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Saturn, as seen by Cassini | |||||||||||||||||||||
Orbital characteristics[1][2] | |||||||||||||||||||||
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Epoch J2000 | |||||||||||||||||||||
Aphelion | 1,513,325,783 km 10.11595804 AU | ||||||||||||||||||||
Perihelion: | 1,353,572,956 km 9.04807635 AU | ||||||||||||||||||||
Semi-major axis: | 1,433,449,370 km 9.58201720 AU | ||||||||||||||||||||
Eccentricity: | 0.055723219 | ||||||||||||||||||||
Orbital period: | 10,832.327 days 29.657 296 yr | ||||||||||||||||||||
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 | ||||||||||||||||||||
Longitude of ascending node: | 113.642811° | ||||||||||||||||||||
Argument of perihelion: | 336.013862° | ||||||||||||||||||||
Satellites: | 60 confirmed (up to 63 seen) | ||||||||||||||||||||
Physical characteristics | |||||||||||||||||||||
Equatorial radius: | 60,268 ± 4 km[4][5] 9.4492 Earths | ||||||||||||||||||||
Polar radius: | 54,364 ± 10 km[4][5] 8.5521 Earths | ||||||||||||||||||||
Flattening: | 0.09796 ± 0.00018 | ||||||||||||||||||||
Surface area: | 4.27×1010 km²[6][5] 83.703 Earths | ||||||||||||||||||||
Volume: | 8.2713×1014 km³[3][5] 763.59 Earths | ||||||||||||||||||||
Mass: | 5.6846×1026 kg[3] 95.152 Earths | ||||||||||||||||||||
Mean density: | 0.687 g/cm³[3][5] (less than water) | ||||||||||||||||||||
Equatorial surface gravity: | 8.96 m/s²[3][5] 0.914 g | ||||||||||||||||||||
Escape velocity: | 35.5 km/s[3][5] | ||||||||||||||||||||
Sidereal rotation period: | 0.439 – 0.449 day[7] (10 h 32 – 47 min) | ||||||||||||||||||||
Rotation velocity at equator: | 9.87 km/s[5] 35,500 km/h | ||||||||||||||||||||
Axial tilt: | 26.73°[3] | ||||||||||||||||||||
Right ascension of North pole: | 2 h 42 min 21 s 40.589°[4] | ||||||||||||||||||||
Declination of North pole: | 83.537°[4] | ||||||||||||||||||||
Albedo: | 0.342 (bond) 0.47 (geom.)[3] | ||||||||||||||||||||
Surface temp.: 1 bar level 0.1 bar |
| ||||||||||||||||||||
Apparent magnitude: | +1.2 to -0.24 [8] | ||||||||||||||||||||
Angular size: | 14.5" — 20.1" [3] (excludes rings) | ||||||||||||||||||||
Adjectives: | Saturnian | ||||||||||||||||||||
Atmosphere [3] | |||||||||||||||||||||
Scale height: | 59.5 km | ||||||||||||||||||||
Composition: |
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- Note: This article contains special characters.
Saturn (pronounced /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]
Contents[hide] |
[edit] Physical characteristics
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]
[edit] 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.[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]
[edit] Internal structure
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]
[edit] Cloud layers
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]
[edit] Magnetic field and magnetosphere
Saturn has an intrinsic magnetic field that has a simple, symmetric shape—a magnetic dipole. Its strength at the equator—0.2 gauss (20 µT)—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]
[edit] Orbit and rotation
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]
[edit] Planetary rings
Saturn is probably best known for its system of planetary rings, which makes it the most visually remarkable object in the solar system.[16]
[edit] 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. 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.
[edit] Physical characteristics
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.
[edit] Spokes of the rings
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]
[edit] Natural satellites
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 | 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) |
- For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellites.
[edit] History and exploration
[edit] Ancient times and observation
- See also: Planet#Etymology
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 Galileo first saw them in 1610.[58] He, though, thought of them as two moons on Saturn's sides. It was not until Christian Huygens used greater telescopic magnification that the rings were assumed to be rings. Huygens also discovered Saturn's moon Titan. Some time later, Jean-Dominique Cassini discovered four other moons: Iapetus, Rhea, Tethys, and Dione. In 1675, Cassini also discovered the gap now known as the Cassini Division.[59]
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.
[edit] 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.[60]
[edit] 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.[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 Keeler gap (a 42 km wide gap in the A Ring).
[edit] Cassini orbiter
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 large lakes and their coastlines with numerous islands 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 last Titan flyby was scheduled for July 19, 2007.
Since early 2005, scientists have been tracking lightning on Saturn, primarily found by Cassini. The power of the lightning is said to be approximately 1000 times than that of the lightning on Earth. In addition, scientists believe that this storm is the strongest of its kind ever seen.[62]
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."[63]
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. [64]
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.[65]
In October 2006, the probe detected a 5,000 km diameter hurricane with an eyewall at Saturn's South Pole.[66]
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.
[edit] Best viewing
Saturn is the most distant of the five planets easily visible to the naked eye, the other four being Mercury, Venus, Mars, and Jupiter (Uranus and occasionally 4 Vesta are 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.[16]
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). During the opposition of December 17, 2002, Saturn appeared at its brightest due to a favorable orientation of the rings relative to the Earth.[47]
[edit] See also
[edit] References
- ^ Yeomans, Donald K. (2006-07-13). HORIZONS System. NASA JPL. Retrieved on 2007-08-08. — At the site, go to the "web interface" then select "Ephemeris Type: ELEMENTS", "Target Body: Saturn Barycenter" and "Center: Sun".
- ^ Orbital elements refer to the barycenter of the Saturn system, and are the instantaneous osculating values at the precise J2000 epoch. Barycenter quantities are given because, in contrast to the planetary centre, they do not experience appreciable changes on a day-to-day basis from to the motion of the moons.
- ^ a b c d e f g h i j k l m n o p q r s Williams, Dr. David R. (September 07, 2006). Saturn Fact Sheet. NASA. Retrieved on 2007-07-31.
- ^ a b c d Seidelmann, P. Kenneth; Archinal, B. A.; A’hearn, M. F.; et.al. (2007). "Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006". Celestial Mech. Dyn. Astr. 90: 155–180. doi:10.1007/s10569-007-9072-y.
- ^ a b c d e f g h Refers to the level of 1 bar atmospheric pressure
- ^ NASA: Solar System Exploration: Planets: Saturn: Facts & Figures
- ^ Than, Ker (September 06, 2007). Length of Saturn's Day Revised. Space.com. Retrieved on 2007-09-06.
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- Introduction to Saturn by NASA's Solar System Exploration
- Saturn Fact Sheet, by NASA
- Cassini-Huygens mission to Saturn, by NASA
- Research News about Saturn
- General information about Saturn
- Studies on the Rings of Saturn
[hide] | ||
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Major Moons | Mimas · Enceladus · Tethys · Dione · Rhea · Titan · Hyperion · Iapetus | |
Characteristics | Dragon Storm · Great White Spot · Rings of Saturn · Moons | |
Exploration | Pioneer 11 · Voyager program · Cassini–Huygens | |
Other | Saturn-crosser asteroid · Delta Octantis · Saturn kilometric radiation · Saturn in fiction |
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Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian |
Small bodies: Meteoroids · Asteroids/Asteroid moons (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud) |
See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the Solar System Portal |
Saturn Statistics | |
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Mass (kg) | 5.688e+26 |
Mass (Earth = 1) | 9.5181e+01 |
Equatorial radius (km) | 60,268 |
Equatorial radius (Earth = 1) | 9.4494e+00 |
Mean density (gm/cm^3) | 0.69 |
Mean distance from the Sun (km) | 1,429,400,000 |
Mean distance from the Sun (Earth = 1) | 9.5388 |
Rotational period (hours) | 10.233 |
Orbital period (years) | 29.458 |
Mean orbital velocity (km/sec) | 9.67 |
Orbital eccentricity | 0.0560 |
Tilt of axis (degrees) | 25.33 |
Orbital inclination (degrees) | 2.488 |
Equatorial surface gravity (m/sec^2) | 9.05 |
Equatorial escape velocity (km/sec) | 35.49 |
Visual geometric albedo | 0.47 |
Magnitude (Vo) | 0.67 |
Mean cloud temperature | -125°C |
Atmospheric pressure (bars) | 1.4 |
Atmospheric composition Hydrogen Helium | 97% 3% |
- Saturn Fly Around.
- Saturn's spokes in rings.
- 60-HST exposures showing a storm in the atmosphere of Saturn.
The Greatest Saturn Portrait ...Yet
While cruising around Saturn in early October 2004, Cassini captured a series of images that have been composed into the largest, most detailed, global natural color view of Saturn and its rings ever made.
This grand mosaic consists of 126 images acquired in a tile-like fashion, covering one end of Saturn's rings to the other and the entire planet in between. The images were taken over the course of two hours on Oct. 6, 2004, while Cassini was approximately 6.3 million kilometers (3.9 million miles) from Saturn. Since the view seen by Cassini during this time changed very little, no re-projection or alteration of any of the images was necessary.
Three images (red, green and blue) were taken of each of 42 locations, or "footprints," across the planet. The full color footprints were put together to produce a mosaic that is 8,888 pixels across and 4,544 pixels tall.
The smallest features seen here are 38 kilometers (24 miles) across. Many of Saturn's splendid features noted previously in single frames taken by Cassini are visible in this one detailed, all-encompassing view: subtle color variations across the rings, the thread-like F ring, ring shadows cast against the blue northern hemisphere, the planet's shadow making its way across the rings to the left, and blue-grey storms in Saturn's southern hemisphere to the right. Tiny Mimas and even smaller Janus are both faintly visible at the lower left. The Sun-Saturn-Cassini, or phase, angle at the time was 72 degrees; hence, the partial illumination of Saturn in this portrait. Later in the mission, when the spacecraft's trajectory takes it far from Saturn and also into the direction of the Sun, Cassini will be able to look back and view Saturn and its rings in a more fully-illuminated geometry. (Courtesy NASA/JPL/Space Science Institute)
Neon Saturn
Flying over the unlit side of Saturn's rings, the Cassini spacecraft captures Saturn's glow, represented in brilliant shades of electric blue, sapphire and mint green, while the planet's shadow casts a wide net on the rings.
On the night side (right side of image), with no sunlight, Saturn's own thermal radiation lights things up. This light at 5.1 microns wavelength (some seven times the longest wavelength visible to the human eye) is generated deep within Saturn, and works its way upward, eventually escaping into space. Thick clouds deep in the atmosphere block that light. An amazing array of dark streaks, spots, and globe-encircling bands is visible instead. Saturn's strong thermal glow at 5.1 microns even allows these deep clouds to be seen on portions of the dayside (left side), especially where overlying hazes are thin and the glint of the sun off of them is minimal. These deep clouds are likely made of ammonium hydrosulfide and cannot be seen in reflected light on the dayside, since the glint of the sun on overlying hazes and ammonia clouds blocks the view of this level.
A pronounced difference in the brightness between the northern and southern hemispheres is apparent. The northern hemisphere is about twice as bright as the southern hemisphere. This is because high-level, fine particles are about half as prevalent in the northern hemisphere as in the south. These particles block Saturn's glow more strongly, making Saturn look brighter in the north. [ more ] (Courtesy NASA/JPL/University of Arizona)
Saturn With Rhea and Dione
NASA's Voyager 2 took this photograph of Saturn on July 21, 1981, when the spacecraft was 33.9 million kilometers (21 million miles) from the planet. Two bright, presumably convective cloud patterns are visible in the mid-northern hemisphere and several dark spoke-like features can be seen in the broad B-ring (left of planet). The moons, Rhea and Dione, appear as blue dots to the south and southeast of Saturn, respectively. Voyager 2 made its closest approach to Saturn on August 25, 1981. (Courtesy NASA/JPL)
The Interior of Saturn
This picture illustrates the internal structure of Saturn. The outer layer is primarily composed of molecular hydrogen. As we go deeper where the presure reaches 100,000 bars, the gas starts to resemble a hot liquid. When the hydrogen reaches a pressure of 1,000,000 bar, hydrogen changes into a new state of metallic hydrogen. In this state it resembles a molten metal. This metalic hydrogen state occurs at about half of Saturn's radius. Below this is a layer dominated by ice where "ice" denotes a soupy liquid mixture of water, methane, and ammonia under high temperatures and pressures. Finally at the center is a rocky or rocky-ice core. (Copyright 2002 Calvin J. Hamilton)
Saturn With Tethys and Dione
Saturn and two of its moons, Tethys (above) and Dione, were photographed by Voyager 1 on November 3, 1980, from a distance of 13 million kilometers (8 million miles). The shadows of Saturn's three bright rings and Tethys are cast onto the cloud tops. The limb of the planet can be seen easily through the 3,500-kilometer-wide (2,170 mile) Cassini Division, which separates ring A from ring B. The view through the much narrower Encke Division, near the outer edge of ring A is less clear. Beyond the Encke Division (at left) is the faintest of Saturn's three bright rings, the C-ring or crepe ring, barely visible against the planet. (Courtesy NASA/JPL)
Saturn's Blue Cranium
Saturn's northern hemisphere is presently a serene blue, more befitting of Uranus or Neptune, as seen in this natural color image from Cassini.
Light rays here travel a much longer path through the relatively cloud-free upper atmosphere. Along this path, shorter wavelength blue light rays are scattered effectively by gases in the atmosphere, and it is this scattered light that gives the region its blue appearance. Why the upper atmosphere in the northern hemisphere is so cloud-free is not known, but may be related to colder temperatures brought on by the ring shadows cast there.
Shadows cast by the rings surround the pole, looking almost like dark atmospheric bands. The ring shadows at higher latitudes correspond to locations on the ringplane that are farther from the planet--in other words, the northernmost ring shadow in this view is made by the outer edge of the A ring. (Courtesy NASA/JPL/Space Science Institute)
Nordic Optical Telescope
This image of Saturn was taken with the 2.6 meter Nordic Optical Telescope, located at La Palma, Canary Islands. (© Copyright Nordic Optical Telescope Scientific Association -- NOTSA)
Saturn's Rings Edge-On
In one of nature's most dramatic examples of "now-you see-them, now-you-don't," NASA's Hubble Space Telescope captured Saturn on May 22, 1995, as the planet's magnificent ring system turned edge-on. This ring-plane crossing occurs approximately every 15 years when the Earth passes through Saturn's ring plane.
The rings do not disappear completely because the edge of the rings reflects sunlight. The dark band across the middle of Saturn is the shadow of the rings cast on the planet (the Sun is almost 3 degrees above the ring plane.) The bright stripe directly above the ring shadow is caused by sunlight reflected off the rings onto Saturn's atmosphere. Two of Saturn's icy moons are visible as tiny starlike objects in or near the ring plane.
Storm on Saturn
This image, taken by the Hubble Space Telescope, shows a rare storm that appears as a white arrowhead-shaped feature near the planet's equator. The storm is generated by an upwelling of warmer air, similar to a terrestrial thunderhead. The east-west extent of this storm is equal to the diameter of the Earth (about 12,700 kilometers or 7,900 miles). The Hubble images are sharp enough to reveal that Saturn's prevailing winds shape a dark "wedge" that eats into the western (left) side of the bright central cloud. The planet's strongest eastward winds, clocked at 1,600 kilometers (1,000 miles) per hour based on Voyager spacecraft images taken in 1980-81, are at the latitude of the wedge.
To the north of this arrowhead-shaped feature, the winds decrease so that the storm center is moving eastward relative to the local flow. The clouds expanding north of the storm are swept westward by the winds at higher latitudes. The strong winds near the latitude of the dark wedge blow over the northern part of the storm, creating a secondary disturbance that generates the faint white clouds to the east (right) of the storm center. The storm's white clouds are ammonia ice crystals that form when an upward flow of warmer gases shoves its way through Saturn's frigid cloud tops.
HST Views Aurora on Saturn
The top image shows the first image ever taken of bright aurorae at Saturn's northern and southern poles, as seen in far ultraviolet light by the Hubble Space Telescope. Hubble resolves a luminous, circular band centered on the north pole, where an enormous auroral curtain rises as far as 2,000 kilometers (1,200 miles) above the cloudtops. This curtain changed rapidly in brightness and extent over the two hour period of HST observations.
The aurora is produced as trapped charged particles precipitating from the magnetosphere collide with atmospheric gases. As a result of the bombardment, Saturn's gases glow at far-ultraviolet wavelengths (110-160 nanometers). These wavelengths are absorbed by the Earth's atmosphere, and can only be observed from space-based telescopes.
For comparison, the bottom image is a visible-light color composite of Saturn as seen by Hubble on December 1, 1994. Unlike the ultraviolet image, Saturn's familiar atmospheric belts and zones are clearly seen. The lower cloud deck is not visible at UV wavelengths because sunlight is reflected from higher in the atmosphere.
Last View of Saturn
Two days after its encounter with Saturn, Voyager 1 looked back on the planet from a distance of more than 5.0 million kilometers (3.0 million miles). This view of Saturn has never been seen by an earth based telescope, since the earth is so close to the Sun only the sunlit face of Saturn can be seen. (Copyright © 2002 Calvin J. Hamilton)
False Color Image of Saturn's Rings
Possible variations in chemical composition from one part of Saturn's ring system to another are visible in this Voyager 2 picture as subtle color variations that can be recorded with special computer-processing techniques. This highly enhanced color view was assembled from clear, orange and ultraviolet frames obtained August 17, 1981 from a distance of 8.9 million kilometers (5.5 million miles). In addition to the previously known blue color of the C-ring and the Cassini Division, the picture shows additional color differences between the inner B-ring and and outer region (where the spokes form) and between these and the A-ring. (Courtesy NASA/JPL)
The Saturn System
This picture of the Saturnian system was prepared from an assemblage of images taken by the Voyager 1 spacecraft during its Saturn encounter in November 1980. This view shows Dione in the forefront, Saturn rising behind, Epimetheus (top-left) and Rhea just to the left of Saturn's rings. To the right and below Saturn's rings are Enceladus, Mimas, Tethys, and Iapetus (bottom-right). The cloud covered Titan is at the top-right. (Copyright Calvin J. Hamilton)
Saturn has 31 officially recognized and named satellites. In addition, there are other unconfirmed satellites. One circles in the orbit of Dione, a second is located between the orbits of Tethys and Dione, and a third is located between Dione and Rhea. The unconfirmed satellites were found in Voyager photographs, but were not confirmed by more than one sighting. Recently, the Hubble Space Telescope imaged four objects that might be new moons.
Several generalizations can be made about the satellites of Saturn. Only Titan has an appreciable atmosphere. Most of the satellites have a synchronous rotation. The exceptions are Hyperion, which has a chaotic orbit, and Phoebe. Saturn has a regular system of satellites. That is, the satellites have nearly circular orbits and lie in the equatorial plane. The two exceptions are Iapetus and Phoebe. All of the satellites have a density of <>3. This indicates they are composed of 30 to 40% rock and 60 to 70% water ice. Most of the satellites reflect 60 to 90% of the light that strikes them. The outer four satellites reflect less than this and Phoebe reflects only 2% of the light that strikes it.
The following table summarizes the radius, mass, distance from the planet center, discoverer and the date of discovery of each of the confirmed satellites of Saturn:
Moon | # | Radius (km) | Mass (kg) | Distance (km) | Discoverer | Date |
---|---|---|---|---|---|---|
Pan | XVIII | 9.655 | ? | 133,583 | Mark R. Showalter | 1990 |
S/2005 S1 | 7 | ? | 136,530 | Cassini Spacecraft | 2005 | |
Atlas | XV | 20x15 | ? | 137,640 | R. Terrile | 1980 |
Prometheus | XVI | 72.5x42.5x32.5 | 2.7e+17* | 139,350 | S. Collins & others | 1980 |
Pandora | XVII | 57x42x31 | 2.2e+17* | 141,700 | S. Collins & others | 1980 |
Epimetheus | XI | 72x54x49 | 5.6e+17* | 151,422 | R. Walker | 1980 |
Janus | X | 98x96x75 | 2.01e+18* | 151,472 | Audouin Dollfus | 1966 |
Mimas | I | 198.6 +- 0.6 | 3.84E+19 | 185,520 | William Herschel | 1789 |
Enceladus | II | 249.4 +- 0.2 | 8.65E+19 | 238,020 | William Herschel | 1789 |
Tethys | III | 529.9 +- 1.5 | 6.176E+20 | 294,660 | Giovanni Domenico Cassini | 1684 |
Telesto | XIII | 17x14x13 | ? | 294,660 | B. Smith & others | 1980 |
Calypso | XIV | 17x11x11 | ? | 294,660 | B. Smith & others | 1980 |
Dione | IV | 559. +- 5 | 1.0959E+21 | 377,400 | Giovanni Domenico Cassini | 1684 |
Helene | XII | 18x16x15 | ? | 377,400 | P. Laques & J. Lecacheus | 1980 |
Rhea | V | 764. +- 4 | 2.3166E+21 | 527,040 | Giovanni Domenico Cassini | 1672 |
Titan | VI | 2575.5 +- 2 | 1.345426E+23 | 1,221,850 | Christiaan Huygens | 1655 |
Hyperion | VII | 205x130x110 | 1.77E+19 | 1,481,000 | William Cranch Bond | 1848 |
Iapetus | VIII | 730 | 1.88E+21 | 3,561,300 | Giovanni Domenico Cassini | 1671 |
Kiviuq | XXIV | 7 | 11,365,000 | B. Gladman | 2000 | |
Ijiraq | XXII | 5 | 11,442,000 | J.J. Kavelaars, B. Gladman | 2000 | |
Phoebe | IX | 115 x 110 x 105 | 4E+18 | 12,952,000 | William Henry Pickering | 1898 |
Paaliaq | XX | 9.5 | 15,198,000 | B. Gladman | 2000 | |
Skathi | XXVII | 3.2 | 15,641,000 | J.J. Kavelaars, B. Gladman | 2000 | |
Albiorix | XXVI | 13 | 16,394,000 | M. Holman, T.B. Spahr | 2000 | |
Erriapo | XXVIII | 4.3 | 17,604,000 | J.J. Kavelaars, B. Gladman | 2000 | |
Siarnaq | XXIX | 16 | 18,195,000 | B. Gladman, J.J. Kavelaars | 2000 | |
Tarvos | XXI | 6.5 | 18,239,000 | J.J. Kavelaars, B. Gladman | 2000 | |
S/2003 S1 | 3.3 | 18,719,000 | S.S. Sheppard | 2003 | ||
Mundilfari | XXV | 2.8 | 18,722,000 | B. Gladman, J.J. Kavelaars | 2000 | |
Suttungr | XXIII | 2.8 | 19,465,000 | B. Gladman, J.J. Kavelaars | 2000 | |
Thrymr | XXX | 2.8 | 20,219,000 | B. Gladman, J.J. Kavelaars | 2000 | |
Ymir | XIX | 8 | 23,130,000 | B. Gladman | 2000 |
Thomas, P., J. Veverka, D. Morrison, M. Davies. and T. V. Johnson. "Saturn's Small Satellites: Voyager Imaging Results." Journal of Geophysical Research, November 1, 1983, 8743-8754.
Soderblom, Laurence A. and Torrence V. Johnson. "The Moons of Saturn." Scientific American, January 1982.
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Resource: www.solarviews.com/eng/saturn.htm
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