Rings of Saturn
977592
225964654
2008-07-16T06:15:04Z
Secret Squïrrel
1868769
/* Physical characteristics */ +wl for first mention of space probes and in pics
[[Image:Saturn eclipse exaggerated.jpg|right|thumb|400px|The full set of rings, photographed as Saturn eclipsed the sun from the vantage of the [[Cassini-Huygens|Cassini spacecraft]] on [[September 15]], [[2006]] (brightness has been exaggerated in this image).]]
[[Saturn]] has the most extensive [[planetary ring]] system of any planet in the [[Solar System]]. The '''rings of Saturn''' consist of countless small particles, ranging in size from [[micrometre|microns]] to [[metre|meters]], that form clumps that in turn [[orbit]] about Saturn. The ring particles are made almost entirely of [[ice|water ice]], with some contamination from [[dust]] and other chemicals.
Although reflection from the rings increases Saturn's [[apparent magnitude|brightness]], they are not visible from Earth with [[naked eye|unaided vision]]. In 1610, the year he first turned a [[telescope]] to the sky, [[Galileo Galilei]] became the very first person to observe Saturn's rings, though he could not see them well enough to discern their true nature. In 1655, [[Christiaan Huygens]] was the first person to describe them as a disk surrounding Saturn.<ref name="solarviews">{{cite web
| url=http://www.solarviews.com/eng/saturnbg.htm
| title=Historical Background of Saturn's Rings
| accessdate=2006-03-08
}}</ref>
Although many people think of Saturn's rings as being made up of a series of tiny ringlets (a concept that goes back to [[Pierre-Simon Laplace|Laplace]]),<ref name=solarviews/> true gaps are few in number. It is more correct to think of the rings as an [[Annulus (mathematics)|annular disk]] with [[concentric]] local [[maxima and minima]] in density and brightness. On the scale of the clumps within the rings there is a lot of empty space, but in general these empty spaces are discontinuous.
There are several gaps within the rings: two opened by known moons embedded within them, and many others at locations of known destabilizing [[orbital resonance]]s with [[Saturn's natural satellites|Saturn's moons]]. Other gaps remain unexplained. Stabilizing resonances, on the other hand, are responsible for the longevity of several rings, such as the [[#Colombo Gap and Titan Ringlet|Titan Ringlet]] and the [[#G Ring|G Ring]].
==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 [[Cosimo II de' Medici, Grand Duke of Tuscany|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.<ref>{{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}}</ref> Then, in 1613, they reappeared again, further confusing Galileo.<ref name="history_of_the_rings">{{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}}</ref>
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."<ref name="history_of_the_rings"/>
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 [[Rings of Saturn#Cassini Division|Cassini Division]]. This division in itself is a 4,800 km wide region between the [[Rings of Saturn#A Ring|A Ring]] and [[Rings of Saturn#B Ring|B Ring]].<ref name="Cassini Division">{{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}}</ref>
In 1787, Pierre Simon Laplace suggested that the rings were composed of a large number of solid ringlets.<ref name=solarviews/>
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.<ref>{{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
|date = March, 2006
|publisher = JOC/EFR}}</ref> Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by [[James Keeler]] of [[Lick Observatory]].
==Physical characteristics==
[[Image:Saturn HST 2004-03-22.jpg|thumb|right|200px|In this image of Saturn taken by the [[Hubble Space Telescope]]'s [[Advanced Camera for Surveys|ACS]] on [[March 22]], [[2004]], the B Ring is the inner of the two wide rings, the A Ring is the outer, and the Cassini Division is the dark band between them. (The less prominent C Ring is also visible just inside the B Ring.)]]
[[Image:Backlit Saturn from Cassini Orbiter 2007 May 9.png|thumb|200px|The unilluminated side of Saturn's rings viewed from the [[Cassini-Huygens|Cassini]] space probe'' on May 9, 2007]]
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]].<ref>{{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}}</ref> They range in size from specks of dust to the size of a small automobile.<ref>{{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
|date = 2005
|accessdate = 2007-07-25
|format = PDF}}</ref>
While the largest gaps in the rings, such as the Cassini Division and [[Rings of Saturn#Encke Gap|Encke Gap]], can be seen from Earth, both [[Voyager program|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 (moon)|Pan]], many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small [[shepherd satellite]]s such as [[Prometheus (moon)|Prometheus]] and [[Pandora (moon)|Pandora]]. Other gaps arise from [[orbital resonance|resonances]] between the orbital period of particles in the gap and that of a more massive moon further out; [[Mimas (moon)|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-Huygens|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 (O<sub>2</sub>) 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 O<sub>2</sub>. According to models of this atmosphere, H<sub>2</sub> is also present. The O<sub>2</sub> and H<sub>2</sub> 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.<ref>{{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 = [[BBC|British Broadcasting Coorperation]]}}</ref>
The rings also have a similarly sparse OH (hydroxide) atmosphere. Like the O<sub>2</sub>, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic [[ion]]s that bombard water molecules ejected by Saturn's moon [[Enceladus (moon)|Enceladus]]. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.<ref>{{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
|date = 2006
|publisher = The [[American Astronomical Society]]}}</ref>
Saturn shows complex patterns in its brightness.<ref>{{cite 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
|date = 2001
|publisher = Georgia Journal of Science}}</ref> Most of the variability is due to the changing aspect of the rings,<ref name="brightness">{{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}}</ref><ref>{{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
|date = 2003
|publisher = Georgia Journal of Science}}</ref> 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.<ref>{{cite web
|url = http://www.britastro.org/jbaa/113-1.htm
|title = The Journal of the British Astronomical Association<!--INSERT TITLE; done by Universe=atom-->
|publisher = [[British Astronomical Association]]
|accessdate=2007-07-07
|year = 2003
|month = February}}</ref>
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.
==Formation==
[[Image:Saturn ring detail art PIA10081-br500.jpg|thumb|right|240px|A 2007 artist impression of the aggregates of icy particles that form the 'solid' portions of Saturn's rings. These elongated clumps are continually forming and dispersing. The largest particles are a few meters across.]]
Saturn's rings may be very old, dating to the formation of Saturn itself. 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 force]]s (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 [[nebula]]r material from which Saturn formed.
It seems likely however that they are composed of debris from the disruption of a moon about 300 km in diameter, bigger than [[Mimas (moon)|Mimas]]. The last time there were collisions large enough to be likely to disrupt a moon that large was during the [[Late Heavy Bombardment]] some four billion years ago.<ref name="Kerr2008a">Kerr, Richard A. 2008. "Saturn's Rings Look Ancient Again", ''[[Science (journal)|Science]]'' 319 (5859), 21.</ref>
The brightness and purity of the water ice in Saturn's rings has been cited as evidence that the rings are much younger than Saturn, perhaps 100 million years old, as the infall of meteoric dust would have led to darkening of the rings. However, new research indicates that the B Ring may be massive enough to have diluted infalling material and thus avoided substantial darkening over the age of the Solar system. Ring material may be recycled as clumps form within the rings and are then disrupted by impacts. This would explain the apparent youth of some of the material within the rings.<ref name="NASAOldTimers">{{cite news
| title = Saturn's Rings May Be Old Timers
| work = NASA/JPL and University of Colorado
|date= 2007-12-12
| url = http://saturn1.jpl.nasa.gov/news/press-release-details.cfm?newsID=798
| accessdate = 2008-01-24 }}</ref>
The ''Cassini'' UVIS team, led by [[Larry Esposito]], used [[occultation|stellar occultation]] to discover 13 objects, ranging from 27 meters to 10 km across, within the [[F ring]]. They are translucent, suggesting they are temporary aggregates of ice boulders a few meters across. Esposito believes this to be the basic structure of the Saturnian rings, particles clumping together, then being blasted apart.
==Subdivisions and structures within the rings==
The densest parts of the Saturnian ring system are the A and B Rings, which are separated by the Cassini Division (discovered in 1675 by [[Giovanni Domenico Cassini]]). Along with the C Ring, which was discovered in 1850 and is similar in character to the Cassini Division, these regions comprise the '''Main Rings'''. The Main Rings are denser and contain larger particles than the tenuous '''Dusty Rings'''. The latter include the D Ring, extending inward to Saturn's cloud tops, the G and E Rings and others beyond the main ring system. The word "dusty" used to characterize these diffuse rings refers to the small size of the particles (often about a [[micrometre|micron]]); their chemical composition is, like the main rings, almost entirely of [[ice|water ice]]. The narrow F Ring, just off the outer edge of the A Ring, is more difficult to categorize; parts of it are very dense, but it also contains a great deal of dust-size particles.
{{wide image|Saturn's rings dark side mosaic.jpg|2200px|Natural-color mosaic of [[Cassini-Huygens|''Cassini'']] narrow-angle camera images of the unilluminated side of Saturn's D, C, B, A and F rings (left to right) taken on [[May 9]], [[2007]].}}
[[Image:Saturn's ring plane.jpg|right|thumb|300px|The illuminated side of Saturn's rings with the major subdivisions labelled]]
===Major subdivisions of the rings===
{| class="wikitable"
|-
! Name<sup>(3)</sup> !! Distance from Saturn's center (km)<sup>(4)</sup> !! Width (km)<sup>(4)</sup> !! Named after
|-
| [[#D Ring|D Ring]] || 66,900 - 74,510 || 7,500 ||
|-
| [[#C Ring|C Ring]] || 74,658 - 92,000 || 17,500 ||
|-
| [[#B Ring|B Ring]] || 92,000 - 117,580 || 25,500 ||
|-
| [[#Cassini Division|Cassini Division]] || 117,580 - 122,170 || 4,700 || [[Giovanni Domenico Cassini|Giovanni Cassini]]
|-
| [[#A Ring|A Ring]] || 122,170 - 136,775 || 14,600 ||
|-
| [[#Roche Division|Roche Division]] || 136,775 - 139,380 || 2,600 || [[Édouard Roche]]
|-
| [[#F Ring|F Ring]] || 140,180 <sup>(1)</sup> || 30-500 ||
|-
| Janus/Epimetheus Ring<sup>(2)</sup> || 149,000 - 154,000 || 5,000 || [[Janus (moon)|Janus]] and [[Epimetheus (moon)|Epimetheus]]
|-
| [[#G Ring|G Ring]] || 170,000 - 175,000 || 5,000 ||
|-
| Pallene Ring<sup>(2)</sup> || 211,000 - 213,500 || 2,500 || [[Pallene (moon)|Pallene]]
|-
| [[#E Ring|E Ring]] || 181,000 - 483,000 || 302,000 ||
|}
===Structures within the C Ring===
{| class="wikitable"
|-
! Name<sup>(3)</sup> !! Distance from Saturn's center (km)<sup>(4)</sup> !! Width (km)<sup>(4)</sup> !! Named after
|-
| [[#Colombo Gap and Titan Ringlet|Colombo Gap]] || 77,870 <sup>(1)</sup> || 150 || [[Giuseppe Colombo|Giuseppe "Bepi" Colombo]]
|-
| [[#Colombo Gap and Titan Ringlet|Titan Ringlet]] || 77,870 <sup>(1)</sup> || 30 || [[Titan (moon)|Titan]], moon of Saturn
|-
| [[#Maxwell Gap|Maxwell Gap]] || 87,491 <sup>(1)</sup> || 270 || [[James Clerk Maxwell]]
|-
|}
===Structures within the Cassini Division===
{| class="wikitable"
|-
! Name<sup>(3)</sup> !! Distance from Saturn's center (km)<sup>(4)</sup> !! Width (km)<sup>(4)</sup> !! Named after
|-
| [[#Huygens Gap|Huygens Gap]] || 117,680 <sup>(1)</sup> || 400 || [[Christiaan Huygens]]
|}
===Structures within the A Ring===
{| class="wikitable"
|-
! Name<sup>(3)</sup> !! Distance from Saturn's center (km)<sup>(4)</sup> !! Width (km)<sup>(4)</sup> !! Named after
|-
| [[#Encke Gap|Encke Gap]] || 133,589 <sup>(1)</sup> || 325 || [[Johann Franz Encke|Johann
Encke]]
|-
| [[#Keeler Gap|Keeler Gap]] || 136,530 <sup>(1)</sup> || 35 || [[James Edward Keeler|James Keeler]]
|}
''Notes:''<br />
<sup>(1)</sup> distance is to centre of gaps, rings and ringlets that are narrower than 1000 km<br />
<sup>(2)</sup> unofficial name<br />
<sup>(3)</sup> Names as designated by the [[International Astronomical Union]], unless otherwise noted. Broader separations between named rings are termed ''divisions'', while narrower separations within named rings are called ''gaps''.<br>
<sup>(4)</sup> Data mostly from the [http://planetarynames.wr.usgs.gov/append8.html Gazetteer of Planetary Nomenclature] and this [http://nssdc.gsfc.nasa.gov/planetary/factsheet/satringfact.html NASA factsheet].
<br style="clear:both;">
{{wide image|Saturn's rings in visible light and radio.jpg|1800px|Oblique (4 degree angle) ''[[Cassini-Huygens|Cassini]]'' images of Saturn's C, B, and A rings (left to right; the F ring is faintly visible in the full size upper image if viewed at sufficient brightness). Upper image: natural color mosaic of Cassini narrow-angle camera photos of the illuminated side of the rings taken on [[December 12]], [[2004]]. Lower image: simulated view constructed from a [[radio occultation]] observation conducted on [[May 3]], [[2005]]. Color in the lower image is used to represent information about ring particle sizes.}}
==D Ring==
[[Image:D ring structure.jpg|thumb|left|240px|A ''Cassini'' image of Saturn's D ring processed to show faint ripples within it; the much brighter C ring appears in the upper left.]]
The D Ring is the innermost ring, and is very faint. In 1980, [[Voyager 1]] detected within this ring three ringlets designated D73, D72 and D68, with D68 being the discrete ringlet nearest to Saturn. Some 25 years later ''Cassini'' images showed that D72 had become significantly fainter and moved planetward by 200 kilometres. Present in the gap between the C ring and D73 is finescale structure with waves 30 kilometres apart.
==C Ring==
The C Ring is a wide but faint ring located inward of the [[#B Ring|B Ring]]. It was discovered in 1850 by [[William Cranch Bond|William]] and [[George Phillips Bond|George Bond]], though [[William Rutter Dawes|William R. Dawes]] and [[Johann Gottfried Galle|Johann Galle]] also saw it independently. [[William Lassell]] termed it the "Crepe Ring" because it seemed to be composed of darker material than the brighter A and B Rings.<ref>David M. Harland, ''Mission to Saturn: Cassini and the Huygens Probe'', Chichester: Praxis Publishing, 2002.</ref>
Its vertical thickness is estimated at 5 metres, its mass at around 1.1{{e|18}} kilograms, and its [[optical depth]] varies from 0.05 to 0.12. [http://www.seds.org/billa/tnp/saturn.html]. That is, 5 and 12 percent of light shining through perpendicular to the ring is blocked, so that when seen from above or below, the ring is close to transparent.
===Colombo Gap and Titan Ringlet===
[[Image:PIA06540 Outer C Ring.jpg|thumb|right|240px|The Maxwell Gap, with the Maxwell Ringlet on its right side, lies above and right of center in this ''Cassini'' image of Saturn's outer C Ring.]]
The Colombo Gap lies in the inner C Ring. Within the gap lies the bright but narrow Colombo Ringlet, centered at 77,883 kilometers from Saturn's center, which is slightly [[ellipse|elliptical]] rather than circular. This ringlet is also called the Titan Ringlet as it is governed by an [[orbital resonance]] with the moon [[Titan (moon)|Titan]]. [http://www.saturntoday.com/news/viewsr.html?pid=14715] At this location within the rings, the time period of a ring particle's [[precession (astronomy)|apsidal precession]] is equal to the time period of Titan's orbital motion, so that the outer end of this eccentric ringlet always points towards Titan.
===Maxwell Gap===
The Maxwell Gap lies within the outer C Ring. It also contains a dense non-circular ringlet, the Maxwell Ringlet.
==B Ring==
[[Image:Spokes in Saturn's B Ring.jpg|thumb|left|240px|Dark B ring spokes are visible in this ''Cassini'' image of the unilluminated side of the rings. Left of center, two dark gaps (the larger being the Huygens Gap) and the bright (from this viewing geometry) ringlets between and immediately beyond them comprise the Cassini Division.]]
The B Ring is the largest, brightest, and most massive of the rings. Its thickness is estimated as 5 to 10 metres, its mass at 2.8{{e|19}} kg, and its [[optical depth]] varies from 0.4 to 2.5, [http://www.seds.org/billa/tnp/saturn.html] meaning that well over 99% of the light passing through some parts of the B Ring is blocked. The B Ring contains a great deal of variation in its density and brightness, nearly all of it unexplained. These are [[concentric]], appearing in the form of narrow ringlets, though the B Ring does not contain any gaps.
===Spokes===
[[Image:Voyager ring spokes.jpg|thumb|right|Dark spokes in the B ring, imaged by [[Voyager 2]] in 1981]]
[[Image:H cassini spokes 02.jpg|thumb|right|''[[Cassini-Huygens|Cassini]]'' images of the unlit side of the rings taken in 2005 at a [[Phase angle (astronomy)|phase angle]] of 145°, with bright spokes.]]
Up until [[1980]], the structure of the rings of Saturn was explained as being caused exclusively by the action of [[gravitation]]al forces. Then images from the Voyager spacecraft showed radial features in the [[Rings of Saturn#B ring|B ring]], known as ''spokes'', which could not be explained in this manner, as their persistence and rotation around the rings was not consistent with [[orbital mechanics]].<ref name="Spokes 2">{{cite web
|url = http://www.planetary.org/explore/topics/saturn/rings.html
|title = The Alphabet Soup of Saturn's Rings
|publisher = The Planetary Society
|date = 2007
|accessdate = 2007-07-24}}</ref> The spokes appear dark in [[backscatter]]ed light, and bright in [[Forward scatter|forward-scattered]] light. The leading theory regarding the spokes' composition is that they consist of [[microscopic]] dust particles suspended away from the main ring by [[electrostatic]] repulsion, as they rotate almost [[synchronization|synchronously]] with the [[magnetosphere]] of Saturn. The precise mechanism generating the spokes is still unknown, although it has been suggested that the electrical disturbances might be caused by either [[lightning]] bolts in Saturn's [[atmosphere]] or [[micrometeoroid]] impacts on the rings.<ref>{{cite web
|url = http://www.solarviews.com/eng/saturnrings.htm
|title = Saturn's Magnificent Rings
|first = Calvin
|last = Hamilton
|date = 2004
|accessdate = 2007-07-25}}</ref>
The spokes were not observed again until some twenty-five years later, this time by the Cassini space probe. 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 their formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and they were next seen in images taken on [[September 5]], [[2005]].<ref name="Spokes">{{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}}</ref>
The spokes appear to be a [[season]]al phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to [[equinox]]. Suggestions that the spokes may be a seasonal effect, varying with Saturn's 29.7-[[year]] [[orbit]], were supported by their gradual reappearance in the later years of the Cassini mission.
==Cassini Division==<!-- This section is linked from [[Orbital resonance]] -->
[[Image:Cassini Division.jpg|thumb|left|200 px|The Cassini division imaged from the Cassini spacecraft, showing the fine detail of the material within it. The Huygens gap can be seen right of centre.]]
The Cassini Division is a 4,800 km (2,980 mile) wide region between the [[#A Ring|A Ring]] and [[#B Ring|B Ring]]. It was discovered in 1675 by [[Giovanni Domenico Cassini|Giovanni Cassini]]. From Earth it appears as a thin black gap in the rings. However, ''[[Voyager program|Voyager]]'' discovered that the gap is itself populated by ring material bearing much similarity to the [[#C Ring|C Ring]]. The division may appear bright in views of the unlit side of the rings, since the relatively low density of material allows more light to be transmitted through the thickness of the rings.
The inner edge of the Cassini Division is governed by a strong [[orbital resonance]]. Ring particles at this location orbit twice for every orbit of the moon [[Mimas (moon)|Mimas]]. The resonance causes Mimas' pulls on these ring particles to accumulate, destabilizing their orbits and leading to a sharp cutoff in ring density. Many of the other gaps between ringlets within the Cassini Division, however, are unexplained.
===Huygens Gap===
The Huygens Gap is at the inner edge of the Cassini Division. It contains the dense Huygens Ringlet, which is non-circular.
==A Ring==<!-- This section is linked from [[Orbital resonance]] -->
The A Ring is the outermost of the large, bright rings. Its inner boundary is
the [[#Cassini Division|Cassini Division]] and its sharp outer boundary is close to the orbit of the small moon [[Atlas (moon)|Atlas]]. The A Ring is interrupted at a location 22% of the ring width from its outer edge by the [[#Encke Gap|Encke Gap]]. A narrower gap 2% of the ring width from the outer edge is called the [[#Keeler Gap|Keeler Gap]].
The thickness of the A Ring is estimated as 10 to 30 metres, its mass as 6.2{{e|18}} kg (about the mass of [[Hyperion (moon)|Hyperion]]), and its [[optical depth]] varies from 0.4 to 1.0. [http://www.seds.org/billa/tnp/saturn.html]
Similarly to the B Ring, the A Ring's outer edge is maintained by an [[orbital resonance]], in this case the 7:6 resonance with [[Janus (moon)|Janus]] and [[Epimetheus (moon)|Epimetheus]]. Other orbital resonances also excite many [[Density wave theory|spiral density waves]] in the A Ring (and, to a lesser extent, other rings as well), which account for most of its structure. These waves are described by the same physics that describes the [[spiral galaxy|spiral arms of galaxies]]. Spiral bending waves, also present in the A Ring and also described by the same theory, are [[Transverse wave|vertical corrugations]] in the ring rather than [[Longitudinal wave|compression waves]].
===Encke Gap===<!-- This section is linked from [[Saturn]] -->
The Encke Gap is a 325-kilometre-wide gap within the [[#A Ring|A Ring]], centered at a distance of 133,590 kilometers from Saturn's center.[http://nssdc.gsfc.nasa.gov/planetary/factsheet/satringfact.html] It is caused by the presence of the small moon [[Pan (moon)|Pan]], which orbits within it. Images from the [[Cassini-Huygens Mission|''Cassini'' probe]] have shown that there are at least three thin, knotted ringlets within the gap.
[[Johann Encke]] himself did not observe this gap; it was named in honour of his ring observations. The gap itself was discovered by [[James Edward Keeler]] in 1888.
The Encke Gap is a ''gap'' because it is entirely within the A Ring. There was some ambiguity between the terms ''gap'' and ''division'' until the IAU clarified the definitions in 2008; prior to that, the separation was sometimes called the "Encke Division".
===Keeler Gap===
The Keeler Gap is a 42-kilometre-wide gap in the [[#A Ring|A Ring]], approximately 250
kilometres from the ring's outer edge. It is named after the astronomer [[James Edward Keeler]]. The small moon [[Daphnis (moon)|Daphnis]], discovered [[May 1]] [[2005]], orbits within it, keeping it clear.
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Image:PIA06534 Encke Division.jpg|The Encke Gap's central ringlet coincides with [[Pan (moon)|Pan]]’s orbit, implying its particles oscillate in [[Horseshoe orbit|horseshoe orbits]]. [[Density wave theory|Spiral density waves]] (induced by [[orbital resonance|resonances]] with nearby [[Moons of Saturn|moons]]), which pervade much of the A Ring, are visible on both sides of the gap.
Image:PIA06099 Enke Gap.jpg|The motion of [[Pan (moon)|Pan]] (not visible) through the Encke Gap induces edge waves and a set of [[spiral]]ing wakes (which are not self-propagating) [http://photojournal.jpl.nasa.gov/catalog/PIA09883 ahead of it in its orbit] on the gap's inner side. The other more tightly wound bands are [[Density wave theory|spiral density waves]].
Image:PIA08319 Daphnis in Keeler Gap.jpg|The passage of [[Daphnis (moon)|Daphnis]] induces waves in the edges of the Keeler gap. Orbital motion is to the lower left; speed decreases going towards the A Ring edge.
</gallery>
</center>
===Moonlets===
In 2006, four tiny "[[moonlet]]s" were found in ''Cassini'' images of the A Ring.<ref name="Tiscareno2006">{{cite journal
| author= Matthew S. Tiscareno ''et al.''
| title= ''100-metre-diameter moonlets in Saturn's A ring from observations of 'propeller' structures''
| journal= [[Nature (journal)|Nature]]
| year= 2006
| volume= 440
| pages= 648–650
| url=http://www.nature.com/nature/journal/v440/n7084/full/nature04581.html
| doi= 10.1038/nature04581
}}</ref> The moonlets themselves are only about a hundred meters in diameter, too small to be seen directly; what ''Cassini'' sees are the "propeller"-shaped disturbances the moonlets create, which are several km across. It is estimated that the A Ring contains thousands of such objects. In 2007, the discovery of eight more moonlets revealed that they are largely confined to a 3000-km belt, about 130,000 km from Saturn's center.<ref name="Sremcevic2007">{{cite journal
| author= Miodrag Sremčević ''et al.''
| title= ''A belt of moonlets in Saturn's A ring''
| journal= [[Nature (journal)|Nature]]
| year= 2007
| volume= 449
| pages= 1019–1021
| url=http://www.nature.com/nature/journal/v449/n7165/full/nature06224.html
| doi= 10.1038/nature06224
}}</ref> Over 150 "propeller" moonlets have now been detected.<ref name="Tiscareno2008">{{cite journal
| author= Matthew S. Tiscareno ''et al.''
| title= ''The population of propellers in Saturn's A Ring''
| journal= [[Astronomical Journal]]
| year= 2008
| volume= 135
| pages= 1083–1091
| doi= 10.1088/0004-6256/135/3/1083
}}</ref>
[[Image:First moonlets PIA07792.jpg|450px]]
''Location of the first four moonlets detected.''
==Roche Division==
The separation between the [[#A Ring|A Ring]] and the [[#F Ring|F Ring]] has been named the Roche Division in honor of the French physicist [[Édouard Roche]].[http://planetarynames.wr.usgs.gov/append8.html] The Roche Division should not be confused with the [[Roche limit]], a physical concept that describes when a large object gets so close to a planet (such as Saturn) that the planet's [[tidal force]]s will pull it apart. Lying at the outer edge of the main ring system, the Roche Division is in fact close to Saturn's Roche limit, which is why the rings have been unable to [[Accretion (astrophysics)|accrete]] into a moon.
Like the [[#Cassini Division|Cassini Division]], the Roche Division is not empty but contains a sheet of material. The character of this material is similar to the tenuous and dusty D, E, and G Rings. Two locations in the Roche Division have a higher concentration of dust than the rest of the region. These were discovered by the [[Cassini-Huygens Mission|Cassini]] probe imaging team and were given [[Astronomical naming conventions#Natural satellites of planets|temporary designations]]: R/2004 S 1, which lies along the orbit of
the moon [[Atlas (moon)|Atlas]]; and R/2004 S 2, centered at 138,900 km from Saturn's center, inward of the orbit of [[Prometheus (moon)|Prometheus]].
==F Ring==<!-- This section is linked from [[Pandora (moon)]] -->
The F Ring is the outermost discrete ring of Saturn and perhaps the most active ring in the Solar system, with features changing on a timescale of hours.<ref name=Murray /> It is located 3000 km beyond the outer edge of the [[#A Ring|A Ring]].<ref name="Karttunen-2000">{{cite book
| author=H. Karttunen, P. Kröger, ed al.
| title=Fundamental Astronomy
| editor=Springer
| ed=3
| address=Helsinky
| year=2000
| pp=221
}}</ref> It was discovered in 1979 by the ''[[Pioneer 11]]'' imaging team.<ref>T.G. Gehrels ''et al'', "Imaging Photopolarimeter on Pioneer Saturn", ''[[Science (journal)|Science]]'' 207, 434-439 (1980)</ref> It is very thin, just a few hundred kilometers wide, and is held together by two [[shepherd satellite|shepherd moons]], [[Prometheus (moon)|Prometheus]] and [[Pandora (moon)|Pandora]], which orbit inside and outside it.
{{wide image|F Ring perturbations PIA08412.jpg|2388px|<big>A mosaic of 107 images covering 255° (about 70%) of the F Ring, as it would appear if straightened out. The radial width (top to bottom) is 1500 km.</big>}}
Recent closeup images from the ''[[Cassini-Huygens Mission|Cassini]]'' probe show that the F Ring consists of one core ring and a [http://photojournal.jpl.nasa.gov/catalog/PIA07717 spiral strand] around it. They also show that when Prometheus encounters the ring at its [[apoapsis]], its gravitational attraction creates kinks and knots in the F Ring as the moon 'steals' material from it, leaving a dark channel in the inner part of the ring. Since Prometheus orbits Saturn more rapidly than the material in the F ring, each new channel is carved about 3.2 degrees in front of the previous one.
<center>
<gallery widths="240px" heights="200px">
Image:PIA07712 - F ring animation.gif|The shepherd moons Pandora (left) and Prometheus orbit on either side of the F ring; Prometheus is followed by dark channels that it has carved into the inner strands of the ring.
Image:Prometheus's effect on the F Ring.jpg|Close-up view of Prometheus and the F Ring. A movie of Prometheus at its [[Apsis|apoapsis]] drawing a streamer of material out of the ring, leaving a dark channel, may be viewed [http://ciclops.org/view.php?id=3806 here] or [http://www.youtube.com/watch?v=6JdzjXlvBYE here].
Image:Prometheus und Pandora.jpg|Prometheus (at center) and Pandora are the inner and outer F Ring shepherds.
</gallery>
</center>
In 2008, further dynamism was detected, suggesting that small unseen moons orbiting within the F Ring are continually passing through its narrow core due to perturbations from Prometheus. One of the small moons was tentatively identified as [[S/2004 S 6]].<ref name=Murray>{{cite journal
| last = Murray
| first = C. D.
| authorlink =
| coauthors = Beurle, K.; Cooper, N. J.; Evans, M. W.; Williams, G. A.; Charnoz, S.
| title = The determination of the structure of Saturn's F ring by nearby moonlets
| journal = [[Nature (journal)|Nature]]
| volume = 453
| issue =
| pages = 739–744
| publisher = [[Nature Publishing Group]]
| location =
| date = [[5 June]] [[2008]]
| url = http://www.nature.com/nature/journal/v453/n7196/abs/nature06999.html
| doi = 10.1038/nature06999
| id =
| accessdate = }}</ref>
[[Image:F Ring Dynamism PIA08290.jpg|center|thumb|541px|Dynamism probably due to perturbing effects of small moonlets orbiting close to or through the ring's core.]]
==Outer Rings==
==="Janus/Epimetheus" Ring===
A faint dust ring is present around the region occupied by the orbits of [[Janus (moon)|Janus]] and [[Epimetheus (moon)|Epimetheus]], as revealed by images taken in forward-scattered light by the [[Cassini spacecraft]] in 2006. The ring has a radial extent of about 5,000 km <ref name="Cassini_eclipse1"> NASA Planetary Photojournal [http://photojournal.jpl.nasa.gov/catalog/PIA08328 PIA08328: Moon-Made Rings]</ref>. Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.<ref name="Cassini_eclipse2"> Cassini-Huygens press release [http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=698 NASA Finds Saturn's Moons May Be Creating New Rings], [[11 October]], [[2006]]. </ref>
===G Ring===
The G Ring is a very thin, faint ring about halfway between the [[#F Ring|F Ring]] and the beginning of the [[#E Ring|E Ring]], with its inner edge about 15000 km inside the orbit of [[Mimas (moon)|Mimas]]. It contains a single distinctly brighter "arc" near its inner edge (similar to the arcs in the [[rings of Neptune]]) that extends about one sixth of its circumference, which is held in place by a 7:6 [[orbital resonance]] with Mimas.<ref name="Hedman2007">{{cite journal
| title=The Source of Saturn's G Ring
| author= M. M. Hedman, J. A. Burns, M. S. Tiscareno, C. C. Porco, G. H. Jones, E. Roussos, N. Krupp, C. Paranicas, S. Kempf
| journal=Science
| volume=317
| pages=653–656
| year=2007
| doi= 10.1126/science.1143964
| pmid=17673659
}}</ref> The arc is believed to be composed of icy particles up to a few meters in diameter, with the rest of the G Ring consisting of dust released by collisions within the arc. The radial width of the arc is about 250 km, compared to a width of 6000 km for the G Ring as a whole.<ref name="Hedman2007"/> The arc is thought to be the remains of a small icy moonlet about a hundred meters in diameter that broke up relatively recently. Dust released from the larger chunks by [[micrometeoroid]] impacts drifts outward from the arc due to interaction with [[Saturn#Magnetic field and magnetosphere|Saturn's]] [[magnetosphere]] (whose [[Plasma (physics)|plasma]] corotates with Saturn's [[magnetic field]], which rotates much more rapidly than the orbital motion of the G Ring). These tiny particles are steadily eroded away by further impacts and dispersed by plasma drag. Over the course of thousands of years the ring will gradually lose mass and eventually disappear.<ref>{{cite news| url=http://space.newscientist.com/article/dn12406-saturn-ring-created-by-remains-of-longdead-moon.html| title=Saturn ring created by remains of long-dead moon| date=02 August 2007| publisher= NewScientist.com news service| first=Anna| last=Davison}}</ref>
==="Pallene" Ring===
A faint dust ring shares [[Pallene (moon)|Pallene's]] orbit, as revealed by images taken in forward-scattered light by the [[Cassini spacecraft]] in 2006.<ref name="Cassini_eclipse1"> NASA Planetary Photojournal [http://photojournal.jpl.nasa.gov/catalog/PIA08328 PIA08328: Moon-Made Rings]</ref> The ring has a radial extent of about 2,500 km. Its source is particles blasted off Pallene's surface by meteoroid impacts, which then form a diffuse ring around its orbital path.<ref name="Cassini_eclipse2"> Cassini-Huygens press release [http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=698 NASA Finds Saturn's Moons May Be Creating New Rings], [[11 October]], [[2006]]. </ref>
===E Ring===<!-- This section is linked from [[Enceladus (moon)]] -->
The E Ring is the outermost ring, and is extremely wide, beginning at the orbit of [[Mimas (moon)|Mimas]] and ending somewhere around the orbit of [[Rhea (moon)|Rhea]]. It is a diffuse disk of icy or dusty material. Unlike the other rings, it is composed of microscopic rather than macroscopic particles. In 2006, [[cryovolcano|cryovolcanism]] on the moon [[Enceladus (moon)|Enceladus]] was determined to be the source of the E Ring's material.
<center>
<gallery widths="260px" heights="260px">
Image:Saturn outer rings labeled.jpg|The outer rings seen back-illuminated by the [[Sun]].
Image:G ring with arc.jpg|The backlit G Ring and its bright inner arc. This image is part of a movie sequence showing the arc's orbital motion that may be viewed [http://www.youtube.com/watch?v=jrmV07lzBsg here] or [http://ciclops.org/view.php?id=2273 here].
Image:E ring with Enceladus.jpg|The backlit E ring, with Enceladus silhouetted against it at center. The moon's south polar jets erupt brightly below it, while tendrils of the E Ring wrap around it.
</gallery>
</center>
==Possible ring system around Rhea==
{{main|Rings of Rhea}}
Saturn's second largest moon [[Rhea (moon)|Rhea]] may have a tenuous ring system of its own consisting of three narrow bands embedded in a disk of solid particles.<ref name="Jones2008">{{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 = [[American Association for the Advancement of Science|AAAS]]
| date = 2008 March 07
| doi = 10.1126/science.1151524
| pmid = 18323452
}}</ref><ref name="LakdawallaE">{{cite web
| last = Lakdawalla
| first = E.
| title = A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea
| work = [http://www.planetary.org/home/ The Planetary Society web site]
| publisher = [[Planetary Society]]
| date = 2008-03-06
| url = http://planetary.org/news/2008/0306_A_Ringed_Moon_of_Saturn_Cassini.html
| accessdate = 2008-03-09}}</ref> These rings have not been imaged, but their existence has been inferred from ''Cassini'' observations in November of 2005 of a depletion of energetic electrons in Saturn's [[magnetosphere]] near Rhea. The [[Cassini–Huygens #Instruments|Magnetospheric Imaging Instrument]] (MIMI) observed a gentle gradient punctuated by three sharp drops in plasma flow on each side of the moon in a nearly symmetric pattern. This could be explained if they were absorbed by solid material in the form of an equatorial disk containing denser rings or arcs, with particles perhaps several decimeters to approximately a meter in diameter. However, not all scientists are convinced that the observations were caused by a ring system.
==References==
{{reflist|2}}
==External links==
* [http://pds-rings.seti.org/saturn/saturn.html Planetary Rings Node: Saturn's Ring System]
* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn&Display=Rings Saturn's Rings] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
==See also==
* [[Édouard Roche]] - French astronomer who described how the breakup of a satellite could form the rings, when it comes within the [[Roche limit]] of a celestial body.
* [[Galileo Galilei]] - the first person to observe Saturn's rings, in 1610
* [[Christian Huygens]] - the first person to propose that there was a ring surrounding Saturn, in 1655
* [[Giovanni Cassini]] - discovered the separation between the A and B rings, in 1675 - (Cassini Division)
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