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 large rings made mainly out of ice and space debris. It was named after the Roman god Saturn. Its symbol is a stylized representation of the god's sickle (Unicode: ♄).

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 not so much so. 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, but Saturn's bands are much fainter and they're also much wider near the equator. Saturn's 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 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. 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.

Rotational behaviour

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). [2] The cause of the change is unknown.

Saturn's rings

Saturn is probably best known for its planetary rings, which make it one of the most visually remarkable objects in the solar system. See rings of Saturn for a list of the planet's rings.

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 Grand 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 of the rings

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 Voyagers 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. [3]

The dark side of the rings

Compare images from the Cassini spacecraft taken in March and October 2004, and a Pioneer 11 picture from 1979:

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.

The 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 dark 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. 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.

As of February 2005, the Cassini spacecraft has not observed any spokes in the rings, despite possessing imaging equipment of higher quality than the Voyagers'. It is possible that the spokes appear and disappear seasonally.

Exploration of Saturn

Pioneer 11 flyby

Saturn was first visited by Pioneer 11 in 1979. It flew within 20,000 km 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. [4]

Voyager flybys

In November, 1980, Voyager 1 probe visited the Saturn system. It send back the first high-resolution images of the planet, rings, and the satellites. Surface features of various moons were seen 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 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.

Saturn's moons

Main article: Saturn's natural satellites

Saturn has a large number of moons, 49 are currently confirmed, 34 of which have names. 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. Saturn's most noteworthy moon is Titan, the only moon in the solar system to have a dense atmosphere.

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.

Best viewing of Saturn

File:Saturnoppositions.png

Saturn Oppositions: 2001-2029

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.

**Saturn's Opposition Periods 2001–2005**
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

Stationary, retrorad	Opposition	Stationary, prograd	Conjunction to sun
October 26th, 2003	December 31st, 2003	March 7th, 2004	July 8th, 2004
November 8th, 2004	January 13th, 2005	March 22nd, 2005	July 23rd, 2005
November 22nd, 2005	January 27th, 2006	April 5th, 2006	August 8th, 2006
December 6th, 2006	February 10th, 2007	April 20th, 2007	August 21st, 2007
December 20th, 2007	February 24th, 2008	May 3rd, 2008	September 4th, 2008
January 1st, 2009	March 8th, 2009	May 17th, 2009	September 17th, 2009
January 14th, 2010	March 22th, 2010	May 31st, 2010	October 1st, 2010
January 27th, 2011	April 3rd, 2011	June 14th, 2011	October 13th, 2011
February 8th, 2012	April 15th, 2012	June 26th, 2012	October 25st, 2012
February 19th, 2013	April 28th, 2013	July 9th, 2013	November 6th, 2013
March 3rd, 2014	May 10th, 2014	July 21st, 2014	November 18th, 2014
March 14th, 2015	May 23rd, 2015	August 2nd, 2015	November 30th, 2015
March 25th, 2016	June 3rd, 2016	August 13th, 2016	December 10th, 2016
April 6th, 2017	June 15th, 2017	August 25th, 2017	December 21st, 2017
April 18th, 2018	June 27th, 2018	September 6th, 2018	January 2nd, 2019
April 30th, 2019	July 9th, 2019	September 18th, 2019	January 13th, 2020
May 11th, 2020	July 20th, 2020	September 29th, 2019	January 24th, 2021

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 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.
Stephen Baxter's novel Titan is focused on the moon Titan, but contains vivid depictions of a journey through the Saturnian system.
Ben Bova's novel Saturn (2003) is about a spacecraft travelling toward the planet, although Saturn itself does not figure greatly in the story.
In Isaac Asimov's short story The Martian Way, Martian colonists use a chunk of ice from Saturn's rings to bring water to the dry world.
Douglas Trumbull's film Silent Running (1972) features an ark-like spacecraft travelling through the Saturnian system.
The film Saturn 3 (1980) is mostly set on one of Saturn's moons, but also features a journey through the planet's rings.
Kurt Vonnegut's novel The Sirens of Titan is partly set on Saturn's best known moon.
Tim Burton's film Beetlejuice 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, is in orbit of Saturn for most of the game.
In Michael McCollum's novel The Clouds of Saturn, 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 Star Trek, Saturn is used for the Starfleet Academy Flight Range.
In the sci-fi anime Cowboy Bebop, in the year 2068 a war was fought on Titan.

Saturn in various cultures

Chinese and Japanese culture designates the planet Saturn with "Earth Star." This is based on Five Elements which was traditionally used to classify natural elements.

Saturn in astrology

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

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