Saturn: Difference between revisions
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==Physical characteristics== |
==Physical characteristics== |
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Saturn's shape is visibly flattened at the poles and bulging at the equator (an [[oblate]] [[spheroid]]); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but to a lesser degree. Saturn is also the only one of the Solar System's planets less dense than water, with an average specific density of 0.69. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water |
Saturn's shape is visibly flattened at the poles and bulging at the equator (an [[oblate]] [[spheroid]]); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but to a lesser degree. Saturn is also the only one of the Solar System's planets less dense than water, with an average specific density of 0.69. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water. |
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Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid [[metallic hydrogen]] layer above that, and a [[molecular hydrogen]] layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 [[Kelvin|K]] 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. |
Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid [[metallic hydrogen]] layer above that, and a [[molecular hydrogen]] layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 [[Kelvin|K]] 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. |
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===History=== |
===History=== |
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The rings were first observed by |
The rings were first observed by [[Galileo Galilei]] in [[1610]] with his [[telescope]], but he clearly did not know what to make of them. He wrote to the Duke of [[Tuscany]] that "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. |
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The riddle of the rings was not solved until [[1655]] by [[Christiaan Huygens]], using a telescope much more powerful than the ones available to Galileo in his time. |
The riddle of the rings was not solved until [[1655]] by [[Christiaan Huygens]], using a telescope much more powerful than the ones available to Galileo in his time. |
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Revision as of 05:14, 8 January 2006
Template:Planet Infobox/Saturn Saturn is the sixth planet from the Sun. It is a gas giant, the second-largest planet in the solar system after Jupiter. Saturn has a prominent system of rings, consisting of mostly ice particles with a smaller amount of rocky debris. It was named after the Roman god Saturn. Its symbol is a stylized representation of the god's sickle (Unicode: ♄). The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the earth star (土星), based on the Five Elements. [1], [2]
Physical characteristics
Saturn's shape is visibly flattened at the poles and bulging at the equator (an oblate spheroid); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but to a lesser degree. Saturn is also the only one of the Solar System's planets less dense than water, with an average specific density of 0.69. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water.
Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 K 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.
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 they're also much wider near the equator. Saturn's winds are among the Solar System's fastest; Voyager data indicates peak easterly winds of 500 m/s (1116 mph).(1) 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. The careful study of these episodes reveal interesting patterns; if it holds another storm will occur in ~2020.(2)
Astronomers using infrared imaging have shown that Saturn has a warm polar vortex, and is the only planet in the solar system known to do so.
(1) Voyager Saturn Science Summary
(2) Patrick Moore, ed., The 1993 Yearbook of Astronomy, Mark Kidger, "The 1990 Great White Spot of Saturn", 176-215, (New York: W.W. Norton & Company, 1992).
Rotational behavior
Since Saturn does not rotate on its axis at a uniform rate, two rotation periods have been assigned to it, like 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 in value 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). [3] The cause of the change is unknown.
Planetary rings
Saturn is probably best known for its planetary rings, which make it one of the most visually remarkable objects in the solar system.
History
The rings were first observed by Galileo Galilei in 1610 with his telescope, but he clearly did not know what to make of them. He wrote to the Duke of Tuscany that "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.
The riddle of the rings was not solved until 1655 by Christiaan Huygens, using a telescope much more powerful than the ones available to Galileo in his time.
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.
Physical characteristics
The rings can be viewed using a quite modest modern telescope or with a good pair of binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, 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 that Saturn formed out of. 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, could 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 undiscovered, 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 indicates 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) and is thought to be a product of the disintegration of water ice from the rings into its components, oxygen and hydrogen. [4]
Dark side of the rings
Compare images from the Cassini spacecraft taken in March and October 2004, and a Pioneer 11 picture from 1979:
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The side of Saturn's rings that is lit by the Sun looks very different to the backlit side, which is darker overall and appears almost black in the thick B ring. From Earth, we cannot appreciate this because the Earth cannot view Saturn from an angle that displays the backlit side of the rings, and our only views of it are from spacecraft. In 2004, the Cassini spacecraft revealed the first views of the backlit side in 25 years.
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. 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 behind the spokes is still unknown.
Twenty-five years later, Cassini observed the spokes again. 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 September 5 2005.
Natural satellites
Saturn has a large number of moons. The precise figure will never be certain 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. Seven of the moons are massive enough to have collapsed into a spheroid under their own gravitation. These are compared to 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.
| Saturn's major moons, compared to Earth's moon Luna | |||||
|---|---|---|---|---|---|
| Name |
Diameter (km) |
Mass (kg) |
Orbital radius (km) | Orbital period (days) | |
| Mimas | mye'-məs ˈmaɪməs |
400 (10% Luna) |
0.4×1020 (0.05% Luna) |
185,000 (50% Luna) |
0.9 (3% Luna) |
| Enceladus | en-sel'-a-dəs ɛnˈsɛlədəs |
500 (15% Luna) |
1.1×1020 (0.2% Luna) |
238,000 (60% Luna) |
1.4 (5% Luna) |
| Tethys | tee'-this ˈtiθəs |
1060 (30% Luna) |
6.2×1020 (0.8% Luna) |
295,000 (80% Luna) |
1.9 (7% Luna) |
| Dione | dye-oe'-nee daɪˈɔʊni |
1120 (30% Luna) |
11×1020 (1.5% Luna) |
377,000 (100% Luna) |
2.7 (10% Luna) |
| Rhea | ree'-a ˈriə |
1530 (45% Luna) |
23×1020 (3% Luna) |
527,000 (140% Luna) |
4.5 (20% Luna) |
| Titan | tye'-tən ˈtaɪtən |
5150 (150% Luna) |
1350×1020 (180% Luna) |
1,222,000 (320% Luna) |
16 (60% Luna) |
| Iapetus | eye-ap'-i-təs aɪˈæpətəs |
1440 (40% Luna) |
20×1020 (3% Luna) |
3,560,000 (930% Luna) |
79 (290% Luna) |
Due to the tidal forces of Saturn, the moons are currently not at the same position as they were when they were first formed (for a timeline of discovery dates, see Timeline of natural satellites).
Exploration of Saturn
Pioneer 11 flyby
Saturn was first visited by Pioneer 11 in September 1979. It flew within 20,000 km of the planet's cloudtops. Low-resolution images were acquired of the planet and few of its moons. Resolution was not good enough to discern surface features, however. 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. It also measured the temperature of Titan. [5]
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 the 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 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 camera stuck 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 and Keeler gaps.
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. 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. As of 2005, Cassini is conducting multiple flybys of Titan and icy satellites. The primary mission ends in 2008 when the spacecraft has completed 74 orbits around the planet.
- For the latest information and news releases, see Cassini website.
Best viewing of Saturn
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 2031, mostly due to a favourable orientation of the rings relative to the Earth.
| Date of Opposition | Distance to Earth (AU) |
Angular diameter |
|---|---|---|
| December 3, 2001 | 8.08 | 20.6 arcsec |
| December 17, 2002 | 8.05 | 20.7 arcsec |
| December 31, 2003 | 8.05 | 20.7 arcsec |
| January 13, 2005 | 8.08 | 20.6 arcsec |
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 29 and a half years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Optical aid (a large pair of binoculars or a telescope) magnifying at least 20X is required to clearly resolve Saturn's rings for most people.
Appearance
Occultation of stars by Saturn between 1800 and 2100
| Date | Time | Star | Brightness of Star | Right ascension of star (Actual Equinox) | Declination of star (Actual Equinox) |
|---|---|---|---|---|---|
| 1818 March 27 | 1h 30,7m UTC | 83 AQUARII | 5,5 | 23 5 8,278 | - 7 41 38,97 |
| 1857 June 30 | 0h 12,6m UTC | DELTA GEMINORUM | 3,6 | 7 20 7,569 | 21 58 57,46 |
| 1965 April 23 | 15h 0,3m UTC | 83 AQUARII | 5,5 | 23 5 9,499 | - 7 41 37,93 |
| 1989 July 03 | 7h 42,1m UTC | 28 SAGITTARII | 5,5 | 18 46 20,584 | -22 23 31,87 |
| 2032 April 07 | 6h 17,6m UTC | 105 TAURI | 5,9 | 5 7 55,440 | 21 42 17,15 |
Saturn in fiction and film
Saturn is a popular setting for science fiction novels and films, although the planet tends to be used as a pretty backdrop rather than as an important part of the plot.
- In Voltaire's Micromégas (1752), the eponymous hero arrives at Saturn first (Uranus and Neptune were unknown then). Saturn's citizens are « only a thousand fathoms high », have 72 senses and live for about 15,000 years. Micromégas forms a close friendship with the secretary of the Academy of Saturn, who accompanies him to Earth.
- The unwitting adventurers in Jules Verne's Off on a Comet (1877) pass within 415,000,000 miles of Saturn while riding on a comet. The book describes Saturn as having 8 satellites and 3 rings. It contains a black and white illustration showing what night might look like from the surface of the planet. The rings are brightly illuminated by the sun, and an elliptical shadow is cast on them by the planet. The drawing shows the surface of Saturn as a rocky, desolate, solid surface.
- In H. P. Lovecraft's Cthulhu Mythos (1928–), Saturn was known as Cykranosh in the Hyperborean Era, both Tsathoggua and Atlach-Nacha came to Earth from there, and Tsathoggua's paternal uncle Hziulquoigmnzhah still resides there.
- In Isaac Asimov's short story The Martian Way (1952), Martian colonists use a chunk of ice from Saturn's rings to bring water to the dry world.
- Kurt Vonnegut's novel The Sirens of Titan (1959) is partly set on Titan, Saturn's best known moon.
- In the Star Trek universe (1966–), Saturn is used for the Starfleet Academy Flight Range.
- In Arthur C. Clarke's novel version of 2001: A Space Odyssey (1968), a spacecraft visits the Saturnian system. Clarke's later novel Imperial Earth (1976) takes place partially at a human colony on Titan.
- Douglas Trumbull's film Silent Running (1972) features an ark-like spacecraft traveling through the Saturnian system.
- In the sixth book of the Yoko Tsuno comic book series (Les Trois soleils de Vinéa, 1976), a small part of the action takes place on a Vinean space station in orbit around Saturn. Saturn's moon Titan is also briefly mentioned and depicted. Other Saturnian moons are visible but not named.
- The film Saturn 3 (1980) is mostly set on one of Saturn's moons, but also features a journey through the planet's rings.
- The science fiction anime series The Super Dimension Fortress Macross (1982–1983) has one episode that takes place in Saturn's rings, and the beginning of the movie adaptation The Super Dimension Fortress Macross: Do You Remember Love? takes place near the moon Titan and Saturn's rings.
- An episode of the cartoon series Transformers from 1985, "The God Gambit," reveals that humanoid aliens have a thriving civilization on the moon Titan. In a later episode from 1986, "Money is Everything," which takes place in the year 2006, Titan has been terraformed by humans.
- Warhammer 40,000's universe (1987) places the headquarters of the Grey Knights and Ordo Malleus in Saturn's moons, owing to their defensive capability.
- Tim Burton's film Beetlejuice (1988) is partly set on a fictional Saturn, populated by giant sandworms.
- The Citadel research and mining space station, setting of the computer game System Shock (1994), is in orbit of Saturn for most of the game.
- Stephen Baxter's novel Titan (1997) is focused on the moon Titan, but contains vivid depictions of a journey through the Saturnian system.
- In Michael McCollum's novel The Clouds of Saturn (1998), SparrowHawk pilots Larson Sands and Halley Trevanon fight against the Northern Alliance during a time when the Sun has flared out of control and boiled Earth's oceans away.
- In the sci-fi anime Cowboy Bebop (1998), in the year 2068 a war was fought on Titan.
- In the anime Bishoujo Senshi Sailor Moon, Sailor Saturn is a guardian representing the planet. Her birth is thought to bring destruction to the world, as she's known as the sailor of death and rebirth. On her forehead is the planet's symbol.
- Ben Bova's novel Saturn (2003) is about a spacecraft traveling toward the planet, although Saturn itself does not figure greatly in the story.
Saturn in various cultures
Chinese and Japanese culture designate the planet Saturn as "Earth Star". This is based on Five Elements which was 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). See: Kabbalah.
See also
External links
- NASA's Saturn fact sheet
- NASA's Cassini mission to Saturn
- Change of seasons on Saturn
- Theoretical description of the rings of Saturn
- A Trip Into Space Description and photos of Saturn
