes one revolution about the Sun in 29.42 years. The equatorial diameter of Saturn is about 75,000 mi (120,540 km), and the polar diameter about 67,600 mi (108,700 km). The volume is 769 (Earth = 1) with a few percent uncertainty. The mass is about 95.2 (Earth = 1) or 1/3500 (Sun = 1). The mean density is 0.70 g/cm3, the lowest mean density of all the planets. The rotation axis of both the planet and the rings is inclined 27° to the perpendicular to the orbital plane. The visible cloud layers of Saturn are much more homogeneous than those of Jupiter. There is no feature comparable to the Great Red Spot, and the contrast of the features that are visible is very low (Fig. 1).
haze layer above the planet's cloud deck. (NASA)">
Saturn, viewed from Voyager 1. The soft, velvety appearance of the low-contrast banded structure is due to scattering by a haze layer above the planet's cloud deck. (NASA)
The optical spectrum of Saturn is characterized by strong absorption bands of methane (CH4) and by much weaker bands of ammonia (NH3). Absorption lines of molecular hydrogen (H2) have also been detected.
The temperature the planet should assume in response to solar heating is calculated to be about 76 K (−323°F), somewhat lower than the measured value of 92 K (−294°F). This suggests that Saturn has an internal heat source of roughly the same magnitude as that on Jupiter. As in the case of Jupiter, a thermal inversion exists in the upper atmosphere. The inversion region is well above the main cloud layer, which is thought to consist primarily of frozen ammonia crystals, with an admixture of some other substances to provide the yellowish color sometimes observed in the equatorial zone.
Theoretical models for the internal structure of Saturn are similar to those for Jupiter, that is, a dense core surrounded by hydrogen compressed to a metallic state which gradually merges into an extremely deep atmosphere. The fact that the two planets radiate comparable amounts of energy despite their difference in size means that smaller Saturn must have some additional energy source besides gravitational contraction. The existence of a magnetic field and belts of trapped electrons has been deduced from observations of nonthermal radiation and mapped out in detail by the Pioneer and Voyager spacecraft.
Jupiter and Saturn are relatively similar bodies. Both seem to have compositions virtually identical with that of the Sun and the other stars—rich in hydrogen and helium. In that sense, they may represent the primitive material from which the entire solar system was formed, whereas the other planets have undergone fractionation processes resulting in the loss of most of the light gases. This conclusion has been strengthened by measurements of the heavy isotope of hydrogen known as deuterium. They demonstrate that the hydrogen that makes up most of the mass of both Jupiter and Saturn was captured directly from the solar nebula when these planets formed 4.5 billion years ago. However, both Jupiter and Saturn show an enhancement of the carbon/hydrogen ratio (as determined from methane and hydrogen) compared with the Sun. This suggests that both of these planets formed in a two-stage process that led initially to formation of a large core with an outgassed, secondary atmosphere, followed by the attraction of an envelope of gases from the surrounding nebula. See also Planetary physics.
The most remarkable feature associated with Saturn is the complex ring system that surrounds the planet (Fig. 2). The system is divided into four main regions, designated A through D. The narrow F ring is located just beyond the edge of ring A, and there are G and E rings still farther out. Each of the four main regions is subdivided into many individual “ringlets,” so that Saturn is actually surrounded by thousands of rings. The ring system is made up of myriad separate particles that move independently in flat, mostly circular orbits in Saturn's equatorial plane. Periodic perturbations by the major satellites are responsible, in part, for the main divisions of Saturn's rings.
Saturn's rings, viewed from Voyager 1. Approximately 95 individual concentric features are visible. One of the satellites discovered by Voyager 1 is visible just inside the narrow F ring. (NASA)
As of July 2004, Saturn had 30 confirmed satellites. The largest and brightest, Titan, is visible with small telescopes; the other satellites are much fainter. Titan's mean apparent diameter corresponds to a linear diameter of approximately 3440 mi (5550 km). But this diameter refers to the satellite's atmosphere, which is filled with a dense aerosol produced photochemically by incident sunlight. The solid surface of Titan has a diameter of 3200 mi (5150 km), making this satellite larger than Mercury but smaller than Jupiter's giant Ganymede. This object contains a large fraction of icy material and is thus quite different from the Moon or the inner planets in composition. Furthermore, it is large and cold enough to retain a thick, nitrogen (N2)-dominated atmosphere that contains a few percent of methane (CH4) and exerts a surface pressure of 1.5 bars (1.5 × 105 Pa), or 1.5 times the sea-level pressure on Earth. The main constituent of this atmosphere is molecular nitrogen. The surface of Titan is so cold [94 ± 2K or (−290 ± 4°F] that takes of liquid ethane may be present. See also Planet; Satellite (astronomy).
For more information on Saturn, visit Britannica.com.
Saturn (Sāturnus), ancient Italian god whose original function remains obscure. He may have been a blight-god. His name has also been connected with satus, ‘sowing’, and he has been thought of as a seed-god, a supposition supported by the date of his festival (the Saturnalia) which occurs in the calendar of Numa on 17 December, between the Consualia for the god of the granary and the Opalia for the god of plenty. Sacrifices to him were made in the Greek manner, with head uncovered; the Romans themselves thought that he had been introduced from Greece, and identified him at an early stage with the Greek god Cronus, the father of Zeus. Thus he came to symbolize the good old days, the Golden Age. His temple stood at the foot of the Capitoline hill and served as the treasury (aerarium) of the Roman people; there too were kept the Tables of the law and the records of decrees of the senate. He was regarded as the husband of Ops and father of Picus.
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 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, 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. Pictures from the Voyager probes show 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 1859 the Scottish physicist James Clerk Maxwell showed that the rings must consist of countless tiny particles each orbiting the planet in accordance with the laws of gravitation. When edgewise to the earth the 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 are lettered from the outermost, A, B, and C. The A, B, and C rings 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 origin of the rings is unknown, although it is believed that they may have been formed from larger satellites that were shattered by the impact of comets and meteoroids.
The Satellite System
Saturn has 48 confirmed natural satellites. 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 eight 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), 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).
There are four named satellites, Pan, Atlas, Prometheus, and Pandora in the shepherd group. The co-orbital group comprises Epimetheus and Janus. The inner large group comprises six satellites, Mimas, Methone, Pallene, Enceladus, Tethys, and Dione. The Trojan 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, four of which—Kiviuq, Ijiraq, Paaliaq, and Siarnaq—have been named. Of the 18 satellites comprising the Norse group, only seven are named: Phoebe, Skathi, Mundilfari, Narvi, Suttungr, Thrymr, and Ymir. The Gallic group consists of three satellites, Albiorix, Erriapo, 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, some sharing orbits with others, are smaller. The two largest of these, the dark-surfaced Phoebe and the irregularly shaped Hyperion, orbit far from the planet; the outermost satellite, Ymir, orbits with retrograde motion, i.e., opposite to that of the planet's rotation, as do Phoebe, Mundilfari, Narvi, Suttungr, Thrymr, and many of the newly discovered, yet unnamed satellites. The smallest, ranging from c.12 to 20 mi (20 to 32 km) in diameter, are Pan and Atlas, the satellites closest to the planet, and Telesto, Calypso, and Helene. Prometheus and Pandora, c.55 mi (90 km) in diameter, share an orbit, as do Epimetheus and Janus.
In astronomy, the second-largest major planet, sixth from the sun. Saturn was named for the Roman god of agriculture. Like Jupiter, Saturn is composed largely of gases and liquids. Saturn is the most distant planet plainly visible to the naked eye. (See solar system; see under “Mythology and Folklore.”)
In 1610, Galileo reported to Johannes Kepler: "SMAISMRMILMEPOETALEUMIBUNENUGTTAUIRAS" This report of a major new discovery with the new telescope was an anagram, a scrambled message. Anagrams were used in the 17th century to establish the priority of a discovery without revealing what had been discovered. (Imagine Watson and Crick sending a colleague an anagram of "DNA IS DOUBLE HELIX.") In this case, Galileo eventually unscrambled the anagram as ALTISSIMUM PLANETAM TERGEMINUM OBSERVAVI, or (loosely) "I have observed the most distant of planets to have a triple form." Note that U and V were considered interchangeable in Latin. This meant that Galileo had found that Saturn appeared to have two satellites.
What Galileo saw was harder to interpret than that, however. In his notebook, the drawing of Saturn looks like a ball with handles. To make matters more complicated, three years later the "handles" had disappeared. Galileo asked, "Has Saturn perhaps devoured his own children?" But by 1616 the planet once again showed its odd triple shape.
Using a better telescope, Christiaan Huygens was able in 1656 to write an anagram announcing that Saturn has a ring. Huygens, however, assumed that the ring is a single, solid structure. Giovanni Cassini observed in 1675 that there were at least two rings separated by a gap and correctly guessed that the rings are made up of many small objects. But Cassini's view was not accepted until James Clerk Maxwell studied the problem mathematically in 1857, more than 200 years after Galileo had observed the rings for the first time.
.jpg) Saturn, as seen by Cassini |
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| Orbital characteristics[1][2] | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Epoch J2000 | |||||||||||||||||||||
| Aphelion | 1,513,325,783 km 10.11595804 AU |
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| Perihelion: | 1,353,572,956 km 9.04807635 AU |
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| Semi-major axis: | 1,433,449,370 km 9.58201720 AU |
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| Eccentricity: | 0.055723219 | ||||||||||||||||||||
| Orbital period: | 10,832.327 days 29.657 296 yr |
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| Synodic period: | 378.09 days[3] | ||||||||||||||||||||
| Avg. orbital speed: | 9.69 km/s[3] | ||||||||||||||||||||
| Mean anomaly: | 320.347750° | ||||||||||||||||||||
| Inclination: | 2.485240° 5.51° to Sun's equator |
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| Longitude of ascending node: | 113.642811° | ||||||||||||||||||||
| Argument of perihelion: | 336.013862° | ||||||||||||||||||||
| Satellites: | 60 confirmed (up to 63 seen) |
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| Physical characteristics | |||||||||||||||||||||
| Equatorial radius: | 60,268 ± 4 km[4][5] 9.4492 Earths |
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| Polar radius: | 54,364 ± 10 km[4][5] 8.5521 Earths |
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| Flattening: | 0.09796 ± 0.00018 | ||||||||||||||||||||
| Surface area: | 4.27×1010 km²[6][5] 83.703 Earths |
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| Volume: | 8.2713×1014 km³[3][5] 763.59 Earths |
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| Mass: | 5.6846×1026 kg[3] 95.152 Earths |
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| Mean density: | 0.687 g/cm³[3][5] (less than water) |
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| Equatorial surface gravity: | 8.96 m/s²[3][5] 0.914 g |
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| Escape velocity: | 35.5 km/s[3][5] | ||||||||||||||||||||
| Sidereal rotation period: | 0.439 – 0.449 day[7] (10 h 32 – 47 min) |
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| Rotation velocity at equator: | 9.87 km/s[5] 35,500 km/h |
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| Axial tilt: | 26.73°[3] | ||||||||||||||||||||
| Right ascension of North pole: | 2 h 42 min 21 s 40.589°[4] |
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| Declination of North pole: | 83.537°[4] | ||||||||||||||||||||
| Albedo: | 0.342 (bond) 0.47 (geom.)[3] |
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| Surface temp.: 1 bar level 0.1 bar |
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| Apparent magnitude: | +1.2 to -0.24 [8] | ||||||||||||||||||||
| Angular size: | 14.5" — 20.1" [3] (excludes rings) |
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| Adjectives: | Saturnian | ||||||||||||||||||||
| Atmosphere [3] | |||||||||||||||||||||
| Scale height: | 59.5 km | ||||||||||||||||||||
| Composition: |
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Saturn (IPA: /ˈsætɚn/) is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Along with the planets Jupiter, Uranus, and Neptune, it is classified as a gas giant (also known as a Jovian planet, after the planet Jupiter). It was named after the Roman god Saturnus, equated to the Greek Kronos (the Titan father of Zeus) and the Babylonian Ninurta. Saturn's symbol represents the god's sickle (Unicode: ♄). The day in the week Saturday gets its name from the planet.
The planet Saturn is primarily composed of hydrogen, with small proportions of helium and trace elements.[9] The interior consists of a small core of rock and ice, surrounded by a thick layer of metallic hydrogen and a gaseous outer layer. The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter. Saturn has a planetary magnetic field intermediate in strength between that of Earth and the more powerful field around Jupiter.
Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. Sixty known moons orbit the planet. Titan, Saturn's largest and the Solar System's second largest moon (after Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.[10]
Due to a combination of its low density, rapid rotation, and fluid state, Saturn is an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10%— 60268 km vs. 54364 km.[3] The other gas planets are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm³ due to the gaseous atmosphere. Saturn is only 95 Earth masses,[3] compared to Jupiter, which is 318 times the mass of the Earth[11] but only about 20% larger than Saturn.[12]
The outer atmosphere of Saturn consists of about 93.2% molecular hydrogen and 6.7% helium. Trace amounts of ammonia, acetylene, ethane, phosphine, and methane have also been detected.[13] The upper clouds on Saturn are composed of ammonia crystals, while the lower level clouds appear to be composed of either ammonium hydrosulfide (NH4SH) or water.[14] The atmosphere of Saturn is significantly deficient in helium relative to the abundance of the elements in the Sun.
The quantity of elements heavier than helium are not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.[15]
Saturn's interior is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar in composition to the Earth, but denser. Above this, there is a thicker liquid metallic hydrogen layer, followed by a layer of liquid hydrogen and helium, and in the outermost 1,000 km a gaseous atmosphere. [16] Traces of various ices are also present. The core region is estimated to be about 9–22 times the mass of the Earth.[17] Saturn has a very hot interior, reaching 11,700 °C at the core, and it radiates 2.5 times more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.[18]
Saturn's celestial body atmosphere exhibits a banded pattern similar to Jupiter's (the nomenclature is the same), but Saturn's bands are much fainter and are also much wider near the equator. At the bottom, extending for 10 km and with a temperature of -23 °C, is a layer made up of water ice. After that comes a layer of ammonium hydrosulfide ice, which extends for another 50 km and is approximately at -93 °C. Eighty kilometers above that are ammonia ice clouds, where the temperatures are about -153 °C. Near the top, extending for some 200 km to 270 km above the clouds, come layers of visible cloud tops and a hydrogen and helium atmosphere.[19] Saturn's winds are among the Solar System's fastest. Voyager data indicate peak easterly winds of 500 m/s (1,800 km/h).[9] Saturn's finer cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made.
Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters, and, in 1994, another smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon which occurs once every Saturnian year, or roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice.[20] Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.[21]
In recent images from the Cassini spacecraft, Saturn's northern hemisphere appears a bright blue, similar to Uranus, as can be seen in the image below. This blue color cannot currently be observed from Earth, because Saturn's rings are currently blocking its northern hemisphere. The color is most likely caused by Rayleigh scattering.
Astronomers using infrared imaging have shown that Saturn has a warm polar vortex and that it is the only such planet known in the solar system. This, they say, is the warmest spot on Saturn. Whereas temperatures on Saturn are normally -185 °C, temperatures on the vortex often reach as high as -122 °C.[23]
A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images.[24][25] Unlike the north pole, HST imaging of the south polar region indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave.[26] However, NASA reported in November 2006 that the Cassini spacecraft observed a 'hurricane-like' storm locked to the south pole that had a clearly defined eyewall.[27] This observation is particularly notable because eyewall clouds had not previously been seen on any planet other than Earth (including a failure to observe an eyewall in the Great Red Spot of Jupiter by the Galileo spacecraft).[28]
The straight sides of the northern polar hexagon are each about 13,800 km long. The entire structure rotates with a period of 10h 39 m 24s, the same period as that of the planet's radio emissions, which is assumed to be equal to the period of rotation of Saturn's interior. The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere.
The pattern's origin is a matter of much speculation. Most astronomers seem to favor some sort of standing-wave pattern in the atmosphere; but the hexagon might be a novel sort of aurora. More extreme speculation has Saturn's radio emissions emanating from the hexagon (something we can see and which has the right rotation period) rather than from the planet's interior (something we cannot see).[29] Polygon shapes have been replicated in spinning buckets of fluid in a laboratory.[30]
Saturn has an intrinsic magnetic field that has a simple, symmetric shape—a magnetic dipole. Its strength at the equator—0.2 Gauss—is approximately one twentieth than that of the field around Jupiter and slightly weaker than Earth's magnetic field.[31] As a result the cronian magnetosphere is much smaller than jovian and extends slightly beyond the orbit of Titan.[32] Most probably, the magnetic field is generated similarly to that of Jupiter—by currents in the metallic-hydrogen layer, which is called a metallic-hydrogen dynamo.[32] Similarly to the those of other planets, this magnetosphere is efficient at deflecting the solar wind particles from the Sun. The moon Titan orbits within the outer part of Saturn's magnetosphere and contributes plasma from the ionized particles in Titan's outer atmosphere.[31]
The average distance between Saturn and the Sun is over 1,400,000,000 km (9 AU). With an average orbital speed of 9.69 km/s,[3] it takes Saturn 10,759 Earth days (or about 29½ years), to finish one revolution around the Sun.[3] The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth.[3] Because of an eccentricity of 0.056, the distance between Saturn and the Sun varies by approximately 155,000,000 km between perihelion and aphelion,[3] which are the nearest and most distant points of the planet along its orbital path, respectively.
The visible features on Saturn rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions (as in Jupiter's case): System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet in the period of the Voyager flybys, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.
However, a precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased appreciably, to approximately 10 h 45 m 45 s (± 36 s).[33] The cause of the change is unknown—it was thought to be due to a movement of the radio source to a different latitude inside Saturn, with a different rotational period, rather than because of a change in Saturn's rotation.
Later, in March 2007, it was found that the rotation of the radio emissions did not trace the rotation of the planet, but rather is produced by convection of the plasma disc, which is dependent also on other factors besides the planet's rotation. It was reported that the variance in measured rotation periods may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and "weighs down" Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet itself. At the time it was stated that there is no currently known method of determining the rotation rate of Saturn's core.[34][35][36]
The latest estimate of Saturn's rotation based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes was reported in September 2007 is 10 hours, 32 minutes, 35 seconds. [37]
![The rings of Saturn (here: as imaged by Cassini in 2007) are the most spectacular in the Solar System.[16]](http://www.answers.com/main/Record2?a=NR&url=http%3A%2F%2Fcommons.wikimedia.org%2Fwiki%2FImage%3ASaturn%2520from%2520Cassini%2520Orbiter%2520%282007-01-19%29.jpg)
.jpg){kind=small}
Saturn is probably best known for its system of planetary rings, which makes it the most visually remarkable object in the solar system.[16]
The rings were first observed by Galileo Galilei in 1610 with his telescope, but he was unable to identify them as such. He wrote to the Duke of Tuscany that "The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one (Saturn itself) is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish. Mystified, Galileo wondered, "Has Saturn swallowed his children?", referring to the myth of the god Saturn eating his own children to prevent them from overthrowing him.[38] Then, in 1613, they reappeared again, further confusing Galileo.[39]
In 1655, Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. Using a telescope that was far superior to those available to Galileo, Huygens observed Saturn and wrote that "It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic."[39]
In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division in itself is a 4,800 km wide region between the A Ring and B Ring.[40]
In 1859, James Clerk Maxwell demonstrated that the rings could not be solid or they would become unstable and break apart. He proposed that the rings must be composed of numerous small particles, all independently orbiting Saturn.[41] Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Lick Observatory.
The rings can be viewed using a quite modest modern telescope or with good binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, average approximately one kilometer in thickness, and are composed of 93 percent water ice with a smattering of tholin impurities, and 7 percent amorphous carbon.[42] They range in size from specks of dust to the size of a small automobile.[43] There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche limit). A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed. This theory is not widely accepted today, since Saturn's rings are thought to be unstable over periods of millions of years and therefore of relatively recent origin.
While the largest gaps in the rings, such as the Cassini Division and Encke Division, can be seen from Earth, the Voyager spacecrafts discovered the rings to have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise from the gravitational pull of Saturn's many moons in several different ways. Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner. Still more structure in the rings consists of spiral waves raised by the moons' periodic gravitational perturbations.
Data from the Cassini space probe indicate that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O2) produced when ultraviolet light from the Sun disintegrates water ice in the rings. Chemical reactions between water molecule fragments and further ultraviolet stimulation create and eject, among other things O2. According to models of this atmosphere, H2 is also present. The O2 and H2 atmospheres are so sparse that if the entire atmosphere were somehow condensed onto the rings, it would be on the order of one atom thick.[44] The rings also have a similarly sparse OH (hydroxide) atmosphere. Like the O2, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic ions that bombard water molecules ejected by Saturn's moon Enceladus. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.[45]
Saturn shows complex patterns in its brightness.[8] Most of the variability is due to the changing aspect of the rings,[46] [47] and this goes through two cycles every orbit. However, superimposed on this is variability due to the eccentricity of the planet's orbit that causes the planet to display brighter oppositions in the northern hemisphere than it does in the southern.[48]
In 1980, Voyager I made a fly-by of Saturn that showed the F-ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.
Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics.[49]The spokes appear dark against the lit side of the rings, and light when seen against the unlit side. It is assumed that they are microscopic dust particles that have levitated away from the ring plane and that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism generating the spokes is still unknown.[50]
Twenty-five years later, the spokes were observed again, this time by Cassini. They appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe spoke formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and the spokes reappeared in images taken on September 5, 2005.[51]
Saturn has a large number of moons. The precise figure is uncertain, as the orbiting chunks of ice in Saturn's rings are all technically moons, and it is difficult to draw a distinction between a large ring particle and a tiny moon. As of 2007, a total of 60 individual moons have been identified, plus 3 unconfirmed moons that could be small dust clumps in the rings. Out of those, 48 have been named. Many of the moons are very small: out of 60, 34 are less than 10 km in diameter, and another 13 less than 50 km.[52] Only seven of them are massive enough to have collapsed into spheroids under their own gravitation. These are compared with Earth's moon in the table below.
Titan, Saturn's largest moon, is the only moon in the Solar System to have a dense atmosphere. While most of the moons in the Saturnian system are small in size, Titan is, relatively speaking, gigantic. After the Sun, the eight planets and Jupiter's moon Ganymede, Titan is the most massive object in the Solar System.[10] Titan comprises more than 90 percent of the mass in orbit around Saturn, including the rings, and the other moons range from one hundredth to one hundred millionth its mass.[53]
Traditionally, most of Saturn's other moons are named after Titans of Greek mythology. This started because John Herschel—son of William Herschel, discoverer of Mimas and Enceladus—suggested doing so in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,[54] because they were the sisters and brothers of Cronos (the Greek Saturn).
| Saturn's major satellites, compared with Earth's Moon. | |||||
|---|---|---|---|---|---|
| Name (Pronunciation key) |
Diameter (km) |
Mass (kg) |
Orbital radius (km) | Orbital period (days) | |
| Mimas | ˈmaɪməs | 400 (10% Luna) |
0.4×1020 (0.05% Luna) |
185,000 (50% Luna) |
0.9 (3% Luna) |
| Enceladus | ɛnˈsɛlədəs | 500 (15% Luna) |
1.1×1020 (0.2% Luna) |
238,000 (60% Luna) |
1.4 (5% Luna) |
| Tethys | ˈtiːθɨs | 1060 (30% Luna) |
6.2×1020 (0.8% Luna) |
295,000 (80% Luna) |
1.9 (7% Luna) |
| Dione | daɪˈoʊni | 1120 (30% Luna) |
11×1020 (1.5% Luna) |
377,000 (100% Luna) |
2.7 (10% Luna) |
| Rhea | ˈriːə | 1530 (45% Luna) |
23×1020 (3% Luna) |
527,000 (140% Luna) |
4.5 (20% Luna) |
| Titan | ˈtaɪtən | 5150 (150% Luna) |
1350×1020 (180% Luna) |
1,222,000 (320% Luna) |
16 (60% Luna) |
| Iapetus | aɪˈæpɨtəs | 1440 (40% Luna) |
20×1020 (3% Luna) |
3,560,000 (930% Luna) |
79 (290% Luna) |
Saturn has been known since prehistoric times.[55] In ancient times, it was the most distant of the five known planets in the solar system (excluding Earth) and thus a major character in various mythologies. In ancient Roman mythology, the god Saturnus, from which the planet takes its name, was the god of the agricultural and harvest sector.[56] The Romans considered Saturnus the equivalent of the Greek god Kronos.[56] The Greeks had made the outermost planet sacred to Kronos,[57] and the Romans followed suit.
In Hindu astrology, there are nine astrological objects, known as Navagrahas. Saturn, one of them, is known as "Sani" or "Shani," the Judge among all the planets, and determines everyone according to their own performed deeds bad or good.[56] Ancient Chinese and Japanese culture designated the planet Saturn as the earth star (土星). This was based on Five Elements which were traditionally used to classify natural elements. In ancient Hebrew, Saturn is called 'Shabbathai'. Its angel is Cassiel. Its intelligence, or beneficial spirit, is Agiel (layga), and its spirit (darker aspect) is Zazel (lzaz). In Ottoman Turkish and in Malay, its name is 'Zuhal', derived from Arabic زحل.
Saturn's rings require at least a 75 mm diameter telescope to resolve and thus were not
known to exist until
No further discoveries of significance were made until 1789 when William Herschel discovered two further moons, Mimas and Enceladus. The irregularly shaped satellite Hyperion, which has a resonance with Titan, was discovered in 1848 by a British team.
In 1899 William Henry Pickering discovered Phoebe, a highly irregular satellite that does not rotate synchronously with Saturn as the larger moons do. Phoebe was the first such satellite found, and it takes more than a year to orbit Saturn in a retrograde orbit. During the early twentieth century, research on Titan led to the confirmation in 1944 that it had a thick atmosphere - a feature unique among the solar system's moons.
Saturn was first visited by Pioneer 11 on September 1979. It flew within 20,000 km of the planet's cloud tops. Low resolution images were acquired of the planet and a few of its moons; the resolution of the images was not good enough to discern surface features. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, or in other words, they are not empty of material. Pioneer 11 also measured the temperature of Titan.[60]
In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan, greatly increasing our knowledge of the atmosphere of the moon. However, it also proved that Titan's atmosphere is impenetrable in visible wavelengths; so, no surface details were seen. The flyby also changed the spacecraft's trajectory out from the plane of the solar system.[61]
Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's turnable camera platform stuck for a couple of days, and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.[61]
The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell gap (a gap within the C Ring) and
![North polar hexagonal cloud feature, discovered by Voyager 1 and confirmed in 2006 by Cassini.[22]](http://content.answers.com/main/content/wp/en/thumb/a/a5/180px-Saturn_hexagonal_north_pole_feature.jpg)
