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Saturn (IPA: /ˈsæ.tÉšn/) 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: ♄). [6] The planet Saturn is primarily composed of hydrogen, with small proportions of helium and trace elements.[7] 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, which is significantly faster than those on Jupiter. Saturn has a planetary magnetic field that is 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. At least 62 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.[8] Physical characteristics Saturn's temperature emissions: The prominent hot spot at the bottom of the image is at Saturn's south pole. Saturn is an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar diameters differ by almost 10% (120,536 km vs. 108,728 km). This is the result of its large size but much lower mass than Jupiter combined with low density, a rapid rotation, and fluid state. 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 compared to Jupiter which is 317 times the mass of the Earth while Jupiter is about 15% larger. Composition 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.[9] 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.[10] Relative to the abundance of the elements in the Sun, the atmosphere of Saturn is significantly deficient in helium. 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.[11] Internal structure Saturn's interior is similar to that of Jupiter, having a small rocky core made up mostly of the elements hydrogen and helium at the center, a thicker liquid metallic hydrogen layer above that, and a molecular hydrogen outer layer.[12] Traces of various ices are also present. The core region is estimated to be about 9–22 times the mass of the Earth.[13] Saturn has a very hot interior, reaching 12,000 kelvins (11,700 °C) at the core, and it radiates 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. With this visual comparison of the physical size of Saturn and Earth, the large mass of Saturn can be clearly seen. Cloud layers Saturn's atmosphere exhibits a banded pattern similar to Jupiter's (in fact, the nomenclature is the same), but Saturn's bands are much fainter and are also much wider near the equator. Saturn's winds are among the Solar System's fastest. Voyager data indicate peak easterly winds of 500 m/s (1116 mph).[7] 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 Saturnian phenomenon with a roughly 30-year periodicity. 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.[14] 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 above. 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. North polar hexagonal cloud feature, discovered by Voyager 1 and confirmed in 2006 by Cassini.[15] Astronomers using infrared imaging have shown that Saturn has a warm polar vortex, and is the only such planet known in the solar system. A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images.[16] [17] 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.[18] 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.[19] This observation is particularly notable because eyewall clouds have not 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).[20] The straight sides of the northern polar hexagon are each about 13,800 kilometers 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).[21] Polygon shapes have been replicated in spinning buckets of fluid in a laboratory.[22] Magnetic field and magnetosphere Saturn has an intrinsic magnetic field that has a simple, symmetric shape—a magnetic dipole. Its strength is one fifth than that of the field around Jupiter (although stronger than Earth's magnetic field), giving the Saturn a smaller magnetosphere. 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.[23] Orbit and rotation Animation of hexagonal cloud feature. The average distance between Saturn and the Sun is over 1.4 billion km, which is approximately 9 times the distance from the Earth to the Sun (or 9 AU). With an average orbital speed of 9.638 km/s, it takes Saturn 10,759 Earth days (or about 29½ years), to finish one revolution around the Sun. The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth. Because of an eccentricity of 0.054, the distance between Saturn and the Sun varies by approximately 155 million km between perihelion and aphelion, which are the nearest and most distant points of the planet along its orbital path, respectively. Since Saturn does not rotate on its axis at a uniform rate, multiple rotation periods have been assigned to it (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, 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. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased slightly, to approximately 10 h 45 m 45 s (± 36 s).[24] The cause of the change is unknown — however, it is thought that this is due to a movement of the radio source to a different latitude inside Saturn, with a different rotational period, rather than an actual change in Saturn's rotation. In March 2007, the discovery was announced that the rotation of the radio emissions did not actually trace the rotation of the planet, but rather is produced by convection of the plasma disc, independent of rotation. It was reported that the variance in measured rotation periods may actually 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. If true, this means that there is no currently known method of determining the actual rotation rate of Saturn's core.[25][26][27] Planetary rings The rings of Saturn (here: as imaged by Cassini in 2007) are one of the most spectacular in the Solar System.[12] Main article: Rings of Saturn Saturn is probably best known for its system of planetary rings, which makes it the most visually remarkable object in the solar system.[12] History 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, and then in 1613 they reappeared again, further confusing Galileo.[28] 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."[28] In 1675, Giovanni Domenico Cassini determined that Saturn's ring was actually 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 (2,980 mi) wide region between the A Ring and B Ring.[29] 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.[30] Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Lick Observatory. Physical characteristics Saturn's rings cut across an eerie scene that is ruled by Titan's luminous crescent and globe-encircling haze, broken by the small moon Enceladus, whose cryovolcanos are dimly visible at its south pole. North is up. Imaged by Cassini in 2006. 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 close to one kilometer in thickness and are composed of silica rock, iron oxide, and ice particles ranging in size from specks of dust to the size of a small automobile. 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 actually 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.[31] 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.[32] Saturn shows complex patterns in its brightness. Most of the variability is due to the changing aspect of the rings, 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 (Henshaw, C., 2003).[33] 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. Spokes of the rings Spokes in the B ring, imaged by Voyager 2 in 1981 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. 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 connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism generating the spokes is still unknown. These are three images of the spokes imaged by Cassini in 2005. 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.[34] Natural Satellites Main article: Saturn's natural satellites Four of Saturn's moons: Dione, Titan, Prometheus (edge of rings), Telesto (top center) 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 59 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 59, 33 are less than 10 km in diameter, and another 13 less than 50 km. 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. Saturn's most noteworthy moon is Titan, the only moon in the solar system to have a dense atmosphere. Traditionally, most of Saturn's other moons are named after actual 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,[35] because they were the sister 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 ˈmaɪmÉ™s 400 (10% Luna) 0.4×1020 (0.05% Luna) 185,000 (50% Luna) 0.9 (3% Luna) Enceladus É›nˈsÉ›ləɾəs 500 (15% Luna) 1.1×1020 (0.2% Luna) 238,000 (60% Luna) 1.4 (5% Luna) Tethys ˈtÊ°iθɪs 1060 (30% Luna) 6.2×1020 (0.8% Luna) 295,000 (80% Luna) 1.9 (7% Luna) Dione daɪˈəʊni 1120 (30% Luna) 11×1020 (1.5% Luna) 377,000 (100% Luna) 2.7 (10% Luna) Rhea ˈriÉ™ 1530 (45% Luna) 23×1020 (3% Luna) 527,000 (140% Luna) 4.5 (20% Luna) Titan ˈtÊ°aɪʔən 5150 (150% Luna) 1350×1020 (180% Luna) 1,222,000 (320% Luna) 16 (60% Luna) Iapetus aɪˈæpəɾəs 1440 (40% Luna) 20×1020 (3% Luna) 3,560,000 (930% Luna) 79 (290% Luna) For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellites. Exploration Main article: Exploration of Saturn A Hubble Space Telescope image, captured in October 1996, shows Saturn's rings from just past edge-on. Credit: NASA/ESA. Pioneer 11 flyby 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.[36] Voyager flybys 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. 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. 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 Keeler gap (a 42 km wide gap in the A Ring). Cassini orbiter Saturn eclipses the Sun, as seen from Cassini. On July 1, 2004, the Cassini-Huygens spacecraft performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had already studied the system extensively. In June 2004, it had conducted a close flyby of Phoebe, sending back high-resolution images and data. Cassini's flyby of Saturn's largest moon, Titan, has captured radar images of a large sea and its coastline with numerous island groups and mountains. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004. Huygens descended onto the surface of Titan on January 14, 2005, sending a flood of data during the atmospheric descent and after the landing. During 2005, Cassini conducted multiple flybys of Titan and icy satellites. Cassini's next Titan flyby is scheduled for July 19, 2007. On March 10, 2006, NASA reported that, through images, the Cassini probe found evidence of liquid water reservoirs that erupt in geysers on Saturn's moon Enceladus. Images had also shown particles of water in its liquid state being emitted by icy jets and towering plumes. According to Dr. Andrew Ingersoll, California Institute of Technology, "Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust. What's different here is that pockets of liquid water may be no more than tens of meters below the surface."[37] On September 20, 2006, a Cassini probe photograph revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings. Apparently, the source of this ring is the result of the crashing of a meteoroid off two of the moons of Saturn. [38] In July 2006, Cassini saw the first proof of hydrocarbon lakes near Titan's north pole, which was confirmed in January 2007. In March 2007, additional images near Titan's north pole discovered hydrocarbon "seas," the largest of which is almost the size of the Caspian Sea.[39] As of 2006, the probe has discovered and confirmed 4 new satellites. Its primary mission will end in 2008 when the spacecraft will be expected to have completed 74 orbits around the planet. The probe, however, is expected to have at least one mission extension. Best viewing Saturn Oppositions: 2001-2029 Saturn has been known since prehistoric times.[40] It is the most distant of the five planets easily visible to the naked eye, the other four being Mercury, Venus, Mars, and Jupiter (Uranus is visible to the naked eye in very dark skies), and was the last planet known to early astronomers until Uranus was discovered in 1781. Saturn appears to the naked eye in the night sky as a bright, yellowish star varying usually between magnitude +1 and 0 and takes approximately 29½ years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Optical aid (large binoculars or a telescope) magnifying at least 20X is required to clearly resolve Saturn's rings for most people.[12] While it is a rewarding target for observation for most of the time it is visible in the sky, Saturn and its rings are best seen when the planet is at or near opposition (the configuration of a planet when it is at an elongation of 180° and thus appears opposite the Sun in the sky). In the opposition on January 13, 2005, Saturn appeared at its brightest until it will in 2031, mostly due to a favorable orientation of the rings relative to the Earth. In various cultures Saturn has been referred to by various ancient cultures. In Hindu astrology, there are nine planets, known as Navagrahas. Saturn, on 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.[41] In ancient Roman mythology, Saturn was the god of the agricultural and harvest sector.[41] Chinese and Japanese culture designate the planet Saturn as the earth star (土星). This is based on Five Elements which were traditionally used to classify natural elements. In 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 زØÙ„. In ancient Greek mythology, Saturn was known as Cronos, who was considered "Father Time."[41] See also Solar System Portal Saturn in astrology Saturn in fiction Dragon Storm (astronomy)
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