Saturn

Infobox Planet
bgcolour = #FFCC66
name = Saturn
symbol =


caption = Saturn, as seen by Cassini
orbit_ref =cite web
last = Yeomans | first = Donald K. | date = 2006-07-13
url = http://ssd.jpl.nasa.gov/?horizons
title = HORIZONS System | publisher = NASA JPL
accessdate = 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.]
epoch = J2000
aphelion = 1 513 325 783 km
10.115 958 04 AU
perihelion = 1 353 572 956 km
9.048 076 35 AU
semimajor = 1 433 449 370 km
9.582 017 20 AU
eccentricity = 0.055 723 219
period = 10 832.327 days
29.657 296 yr
synodic_period = 378.09 days
avg_speed = 9.69 km/s
inclination = 2.485 240°
5.51° to Sun's equator
asc_node = 113.642 811°
arg_peri = 336.013 862°
mean_anomaly = 320.346 750°
satellites = ~ 200 observed (60 with secure orbits)
maximum distance from sun = 1.5 billion km (938 million miles)
minimum distance from sun = 1.35 billion km (840 million miles)
physical_characteristics = yes
flattening = 0.097 96 ± 0.000 18
equatorial_radius = 60 268 ± 4 kmcite journal|last= Seidelmann|first= P. Kenneth|coauthors= Archinal, B. A.; A’hearn, M. F.; et.al.|title= Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006|journal= Celestial Mech. Dyn. Astr. |volume=90|pages=155–180|year=2007|doi=10.1007/s10569-007-9072-y|url=http://adsabs.harvard.edu/doi/10.1007/s10569-007-9072-y] Refers to the level of 1 bar atmospheric pressure]
9.4492 Earths
polar_radius = 54 364 ± 10 km
8.5521 Earths
surface_area = 4.27e|10 km² [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn&Display=Facts NASA: Solar System Exploration: Planets: Saturn: Facts & Figures] ]
83.703 Earths
volume = 8.2713e|14 km³cite web
url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html
title = Saturn Fact Sheet
publisher = NASA
last = Williams
first = Dr. David R.
accessdate = 2007-07-31
date = September 7, 2006]
763.59 Earths
mass = 5.6846e|26 kg
95.152 Earths
density = 0.687 g/cm³
(less than water)
surface_grav = 8.96 m/s²
0.914 "g"
escape_velocity = 35.5 km/s
sidereal_day = 0.439 – 0.449 daycite web
url = http://www.space.com/scienceastronomy/070906_saturn_day.html
title = Length of Saturn's Day Revised
publisher = Space.com
last = Than
first = Ker
accessdate = 2007-09-06
date = September 6, 2007]
(10 h 32 – 47 min)
rot_velocity = 9.87 km/s
35 500 km/h
axial_tilt = 26.73°
right_asc_north_pole = 2 h 42 min 21 s
40.589°
declination = 83.537°
albedo = 0.342 (bond) 0.47 (geom.)
magnitude = +1.2 to -0.24cite web
url = http://findarticles.com/p/articles/mi_qa4015/is_200101/ai_n8933308
title = Wideband photoelectric magnitude measurements of Saturn in 2000
accessdate = 2007-10-14
last = Schmude
first = Richard W Junior
year = 2001
publisher = Georgia Journal of Science]
angular_size = 14.5" — 20.1"
(excludes rings)
temperatures = yes
temp_name1 = 1 bar level
min_temp_1 =
mean_temp_1 = 134 K
max_temp_1 =
temp_name2 = 0.1 bar
min_temp_2 =
mean_temp_2 = 84 K
max_temp =
adjectives = Saturnian
atmosphere = yes
atmosphere_ref =
scale_height = 59.5 km
atmosphere_composition =

~96%	Hydrogen (H2)
~3%	Helium
~0.4%	Methane
~0.01%	Ammonia
~0.01%	Hydrogen deuteride (HD)
0.000 7%	Ethane
Ices:
	Ammonia
	water
	ammonium hydrosulfide(NH4SH)

Saturn (Audio-IPA|en-us-Saturn.ogg|/ˈsætɚn/ [cite book
first=Elizabeth | last=Walter | date=April 21, 2003
title=Cambridge Advanced Learner's Dictionary
publisher=Cambridge University Press
edition=Second Edition | id=ISBN 0521531063 ] ) is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian planets, where Jovian is the adjectival form of Jupiter.

Saturn is 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: Unicode|♄).

The planet Saturn is composed of hydrogen, with small proportions of helium and trace elements. 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 1800 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 Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.cite web
url = http://saturn.jpl.nasa.gov/news/features/saturn-story/moons.cfm
title = The Story of Saturn
accessdate = 2007-07-07
last = Munsell
first = Kirk
date = April 6, 2005
publisher = NASA Jet Propulsion Laboratory; California Institute of Technology]

Physical characteristics

Due to a combination of its lower 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%—60 268 km vs. 54 364 km. 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 318 times the mass of the Earthcite web
url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html
title = Jupiter Fact Sheet
publisher = NASA
last = Williams
first = Dr. David R.
date = November 16, 2004
accessdate = 2007-08-02] but only about 20% larger than Saturn. [cite web
url = http://ase.arc.nasa.gov/projects/bayes-group/Atlas/size/Jupiter/Saturn.html
title = Jupiter compared to Saturn
publisher = NASA
accessdate = 2007-07-15]

Internal structure

Though there is little direct information about Saturn's internal structure, it is thought that its 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 1000 km a gaseous atmosphere. Traces of various ices are also present. The core region is estimated to be about 9–22 times the mass of the Earth. [cite journal
last = Fortney
first = Jonathan J.
title=Looking into the Giant Planets
journal=Science
year=2004
volume=305
issue=5689
pages=1414–1415
url=http://www.sciencemag.org/cgi/content/full/305/5689/1414
accessdate=2007-04-30
doi=10.1126/science.1101352
pmid=15353790 ] 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. [cite web
url = http://www.nasa.gov/worldbook/saturn_worldbook.html
title = NASA - Saturn
publisher = NASA
accessdate = 2007-07-27
year = 2004]

Atmosphere

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. [cite journal
author=Courtin, R.; Gautier, D.; Marten, A.; Bezard, B.
title=The Composition of Saturn's Atmosphere at Temperate Northern Latitudes from Voyager IRIS spectra
journal=Bulletin of the American Astronomical Society
year=1967

volume=15
pages=831
url=http://adsabs.harvard.edu/abs/1983BAAS...15..831C
accessdate=2007-02-04 ] 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. [cite web
last = Martinez
first = Carolina
date =September 5, 2005
url = http://www.nasa.gov/mission_pages/cassini/whycassini/cassini-090505-clouds.html
title =Cassini Discovers Saturn's Dynamic Clouds Run Deep
publisher =NASA
accessdate = 2007-04-29] 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.cite journal
last = Guillot
first = Tristan
title=Interiors of Giant Planets Inside and Outside the Solar System
journal=Science
year=1999
volume=286
issue=5437
pages=72–77
url=http://www.sciencemag.org/cgi/content/full/286/5437/72
accessdate=2007-04-27
doi=10.1126/science.286.5437.72
pmid=10506563 ]

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. [cite web
url = http://www.mira.org/fts0/planets/100/text/txt002x.htm
title = Saturn
accessdate = 2007-07-27
publisher = MIRA] Saturn's winds are among the Solar System's fastest. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h).cite web
title = Voyager Saturn Science Summary
url = http://www.solarviews.com/eng/vgrsat.htm
first = Calvin
middle = J.
last = Hamilton
accessdate = 2007-07-05
year = 1997
publisher = Solarviews] 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. [cite web|title=Saturn’s cloud structure and temporal evolution from ten years of Hubble Space Telescope images (1994–2003)|author=S. Pérez-Hoyos, A. Sánchez-Lavega, R.G. Frenchb, J.F. Rojas|url=http://www.ajax.ehu.es/sph/principal/tesis/docs/sph.etal.2005.pdf|format=PDF|year=2005|accessdate=2007-07-24] 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. [Patrick Moore, ed., "1993 Yearbook of Astronomy", (London: W.W. Norton & Company, 1992), Mark Kidger, "The 1990 Great White Spot of Saturn", pp. 176-215.]

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.

. [cite web
url = http://www.nasa.gov/mission_pages/cassini/multimedia/pia09188.html
title = Saturn's Strange Hexagon
accessdate = 2007-07-06
date = March 27, 2007
last = Watanabe
first = Susan
publisher = NASA] ]

Astronomers using infrared imaging have shown that Saturn has a warm polar vortex and that it is the only such feature 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. [cite web
url = http://www.mcpstars.org/node/353
title = Warm Polar Vortex on Saturn
year = 2007
publisher = Merrillville Community Planetarium
accessdate = 2007-07-25]

A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images. [cite web
url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1988Icar...76..335G&db_key=AST&data_type=HTML&format=
title = A hexagonal feature around Saturn's North Pole
last = Godfrey
publisher = Icarus
accessdate = 2007-07-09] [cite web
url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993Sci...260..329S&db_key=AST&data_type=HTML&format=
title = Ground-based observations of Saturn's north polar SPOT and hexagon
last = Sanchez-Lavega
first = A.
accessdate = 2007-07-30
publisher = Bulletin of the American Astronomical Society] 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". [cite web
url = http://www.aas.org/publications/baas/v34n3/dps2002/10.htm
title = Hubble Space Telescope Observations of the Atmospheric Dynamics in Saturn's South Pole from 1997 to 2002
accessdate = 2007-07-06
publisher = The American Astronomical Society
date = October 8, 2002] 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. [cite web
url = http://photojournal.jpl.nasa.gov/catalog/PIA09187
title = NASA catalog page for image PIA09187
accessdate = 2007-05-23
publisher = NASA Planetary Photojournal] 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). [cite web
url = http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=703
title = NASA Sees into the Eye of a Monster Storm on Saturn
publisher = NASA
date = November 9, 2006
accessdate = 2006-11-20]

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 think some sort of standing-wave pattern in the atmosphere; but the hexagon might be a novel sort of aurora. Polygon shapes have been replicated in spinning buckets of fluid in a laboratory. [cite web
url = http://www.nature.com/news/2006/060515/full/060515-17.html
title = Geometric whirlpools revealed
publisher=Nature
date = May 19, 2006
accessmonthday=April 27
accessyear=2007 Bizarre geometric shapes that appear at the centre of swirling vortices in planetary atmospheres might be explained by a simple experiment with a bucket of water but correlating this to Saturn's pattern is by no means certain.]

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. As a result the cronian magnetosphere is much smaller than the jovian and extends slightly beyond the orbit of Titan.cite web
url = http://library.thinkquest.org/C005921/Saturn/satuAtmo.htm
title = Saturn: Atmosphere and Magnetosphere
publisher = Thinkquest Internet Challenge
accessdate = 2007-07-15
last = McDermott
first = Matthew
year = 2000] 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. Similarly to 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.cite web
author=Russell, C. T.; Luhmann, J. G.
year=1997
url =http://www-ssc.igpp.ucla.edu/personnel/russell/papers/sat_mag.html
title =Saturn: Magnetic Field and Magnetosphere
publisher =UCLA - IGPP Space Physics Center
accessdate = 2007-04-29]

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, 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 0.056, the distance between Saturn and the Sun varies by approximately 155 000 000 km between perihelion and aphelion, 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). [cite web
url = http://www.nasa.gov/mission_pages/cassini/media/cassini-062804.html
title = Scientists Find That Saturn's Rotation Period is a Puzzle
date = June 28, 2004
publisher = NASA
accessdate = 2007-03-22] 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. [cite press release
url = http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=733
title = Enceladus Geysers Mask the Length of Saturn's Day
date = March 22, 2007
publisher = NASA Jet Propulsion Laboratory
accessdate=2007-03-22] [cite web
url = http://www.sciencemag.org/cgi/content/abstract/316/5823/442
title = The Variable Rotation Period of the Inner Region of Saturn's Plasma Disk
date = March 22, 2007
publisher = "Science"
accessdate = 2007-04-24] [cite web
url = http://www.sciencemag.org/cgi/content/abstract/316/5823/442
title = A New Spin on Saturn's Rotation
date = April 20, 2007
publisher="Science"
accessdate=2007-04-24]

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.cite journal|author=J.D. Anderson and G. Schubert|journal=Science|title=Saturn's gravitational field, internal rotation, and interior structure|volume=317|pages=1384–1387|year=2007|doi= 10.1126/science.1144835|pmid=17823351]

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.cite web
title = Saturn
url = http://www.nmm.ac.uk/server/show/conWebDoc.286
publisher = National Maritime Museum
accessdate = 2007-07-06]

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. [cite web|title=NightSky Friday: See Saturn closest to Earth in 30 Years|work=space.com|author=Joe Rao|url=http://www.space.com/spacewatch/saturn_guide_031205.html|year=2003|accessdate=2007-07-28] Then, in 1613, they reappeared again, further confusing Galileo.cite web|url=http://www2.jpl.nasa.gov/saturn/back.html|title=Historical Background of Saturn's Rings|accessdate=2007-05-23|work=Saturn Ring Plane Crossings of 1995-1996|first=Ron|last=Baalke|publisher=Jet Propulsion Laboratory]

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."

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 4800 km-wide region between the A Ring and B Ring.cite web
title = Saturn's Cassini Division
url = http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level2/cassini_division.html
accessdate = 2007-07-06
publisher = StarChild]

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. [cite web
url=http://www-history.mcs.st-andrews.ac.uk/~history/Extras/Maxwell_Saturn.html
title=James Clerk Maxwell on the nature of Saturn's rings
accessdate = 2007-07-08
month = March | year = 2006
publisher = JOC/EFR] Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Lick Observatory.

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 20 meters in thickness, and are composed of 93 percent water ice with a smattering of tholin impurities, and 7 percent amorphous carbon. [cite web|title= The Composition of Saturn's Rings|author=Poulet F.; Cuzzi J.N.|work=NASA Ames Research Center|url=http://www.ingentaconnect.com/content/ap/is/2002/00000160/00000002/art06967|year=2002|accessdate=2007-07-28] They range in size from specks of dust to the size of a small automobile. [cite web
url = http://www.ee.kth.se/php/modules/publications/reports/2005/TRITA-ALP-2005-03.pdf
title = Dusty Plasma Response to a Moving Test Change
first = Muhammad
last = Shafiq
year = 2005
accessdate = 2007-07-25
format = PDF] 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.

While the largest gaps in the rings, such as the Cassini Division and Encke Gap, can be seen from Earth, both Voyager spacecraft discovered that the rings have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise, in several different ways, from the gravitational pull of Saturn's many moons. 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 interacts with 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. [cite web
url = http://news.bbc.co.uk/1/hi/sci/tech/4640641.stm
title = Saturn rings have own atmosphere
date = July 1, 2005
accessdate = 2007-07-06
last = Rincon
first = Paul
publisher = British Broadcasting Corporation] 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. [cite web
url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006ApJ...644L.137J&db_key=AST&data_type=HTML&format=&high=42bf06f4d906731
title = The Enceladus and OH Tori at Saturn
last = Johnson
first = R. E.
accessdate = 2007-07-07
year = 2006
publisher = The American Astronomical Society]

Saturn shows complex patterns in its brightness. Most of the variability is due to the changing aspect of the rings,cite web
url = http://goliath.ecnext.com/coms2/summary_0199-5991060_ITM
title = Wideband photometric magnitude measurements of Saturn made during the 2005-06 Apparition
accessdate = 2007-10-14
last = Schmude
first = Richard, Jr.
date = September 22, 2006
publisher = Georgia Journal of Science] 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. [cite web
url = http://www.britastro.org/jbaa/113-1.htm
title = The Journal of the British Astronomical Association
publisher = British Astronomical Association
accessdate=2007-07-07
year = 2003
month = February]

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.

pokes 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.cite web
url = http://www.planetary.org/explore/topics/saturn/rings.html
title = The Alphabet Soup of Saturn's Rings
publisher = The Planetary Society
year = 2007
accessdate = 2007-07-24] The spokes appear dark in backscattered light, and bright in forward-scattered light. 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. [cite web
url = http://www.solarviews.com/eng/saturnrings.htm
title = Saturn's Magnificent Rings
first = Calvin
last = Hamilton
year = 2004
accessdate = 2007-07-25]

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.cite web
url = http://www.space.com/scienceastronomy/050915_cassini_spokes.html
title = Cassini Probe Spies Spokes in Saturn's Rings
accessdate = 2007-07-06
date = 2005-09-15
publisher = Imaginova Corp.
first = Tarig
last = Malik]

Natural satellites

Saturn has a large number of moons. The precise figure is indeterminate, 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, 60 moons had been identified, plus 3 unconfirmed moons that could be large dust clumps in the rings. Of those, 52 had been given proper names. Many of the moons are very small: 34 are less than 10 km in diameter, and another 13 less than 50 km.cite web
url = http://www.ifa.hawaii.edu/~sheppard/satellites/satsatdata.html|title=www.ifa.hawaii.edu/~sheppard/satellites/satsatdata.html
title = Saturn's Known Satellites
accessdate=2007-05-23] Only seven are massive enough to have collapsed into hydrostatic equilibrium 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. 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. [cite book|title=Solar System Voyage|author=Serge Brunier|pages=164|publisher=Cambridge University Press|year=2005]

Saturn's second largest moon Rhea may have a tenuous ring system of its own.cite journal
last = Jones
first = Geraint H.
coauthors = "et al."
title = The Dust Halo of Saturn's Largest Icy Moon, Rhea
url = http://www.sciencemag.org/cgi/content/short/319/5868/1380
journal = Science
volume = 319
issue = 5868
pages = 1380–1384
publisher = AAAS
date = 2008 March 7
doi = 10.1126/science.1151524
pmid = 18323452]

Traditionally, most of Saturn's moons have been 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", [Herschel, J.; "Results of Astronomical Observations made at the Cape of Good Hope", 1847 —as reported by Lassell, W.; [http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html Monthly Notices of the Royal Astronomical Society] , Vol. 8, No. 3 (January 14, 1848), pp. 42–43] because they were the sisters and brothers of Cronos (the Greek Saturn).

:"For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellites".

History and exploration

Ancient times and observation

Saturn has been known since prehistoric times.cite web
title = Saturn > Observing Saturn
url = http://www.nmm.ac.uk/server/show/conWebDoc.13852/viewPage/5
publisher = National Maritime Museum
accessdate = 2007-07-06] 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. The Romans considered Saturnus the equivalent of the Greek god Kronos. The Greeks had made the outermost planet sacred to Kronos, [cite book|title=The History and Practice of Ancient Astronomy|author=James Evans|publisher=Oxford University Press|year=1998|pages=296-7] 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 by everyone accordingly to their own performed deeds bad or good.cite web
title = Starry Night Times
url = http://www.starrynight.com/sntimes/2006/2006-01-full.html
accessdate = 2007-07-05
year = 2006
publisher = Imaginova Corp.] 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, Urdu and Malay, its name is 'Zuhal', derived from Arabic زحل.

Saturn's rings require at least a 15 mm diameter telescope [cite web
last=Eastman
first=Jack
url=http://www.thedas.org/dfiles/eastman_saturn.html
title=Saturn in Binoculars
year = 1998
publisher=The Denver Astronomical Society
accessdate = 2008-09-03] to resolve and thus were not known to exist until Galileo first saw them in 1610. [cite web
url = http://library.thinkquest.org/C005921/Saturn/satuHist.htm
title = Saturn: History Timeline
accessdate = 2007-07-16
year = 2000
last = Chan
first = Gary] 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. [cite web
url = http://huygensgcms.gsfc.nasa.gov/Shistory.htm
title = Saturn: History of Discoveries
accessdate = 2007-07-15
last = Micek
last = Catherine]

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.

Pioneer 11 flyby

Saturn was first visited by Pioneer 11 in 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. [cite web
url = http://web.archive.org/web/20060130100401/http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PN10&11.html
title = The Pioneer 10 & 11 Spacecraft
accessdate=2007-07-05
publisher = Mission Descriptions]

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.cite web
url = http://www.planetary.org/explore/topics/saturn/missions.html
title = Missions to Saturn
publisher = The Planetary Society
year = 2007
accessdate = 2007-07-24]

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

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 commenced on March 23, 2008.

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. [cite web
url = http://www.sciencedaily.com/releases/2006/02/060215090726.htm
title = Astronomers Find Giant Lightning Storm At Saturn
year = 2007
accessdate = 2007-07-27
publisher = ScienceDaily LLC]

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." [cite web
url = http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=639
title = NASA's Cassini Discovers Potential Liquid Water on Enceladus
accessdate = 2007-07-08
date = March 9, 2006
publisher = NASA Jet Propulsion Laboratory
first = Michael
last = Pence]

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. [cite web
url = http://space.newscientist.com/channel/solar-system/cassini-huygens/dn10124-faint-new-ring-discovered-around-saturn.html
title = Faint new ring discovered around Saturn
accessdate = 2007-07-08
date = September 20, 2007
last = Shiga
first = David
publisher = NewScientist.com]

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. [cite web
url = http://news.bbc.co.uk/2/hi/science/nature/6449081.stm
title = Probe reveals seas on Saturn moon
publisher = BBC
last =
first =
accessdate = 2007-09-26
date = March 14, 2007]

In October 2006, the probe detected a 5,000 km diameter hurricane with an eyewall at Saturn's South Pole. [cite web
url = http://news.bbc.co.uk/2/hi/science/nature/6135450.stm
title = Huge 'hurricane' rages on Saturn
publisher = BBC
last = Rincon
first = Paul
accessdate = 2007-07-12
date = November 10, 2006]

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 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 point of light whose magnitude is usually between +1 and 0 and takes approximately 29½ years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Most people will require optical aid (large binoculars or a telescope) magnifying at least 20X to clearly resolve Saturn's rings.

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 its rings relative to the Earth.cite web
url = http://findarticles.com/p/articles/mi_qa4015/is_200301/ai_n9338203
title = SATURN IN 2002-03
accessdate = 2007-10-14
last = Schmude
first = Richard W Jr
year = 2003
publisher = Georgia Journal of Science]

See also

*Dragon Storm (astronomy)
*Saturn in astrology
*Saturn in fiction

References

*
*

External links

* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn Introduction to Saturn] by NASA's Solar System Exploration
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html Saturn Fact Sheet] , by NASA
* [http://saturn.jpl.nasa.gov/home/index.cfm Cassini-Huygens mission] to Saturn, by NASA
* [http://www.sciencedaily.com/news/space_time/saturn/ Research News] about Saturn
* [http://www.solarviews.com/eng/saturn.htm General information] about Saturn
* [http://www.affs.org/html/studies_on_the_rings_of_saturn.html Studies on the Rings] of Saturn
* [http://www.astronomycast.com/astronomy/episode-59-saturn/ Astronomy Cast: Saturn]