Moons of Saturn
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[[Image:Saturn system.jpg|right|thumb||250px|The Saturnian system (photographic montage)]]
[[Image:Moons of Saturn 2007.jpg|right|thumb||250px|Moons of Saturn (photographic montage)]]
[[Image:PIA08235.jpg|thumb|300px|Saturn's rings cut across a scene that is ruled by Titan's luminous crescent and globe-encircling haze, broken by the small moon Enceladus, whose icy jets are dimly visible at its south pole. North is up.]]
[[Saturn (planet)|Saturn]] has 60 named<!--- confirmation = naming and numbering, last one as of May 2007 is Saturn XLVIII (Surtur) ---> '''[[natural satellite]]s''', many of which were discovered only recently and may be as small as two-to-three kilometers across, plus hundreds of observed "moonlets" only a few tens or hundreds of meters across in the A Ring. Seven of the moons are large enough to have achieved [[hydrostatic equilibrium]] and so would be considered [[dwarf planet]]s if they were in orbit about the Sun; indeed one of them, [[Titan (moon)|Titan]], is massive enough to retain an atmosphere denser than that of [[Earth]]. A precise number of moons can not be given, as there is no objective boundary between the countless small anonymous objects that form Saturn's ring system and the larger objects that have been named as moons.
==Table of known moons==
The Saturnian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have [[gravitational collapse|collapsed]] into a [[spheroid]] are highlighted in light blue. Titan, which is planetary in size, has darker highlighting. The irregular (captured) moons are indicated in grey: light grey for prograde satellites, darker grey for retrograde satellites.
{{Saturnian satellites}}
The following objects have not been confirmed as satellites:
{| class="wikitable"
|-
! Order
! colspan="3" | Name
! Image
! Diameter (km)
! Semi-major<br>axis (km)
! Orbital<br>period (d)
! Inclination
! Position
! Discovered
|-
|*|| || [[S/2004 S 6]] <ref name="note_2">It is not yet clear if these are real satellites or merely persistent clumps within the F Ring</ref> || || bgcolor=black|[[Image:S2004 S6.jpg|50px|center]] || ~3−5 || 140,130 <ref name="Spitale06" /> || +0.61801 <ref name="Spitale06" /> || || rowspan="3"| uncertain objects around the F-Ring || 2004
|-
|*|| || [[S/2004 S 4]] <ref name="note_2a">S/2004 S4 was most likely a transient clump − it has not been recovered since the first sighting.</ref> || || || ~3−5 || ~140,100 <ref name="PGJ"> [http://perso.wanadoo.fr/pgj/planetes/satsat.htm PGJ Astronomie webpage (Gilbert Javaux)]. Note that the F ring is centered at ~140,180 km </ref> || +0.619 <ref name="note_1">Computed from the semi-major axis using the IAU-MPC Natural Satellites Ephemeris Service µ value</ref> || || 2004
|-
|*|| || [[S/2004 S 3]] <ref name="note_2" /> || || bgcolor=black| [[Image:S2004 S 3 - PIA06115.png|50px|center]] || ~3−5 || ~140,300 <ref name="see S3"> See references in [[S/2004 S 3]]</ref> || ~ +0.62 || || 2004
|}
==Grouping the moons==
Although the borders may be somewhat nebulous, Saturn's moons can be divided into ten groups.
===A-Ring 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 disturbance 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>
===Ring shepherds===
[[Shepherd satellite]]s are moons that orbit within, or just beyond, a planet's [[planetary ring|ring system]]. They have the effect of sculpting the rings: giving them sharp edges, and creating gaps between them. Saturn's shepherd moons are the moonlets, [[Pan (moon)|Pan]], [[Daphnis (moon)|Daphnis]], [[Atlas (moon)|Atlas]], [[Prometheus (moon)|Prometheus]], [[Pandora (moon)|Pandora]], in addition to the unconfirmed moons [[S/2004 S 4]], [[S/2004 S 6]] and [[S/2004 S 3]].
===Co-orbitals===
[[Janus (moon)|Janus]] and [[Epimetheus (moon)|Epimetheus]] are [[co-orbital moon]]s. These two moons are of roughly equal size and have orbits with only a few kilometers difference in diameter, close enough that they would collide if they attempted to pass each other. Instead of colliding, however, their gravitational interaction causes them to swap orbits every four years. See [[Epimetheus (moon)|Epimetheus' article]] for a more detailed explanation of this arrangement.
===Inner large moons===
The innermost large moons of Saturn orbit within its tenuous [[E Ring]]. They are:
*[[Mimas (moon)|Mimas]], the "Death Star" moon, with a deep [[Herschel (crater on Mimas)|crater]] one fifth its diameter;
*[[Enceladus (moon)|Enceladus]], a bright, [[Tiger Stripes (Enceladus)|striped]] moon whose geysers are the source of the E Ring;
*[[Tethys (moon)|Tethys]], with a [[Ithaca Chasma|canyon]] running three-quarters its circumference that formed when an internal ocean froze;
*[[Dione (moon)|Dione]], with [[Linea|wispy terrain]] on its trailing hemisphere.
===Alkyonides===
{{see also|Alkyonides}}
Three recently discovered tiny moons also orbit within this group: [[Methone (moon)|Methone]], [[Pallene (moon)|Pallene]], and [[Anthe (moon)|Anthe]]. So too do the co-orbital moons that form a group of their own (see below).
===Trojan moons===
{{main|Trojan moon}}
Trojan moons are another kind of co-orbital. Like the other co-orbitals, they are a feature unique to the Saturnian system. They are moons that orbit at exactly the same distance from Saturn as another moon, but at such a distance from the other moon that they never collide. Tethys has two tiny co-orbitals [[Telesto (moon)|Telesto]] and [[Calypso (moon)|Calypso]], and Dione also has two, [[Helene (moon)|Helene]] and [[Polydeuces (moon)|Polydeuces]]. All four of these moons orbit in the larger moons' L<sub>4</sub> or L<sub>5</sub> [[Lagrangian point]]s, one in each point.
===Outer large moons===
Saturn's largest moons all orbit beyond its E Ring and can thus be considered a distinct group. They are:
*[[Rhea (moon)|Rhea]], which may have its own [[Rings of Rhea|ring system]];
*[[Titan (moon)|Titan]], a huge moon, with [[Lakes of Titan|methane lakes]], [[dune|sand dunes]] made of organic material, and an atmosphere thicker than Earth's;
*[[Hyperion (moon)|Hyperion]], a smallish, tumbling moon that looks like a sponge;
*[[Iapetus (moon)|Iapetus]], a two-toned, walnut-shaped moon with an [[equatorial ridge]].
===Irregular moons===
[[Image:TheIrregulars SATURN.svg|thumb|300px|Irregular satellites of Saturn.]]
[[Irregular moons]] are small satellites with large-radius, inclined, and sometimes [[Retrograde and direct motion|retrograde]] orbits, believed to have been acquired by the parent planet through a capture process. The largest Saturnian irregular by far, and the only one known in any detail, is [[Phoebe (moon)|Phoebe]].
====Inuit group====
{{main|Saturn's Inuit group of satellites}}
The Inuit group are five [[prograde]] outer moons that are similar enough in their distances from Saturn and their orbital inclinations that they can be considered a group. They are [[Ijiraq (moon)|Ijiraq]], [[Kiviuq (moon)|Kiviuq]], [[Paaliaq (moon)|Paaliaq]], [[Siarnaq (moon)|Siarnaq]], and [[Tarqeq (moon)|Tarqeq]].
====Norse group====
{{main|Saturn's Norse group of satellites}}
The Norse group are 29 [[Retrograde motion|retrograde]] outer moons that are similar enough in their distance from Saturn to be considered a group. They are [[Aegir (moon)|Aegir]], [[Bergelmir (moon)|Bergelmir]], [[Bestla (moon)|Bestla]], [[Farbauti (moon)|Farbauti]], [[Fenrir (moon)|Fenrir]], [[Fornjot (moon)|Fornjot]], [[Greip (moon)]], [[Hati (moon)|Hati]], [[Hyrrokkin (moon)|Hyrrokkin]], [[Jarnsaxa (moon)|Jarnsaxa]], [[Kari (moon)|Kari]], [[Loge (moon)|Loge]], [[Mundilfari (moon)|Mundilfari]], [[Narvi (moon)|Narvi]], [[Phoebe (moon)|Phoebe]], [[Skathi (moon)|Skathi]], [[Skoll (moon)|Skoll]], [[Surtur (moon)|Surtur]], [[Suttungr (moon)|Suttungr]], [[Thrymr (moon)|Thrymr]], [[Ymir (moon)|Ymir]], [[S/2004 S 7]], [[S/2004 S 12]], [[S/2004 S 13]], [[S/2004 S 17]], [[S/2006 S 1]], [[S/2006 S 3]], [[S/2007 S 2]], and [[S/2007 S 3]].
====Gallic group====
{{main|Saturn's Gallic group of satellites }}
The Gallic group are four prograde outer moons that are similar enough in their distance from Saturn and their orbital inclination that they can be considered a group. They are [[Albiorix (moon)|Albiorix]], [[Bebhionn (moon)|Bebhionn]], [[Erriapus (moon)|Erriapus]], and [[Tarvos (moon)|Tarvos]].
The diagram illustrates the orbits of the [[irregular natural satellite|irregular satellites]] of Saturn discovered so far<sup>1</sup>. The eccentricity of the orbits is represented by the segments (extending from the [[apsis|pericentre]] to the [[apsis|apocentre]]) with the inclination represented on Y axis. The satellites above the axis are [[prograde]], the satellites beneath are [[Prograde and retrograde motion|retrograde]]. The X axis is labelled in [[gigametre|Gm]] (million km) and the fraction of the [[Hill sphere]]'s (gravitational influence) radius (~65 Gm for Saturn). Prograde groups: Inuit and Gallic and the retrograde Norse group are clearly identifiable (from top to bottom).
<sup>1</sup><small>Named satellites are plotted in yellow; the unnamed satellites S/2004 Sxx (announced in 2005 and 2006) are plotted in white and S/2006 Sxx in grey.</small>
==Discovery==
Before the advent of telescopic photography, eight moons of Saturn were discovered by direct observation using an optical [[telescope]]:
* [[Titan (moon)|Titan]], discovered in 1655 by [[Christiaan Huygens]];
* [[Tethys (moon)|Tethys]], [[Dione (moon)|Dione]], [[Rhea (moon)|Rhea]] and [[Iapetus (moon)|Iapetus]] (the "[[Sidera Lodoicea]]") discovered 1671-1684 by [[Giovanni Domenico Cassini]];
* [[Mimas (moon)|Mimas]] and [[Enceladus (moon)|Enceladus]], discovered 1789 by [[William Herschel]];
* [[Hyperion (moon)|Hyperion]], discovered 1848 by [[William Cranch Bond|W.C. Bond]], [[George Phillips Bond|G.P. Bond]] and [[William Lassell|Lassell]].
* The spurious satellite [[Chiron (hypothetical moon)|Chiron]], "discovered" in 1861, is now known not to exist.
The use of long-exposure photographic plates made it possible to discover additional moons:
* [[Phoebe (moon)|Phoebe]] was the first satellite discovered by telescopic photograph in 1899 by [[William Henry Pickering|W.H. Pickering]].
* [[Themis (moon)|Themis]], "discovered" in 1905, also was later proven not to exist.
* In 1966, the satellites [[Janus (moon)|Janus]] and [[Epimetheus (moon)|Epimetheus]] were observed, but not confirmed, and it was not realized that there were two distinct moons sharing an orbit.
===Voyager===
The study of the outer planets has since been revolutionized, first by the use of unmanned space probes, and then by advances in telescopy:
*From 1980, when the first of the ''[[Voyager program|Voyager]]'' space probes arrived at Saturn, to 1990, analysis of Voyager images revealed eight more moons in the inner Saturnian system (bringing the total from 10 to 18). Epimethueus was confirmed as distinct from Janus. The last discovered was [[Pan (moon)|Pan]].
*A survey starting in late 2000 found thirteen<!--counting the 2003 discovery of Narvi--> new moons orbiting Saturn at a great distance in orbits that suggest they are fragments of larger bodies captured by Saturn's gravitational pull (''Nature'' vol. 412, pp. 163–166).
===Cassini===
*The ''[[Cassini-Huygens|Cassini]]'' mission, which arrived at Saturn in the summer of 2004, discovered three small moons in the inner Saturnian system ([[Methone (moon)|Methone]], [[Pallene (moon)|Pallene]] and [[Polydeuces (moon)|Polydeuces]]) as well as three suspected but unconfirmed moons in the [[F Ring]]. This increased the total to thirty-four confirmed moons, and three unconfirmed.
*On [[November 16]], [[2004]], Cassini scientists announced that the structure of [[Rings of Saturn|Saturn's rings]] indicates the presence of several more moons orbiting within the rings, but only one, [[Daphnis (moon)|Daphnis]], has been visually confirmed so far (its confirmation was announced on [[May 6]], [[2005]]).[http://www.space.com/scienceastronomy/saturn_update_041116.html].
*On [[May 3]], [[2005]], astronomers using the [[Mauna Kea Observatory]] announced the discovery of twelve more small outer moons [http://www.ifa.hawaii.edu/~jewitt/saturn2005.html] [http://www.planetary.org/news/2005/0503_Twelve_New_Moons_for_Saturn.html].
*On [[June 30]], [[2006]], astronomers using the [[Subaru (telescope)|Subaru 8.2 m telescope]] announced the discovery of nine more small outer moons [http://www.ifa.hawaii.edu/~sheppard/satellites/satsatdata.html].
*On [[April 13]], [[2007]], [[Tarqeq (moon)|Tarqeq]] was announced.
*On [[May 1]], [[2007]], [[S/2007 S 2]] and [[S/2007 S 3]] were announced.
*On [[July 18]], [[2007]], [[Anthe (moon)|Anthe]] was announced.
*On [[March 6]], [[2008]], it was announced that ''Cassini'' observations of a depletion of energetic electrons in Saturn's [[magnetosphere]] near [[Rhea (moon)|Rhea]] might be the signature of a [[Rings of Rhea|tenuous ring system]] around Saturn's second largest moon.<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>
==Naming notes==
Some [[asteroid]]s share the same names as moons of Saturn: [[55 Pandora]], [[106 Dione]], [[577 Rhea]], [[1809 Prometheus]], [[1810 Epimetheus]], [[4450 Pan]]. See also ''[[Name conflicts of solar system objects]]''.
==See also==
* [[Saturn's moons in fiction]]
* Natural satellites of [[Mars' natural satellites|Mars]]{{·}} [[Jupiter's natural satellites|Jupiter]]{{·}} [[Uranus' natural satellites|Uranus]]{{·}} [[Neptune's natural satellites|Neptune]]{{·}} [[Pluto's natural satellites|Pluto]]
* [[Timeline of discovery of Solar System planets and their natural satellites]]
* [[Naming of natural satellites]]
* [http://www.youtube.com/watch?v=xxXa9pxwzoY Carolyn Porco: Fly me to the moons of Saturn]
== References ==
{{Reflist|2}}
==External links==
*[http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html Saturnian Satellites Fact Sheet]
*[http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn&Display=Rings Saturn's Rings] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
*[http://planetarynames.wr.usgs.gov/append7.html USGS Astrogeology: Gazetteer of Planetary Names - Planetary Body Names and Discoverers]
*[http://www.astronomycast.com/astronomy/episode-61-saturns-moons/ Saturn's Moons] [[Astronomy Cast]] episode #61, includes full transcript.
{{Saturn}}
{{Moons of Saturn}}
{{Solar System moons (compact)}}
{{Solar System}}
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