Moons of Saturn

An artist's concept of Saturn, its rings and major icy moons—from Mimas to Rhea.

The moons of Saturn are numerous and diverse, ranging from tiny moonlets less than 1 kilometer (0.62 mi) across to the enormous Titan, which is larger than the planet Mercury. Saturn has sixty-two moons with confirmed orbits, fifty-three of which have names, and only thirteen of which have diameters larger than 50 kilometers (31 mi).^[1][2] Saturn has seven moons that are large enough to become spherical, and dense rings with complex orbital motions of their own. Particularly notable among Saturn's moons are Titan, the second-largest moon in the Solar System, with an Earth-like atmosphere and a landscape including hydrocarbon lakes and dry river networks, and Enceladus, which emits jets of gas and dust and may harbor liquid water under its south pole region.

Twenty-four of Saturn's moons are regular satellites; they have prograde orbits not greatly inclined to the Saturn's equatorial plane. These include the seven major satellites, four small moons which exist in a Trojan orbit with larger moons, two mutually co-orbital moons and two moons which act as shepherds of Saturn's F Ring. Two other known regular satellites orbit within gaps in Saturn's rings. The relatively large Hyperion is locked in a resonance with Titan. The remaining regular moons orbit near the outer edge of the A Ring, within G Ring and between the major moons Mimas and Enceladus. The regular satellites are traditionally named after Titans and Titanesses or other figures associated with the mythological Saturn.

The remaining thirty-eight, all small except one, are irregular satellites, whose orbits are much farther from Saturn, have high inclinations, and are mixed between prograde and retrograde. These moons are probably captured minor planets, or debris from the breakup of such bodies after they were captured, creating collisional families. The irregular satellites have been classified by their orbital characteristics into the Inuit, Norse, and Gallic groups, and their names are chosen from the corresponding mythologies. The largest of the irregular moons is Phoebe, the ninth moon of Saturn, discovered at the end of the 19th century.

The rings of Saturn are made up of objects ranging in size from microscopic to hundreds of meters, each of which is on its own orbit about the planet.^[3] Thus a precise number of Saturnian moons cannot 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. At least 150 moonlets embedded in the rings have been detected by the disturbance they create in the surrounding ring material, though this is thought to be only a small sample of the total population of such objects.^[4]

Discovery and naming

Saturn (overexposed) and the moons Iapetus, Titan, Dione, Hyperion, and Rhea viewed through a 12.5″ telescope

Early observations

Before the advent of telescopic photography, eight moons of Saturn were discovered by direct observation using optical telescopes. Saturn's largest moon, Titan, was discovered in 1655 by Christiaan Huygens using a 57-millimeter (2.2 in) objective lens^[5] on a refracting telescope of his own design.^[6] Tethys, Dione, Rhea and Iapetus (the "Sidera Lodoicea") were discovered in 1671–1672 by Giovanni Domenico Cassini.^[7] Mimas and Enceladus were discovered in 1789 by William Herschel.^[7] Hyperion was discovered in 1848 by W.C. Bond, G.P. Bond^[8] and William Lassell.^[9]

The use of long-exposure photographic plates made possible the discovery of additional moons. The first to be discovered in this manner, Phoebe, was found in 1899 by W.H. Pickering.^[10] In 1966 the tenth satellite of Saturn was discovered by Audouin Dollfus, when the rings were observed edge-on near an equinox.^[11] It was later named Janus. A few years later it was realized that all observations of 1966 could only be explained if another satellite had been present and that it had an orbit similar to that of Janus.^[11] This object is now known as Epimetheus, the eleventh moon of Saturn. It shares the same orbit with Janus—the only known example of co-orbitals in the Solar System.^[12] In 1980 three additional Saturnian moons were discovered from the ground and later confirmed by the Voyager probes. They are Trojan moons of Dione (Helene) and Tethys (Telesto and Calypso).^[12]

Observations by spacecraft

Four moons of Saturn can be seen on this image of the Cassini spacecraft: Huge Titan and Dione at the bottom, small Prometheus (under the rings) and tiny Telesto above center.

The study of the outer planets has since been revolutionized by the use of unmanned space probes. The arrival of the Voyager spacecraft at Saturn in 1980–1981 resulted in the discovery of three additional moons—Atlas, Prometheus and Pandora, bringing the total to 17.^[12] In addition, Epimetheus was confirmed as distinct from Janus. In 1990, Pan was discovered in archival Voyager images.^[12]

The Cassini mission, which arrived at Saturn in the summer of 2004, initially discovered three small inner moons including Methone and Pallene between Mimas and Enceladus as well as the second Lagrangian moon of Dione—Polydeuces. It also observed three suspected but unconfirmed moons in the F Ring.^[13] In November 2004 Cassini scientists announced that the structure of Saturn's rings indicates the presence of several more moons orbiting within the rings, although only one, Daphnis, has been visually confirmed so far (in 2005).^[14] In 2007 Anthe was announced.^[15] In 2008 it was reported that Cassini observations of a depletion of energetic electrons in Saturn's magnetosphere near Rhea might be the signature of a tenuous ring system around Saturn's second largest moon.^[16] In March 2009, Aegaeon, a moonlet within the G Ring, was announced.^[17] In July of the same year, S/2009 S 1, the first moonlet within the B Ring, was observed.^[2]

Outer moons

Study of Saturn's moons has also been aided by advances in telescope instrumentation, primarily the introduction of digital charge-coupled devices which replaced photographic plates. For the entire 20th century, Phoebe stood alone among Saturn's known moons with its highly irregular orbit. Beginning in 2000, however, three dozen additional irregular moons have been discovered using ground-based telescopes.^[18] A survey starting in late 2000 and conducted using three medium-size telescopes found thirteen new moons orbiting Saturn at a great distance, in eccentric orbits, which are highly inclined to both the equator of Saturn and the ecliptic.^[19] They are probably fragments of larger bodies captured by Saturn's gravitational pull.^[18][19] In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons.^[20][21] In 2006, astronomers using the Subaru 8.2 m telescope reported the discovery of further nine irregular moons.^[22] In April 2007, Tarqeq (S/2007 S 1) was announced. In May of the same year S/2007 S 2 and S/2007 S 3 were reported.^[23]

Naming

The modern names for Saturnian moons were suggested by John Herschel in 1847.^[7] He proposed to name them after mythological figures associated with the Roman god of agriculture and harvest, Saturn (equated to the Greek Kronos).^[7] In particular, the then known seven satellites were named after Titans and Titanesses—brothers and sisters of Saturn.^[10] In 1848 Lassell proposed that the eighth satellite of Saturn was named Hyperion after another Titan.^[9] When in the 20th century the names of Titans were exhausted, the moons were named after different characters of the Greco-Roman mythology or giants from other mythologies.^[24] All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and after Norse ice giants.^[25]

Some asteroids share the same names as moons of Saturn: 55 Pandora, 106 Dione, 577 Rhea, 1809 Prometheus, 1810 Epimetheus, and 4450 Pan. In addition, two more asteroids previously shared the names of Saturnian moons until spelling differences were made permanent by the International Astronomical Union (IAU): Calypso and asteroid 53 Kalypso; and Helene and asteroid 101 Helena.

Sizes

The relative masses of Saturn's moons. Mimas and the other moons smaller than Enceladus are invisible at this scale.

The Saturnian moon system is very lopsided: one moon, Titan, comprises more than 96% of the mass in orbit around the planet. The six other oblate moons constitute roughly four percent, while the remaining 54 small moons, together with the rings, comprise only 0.04%.^{[note 1]}

Orbital groups

Although the borders may be somewhat nebulous, Saturn's moons can be divided into ten groups according to their orbital characteristics. Many of them, such as Pan and Daphnis, orbit within Saturn's ring system and have orbital periods only slightly longer than the planet's rotation period.^[29] The innermost moons and most regular satellites all have orbital inclinations ranging from less than a degree to about 1.5 degrees (except Iapetus, which has an inclination of 7.57 degrees) and small orbital eccentricities.^[28] On the other hand, irregular satellites in the outermost regions of Saturn's moon system, in particular the Norse group, have orbital radii of millions of kilometers and orbital periods lasting several years. The moons of the Norse group also orbit in the opposite direction to Saturn's rotation.^[25]

Ring moonlets

Daphnis in the Keeler gap

During late July in 2009, a moonlet was discovered in the B Ring,^[2] 480 km from the outer edge of the ring, by the shadow it cast. It is estimated to be 300 m in diameter. Unlike the A Ring moonlets (see below), it does not induce a 'propeller' feature, probably due to the density of the B Ring.^[30]

In 2006, four tiny "moonlets" were found in Cassini images of the A Ring.^[31] Before this discovery only two larger moons had been known within gaps in the A Ring: Pan and Daphnis. These are large enough to clear continuous gaps in the ring.^[31] In contrast, a moonlet is only massive enough to clear two small—about 10 km across—partial gaps in the immediate vicinity of the moonlet itself creating a structure shaped like an airplane propeller.^[32] The moonlets themselves are tiny, ranging from about 40 to 500 meters in diameter, and are too small to be seen directly.^[4] In 2007, the discovery of 150 more moonlets revealed that they (with the exception of two that have been seen outside the Encke gap) are confined to three narrow bands in the A Ring between 126,750 and 132,000 km from Saturn's center. Each band is about a thousand kilometers wide, which is less than 1% the width of Saturn's rings.^[4] This region is relatively free from the disturbances related to resonances with larger satellites,^[4] although other areas of the A Ring without disturbances are apparently free of moonlets. The moonlets were probably formed from the breakup of a larger satellite.^[32] It is estimated that the A Ring contains 7,000–8,000 propellers larger than 0.8 km in size and millions larger than 0.25 km.^[4]

Similar moonlets may reside in the F Ring.^[4] There, "jets" of material may be due to collisions, initiated by perturbations from the nearby small moon Prometheus, of these moonlets with the core of the F Ring. One of the largest F-Ring moonlets may be the as-yet unconfirmed object S/2004 S 6. The F Ring also contains transient "fans" which are thought to result from even smaller moonlets, about 1 km in diameter, orbiting near the F Ring core.^[33]

One of the recently discovered moons, Aegaeon, resides within the bright arc of G Ring and is trapped in the 7:6 mean motion resonance with Mimas.^[17] This means that it makes exactly seven revolutions around Saturn while Mimas makes exactly six. The moon is the largest among the population of bodies that are sources of dust in this ring.^[34]

Ring shepherds

Shepherd satellites are small moons that orbit within, or just beyond, a planet's ring system. They have the effect of sculpting the rings: giving them sharp edges, and creating gaps between them. Saturn's shepherd moons are Pan (Encke gap), Daphnis (Keeler gap), Atlas (A Ring), Prometheus (F Ring) and Pandora (F Ring).^[13][17] These moons together with co-orbitals (see below) probably formed as a result of accretion of the friable ring material on preexisting denser cores. The cores with sizes from one third to one-half the present day moons may be themselves collisional shards formed when a parental satellite of the rings disintegrated.^[29]

Co-orbitals

Janus and Epimetheus are called co-orbital moons.^[12] They are of roughly equal size, with Janus being slightly larger than Epimetheus.^[29] Janus and Epimetheus have orbits with only a few kilometers difference in semi-major axis, 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.^[35]

Inner large moons

The innermost large moons of Saturn orbit within its tenuous E Ring, along with three smaller moons of the Alkyoniods group.

• Mimas is the smallest and least massive of the inner round moons,^[27] although its mass is sufficient to alter the orbit of Methone.^[35] It is noticeably ovoid-shaped, having been made shorter at the poles and longer at the equator (by about 20 km) by the effects of Saturn's gravity.^[26] Mimas has a large impact crater one third its diameter, Herschel, situated on the moon's leading hemisphere.^[36] The moon has no known past or present geologic activity, and its surface is dominated by impact craters. The only tectonic features known are a few arcuate and linear troughs, which probably formed when Mimas was shattered by the Herschel impact.^[36]

Tiger stripes on Enceladus

• Enceladus is one of the smallest of Saturn's moons that is spherical in shape—only Mimas is smaller^[26]—yet is the only small Saturnian moon that is currently endogenously active, and the smallest known body in the Solar System that is geologically active today.^[37] The surface of the moon is morphologically diverse; it includes ancient heavily cratered terrain as well as younger smooth areas with few impact craters. Many plains on Enceladus are fractured and intersected by systems of lineaments.^[37] The area around the south pole of the moon was found by Cassini to be unusually warm and cut by a system of fractures about 130 km long called "tiger stripes", some of which emit jets of water vapor and dust.^[37] These jets form a large plume off the south pole of the moon, which replenishes Saturn's E ring^[37] and serves as the main source of ions in the magnetosphere of Saturn.^[38] The gas and dust are released with a rate of more than 100 kg/s. Enceladus may have liquid water underneath the south-polar surface.^[37] The source of the energy for this cryovolcanism is thought to be a mean motion resonance with Dione.^[37] The pure ice on the surface makes Enceladus one of the brightest known objects in the Solar System—its geometrical albedo is more than 140%.^[37]
• Tethys is the third largest of Saturn's inner moons.^[27] Its most prominent features are a large (400 km diameter) impact crater named Odysseus on its leading hemisphere and a vast canyon system named Ithaca Chasma extending at least 270° around the moon.^[36] The Ithaca Chasma is concentric with Odysseus, and these two features may be related. Tethys appears to have no current geological activity. A heavily cratered hilly terrain occupies the majority of its surface, while a smaller and smoother plains region lies on the hemisphere opposite to that of Odysseus.^[36] The plains contain fewer craters and are apparently younger. A sharp boundary separates them from the cratered terrain. There is also a system of extensional troughs radiating away from Odysseus.^[36] The density of Tethys (0.97 g/cm³) is less than that of water, indicating that the moon is made mainly of water ice with only a small fraction of rock.^[26]
• Dione is the second-largest inner moon of Saturn. It has a higher density than geologically dead Rhea, the largest inner moon, but lower than that of active Enceladus.^[26] While the majority of Dione's surface is heavily cratered old terrain, this moon is also covered with an extensive network of troughs and lineaments, indicating that in the past it had global tectonic activity.^[39] The troughs and lineaments are especially prominent on the trailing hemisphere, where several intersecting sets of fractures form what is called "wispy terrain".^[39] The cratered plains have a few large impact craters reaching 250 km in diameter.^[36] Smooth plains with low impact crater counts are present as well on a small fraction of the moon's surface.^[40] They were probably tectonically resurfaced relatively later in the geological history of Dione. At two locations within smooth plains strange landforms (depressions) resembling oblong impact craters have been identified, both of which lie at the centers of radiating networks of cracks and troughs;^[40] these features may be cryovolcanic in origin. Dione may be geologically active even now, although on a scale much smaller than the cryovolcanism of Enceladus. This follows from Cassini magnetic measurements that show Dione is a net source of plasma in the magnetosphere of Saturn, much like Enceladus.^[40]

The Alkyonides

Three small moons orbit between Mimas and Enceladus: Methone, Anthe, and Pallene. Named after the Alkyonides of Greek mythology, they are some of the smallest moons in the Saturn system. All Alkyonides possess very faint ring arcs along their orbits.^[41]

Trojan moons

Trojan moons are a unique feature not found outside the Saturnian system. A Trojan body orbits at either the leading L₄ or trailing L₅ Lagrange point of a much larger object, such as a large moon or planet. Tethys has two Trojan moons, Telesto (leading) and Calypso (trailing), and Dione also has two, Helene (leading) and Polydeuces (trailing).^[13] Helene is by far the largest Trojan moon,^[26] while Polydeuces is the smallest and has the most chaotic orbit.^[35]

Outer large moons

These moons all orbit beyond the E Ring. They are:

Inktomi or "The Splat", a relatively young crater with prominent butterfly-shaped ejecta on Rhea's leading hemisphere

• Rhea is the second-largest of Saturn's moons.^[26] In 2005 Cassini detected a depletion of electrons in the plasma wake of Rhea, which forms when the co-rotating plasma of Saturn's magnetosphere is absorbed by the moon.^[16] The depletion is most likely caused by the presence of dust-sized particles concentrated in a few faint equatorial rings.^[16] Such a ring system is unlikely to be detected by optical imaging in the foreseeable future because it is so tenuous. This discovery would make Rhea the only known moon in the solar System to have rings.^[16] Otherwise Rhea has rather a typical heavily cratered surface,^[36] with the exceptions of a few large Dione-type fractures (wispy terrain) on the trailing hemisphere^[42] and a very faint "line" of material at the equator that may have been deposited by material deorbiting from its rings.^[43] Rhea also has two very large impact basins on its anti-Saturnian hemisphere, which are about 400 and 500 km across.^[42] The first, Tirawa, is roughly comparable to the Odysseus basin on Tethys.^[36] There is also a 48 km-diameter impact crater called Inktomi^{[44][note 2]} at 112°W that is prominent because of an extended system of bright rays,^[45] which may be one of the youngest craters on the inner moons of Saturn.^[42] No evidence of any endogenic activity has been discovered on the surface of Rhea.^[42]
• Titan, at 5,151 km diameter, is the second largest moon in the Solar System.^[27] Out of all the large moons, Titan is the only one with a dense (surface pressure of 1.5 atm), cold atmosphere, primarily made of nitrogen with a small fraction of methane.^[46] The dense atmosphere frequently produces bright white convective clouds, especially over the south pole region.^[46] The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters and is probably very young.^[46] It contains a pattern of light and dark regions, flow channels and possibly cryovolcanoes.^[46][47] Some dark regions are covered by longitudinal dune fields shaped by tidal winds, where sand is made of frozen water or hydrocarbons.^[48] Titan is the only moon with large bodies of a liquid on its surface, in the form of methane lakes at Titan's north and south poles.^[49] The largest lake, Kraken Mare, is larger than the Caspian Sea.^[50] Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.^[47]
• Hyperion is Titan's nearest neighbor in the Saturn system. The two moons are locked in a 4:3 mean motion resonance with each other, meaning that while Titan makes four revolutions around Saturn, Hyperion makes exactly three.^[27] With an average diameter of about 270 km, Hyperion is smaller and lighter than Mimas.^[51] It has an extremely irregular shape, and a very odd, tan-colored icy surface resembling a sponge, though its interior may be partially porous as well.^[51] The average density of about 0.55 g/cm^3[51] indicates that the porosity exceeds 40% even assuming it has a purely icy composition. The surface of Hyperion is covered with numerous impact craters—those with diameters 2–10 km are especially abundant.^[51] It is the only moon known to have a chaotic rotation, which means Hyperion has no well-defined poles or equator. While on short timescales the satellite approximately rotates around its long axis at a rate of 72–75° per day, on longer timescales its axis of rotation (spin vector) wanders chaotically across the sky.^[51] This makes the rotational behavior of Hyperion essentially unpredictable.^[52]

Equatorial ridge on Iapetus

• Iapetus is the third-largest of Saturn's moons.^[26] Orbiting the planet at 3.5 million km, it is by far the most distant of Saturn's large moons, and also possesses the greatest orbital inclination, at 7.5 degrees.^[28] Iapetus has long been known for its unusual two-toned surface; its leading hemisphere is pitch-black and its trailing hemisphere is almost as bright as fresh snow.^[53] Cassini images showed that the dark material is confined to a large near equatorial area on the leading hemisphere called Cassini Regio, which extends approximately from 40°N to 40°S.^[53] The pole regions of Iapetus are as bright as its trailing hemisphere. Cassini also discovered a 20 km tall equatorial ridge, which spans nearly the moon's entire equator.^[53] Otherwise both dark and bright surfaces of Iapetus are old and heavily cratered. The images revealed at least four large impact basins with diameters from 380 to 550 km and numerous smaller impact craters.^[53] No evidence of any endogenic activity has been discovered.^[53] The source of the dark material for the Iapetus' starkly dichromatic surface may have been found in 2009, when NASA's Spitzer Space Telescope discovered a vast, nearly invisible ring around Saturn, just inside the orbit of the moon Phoebe—the Phoebe ring.^[54] Scientists believe that the ring originates from dust and ice particles kicked up from Phoebe as it collides with comets, which plume off the moon's surface into space. Because the ring particles, like Phoebe itself, orbit in the opposite direction to Iapetus, the moon collides with them as they drift in the direction of Saturn, leaving a dark coating on its leading hemisphere.^[54]

Irregular moons

Diagram illustrating the orbits of the irregular satellites of Saturn. The inclination and semi-major axis are represented on the Y and X-axis, respectively. The eccentricity of the orbits is shown by the segments extending from the pericenter to apocenter. The satellites with positive inclinations are prograde, those with negative are retrograde. The X-axis is labeled in km. The prograde Inuit and Gallic groups and the retrograde Norse group are identified.

Irregular moons are small satellites with large-radii, inclined, and frequently retrograde orbits, believed to have been acquired by the parent planet through a capture process. They often occur as collisional families or groups.^[18] The precise size as well as albedo of the irregular moons are not known for sure because the moons are very small to be resolved by a telescope, although the latter is usually assumed to be quite low—around 6% (albedo of Phoebe) or less.^[19] The irregulars generally have featureless visible and near infrared spectra dominated by water absorption bands.^[18] They are neutral or moderately red in color—similar to C-type, P-type, or D-type asteroids,^[25] though they are much less red than Kuiper Belt objects.^{[18][note 3]}

Inuit group

The Inuit group includes five prograde outer moons that are similar enough in their distances from the planet (186–297 radii of Saturn), their orbital inclinations (45–50°) and their colors that they can be considered a group.^[19][25] The moons are Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq.^[25] The largest among them is Siarnaq with an estimated size of about 40 km.

Gallic group

The Gallic group are four prograde outer moons that are similar enough in their distance from the planet (207–302 radii of Saturn), their orbital inclination (35–40°) and their color that they can be considered a group.^[19][25] They are Albiorix, Bebhionn, Erriapus, and Tarvos.^[25] Tarvos, as of 2009, is the most distant of Saturn's moons with a prograde orbit. The largest among these moons is Albiorix with an estimated size of about 32 km.

Norse group

The Norse (or Phoebe) group consists of 29 retrograde outer moons.^[19][25] They are Aegir, Bergelmir, Bestla, Farbauti, Fenrir, Fornjot, Greip, Hati, Hyrrokkin, Jarnsaxa, Kari, Loge, Mundilfari, Narvi, Phoebe, Skathi, Skoll, Surtur, Suttungr, Thrymr, 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.^[25] After Phoebe, Ymir is the largest of the known retrograde irregular moons, with an estimated diameter of only 18 km. The Norse group may itself consist of several smaller subgroups.^[25]

• Phoebe, at 214 km in diameter, is by far the largest of Saturn's irregular satellites.^[18] It has a retrograde orbit and rotates on its axis every 9.3 hours.^[55] Phoebe was the first moon of Saturn to be studied in detail by Cassini, in June 2004; during this encounter Cassini was able to map nearly 90% of the moon's surface. Phoebe has a nearly spherical shape and a relatively high density of about 1.6 g/cm³.^[18] Cassini images revealed a dark surface scarred by numerous impacts—there are about 130 craters with diameters exceeding 10 km. Spectroscopic measurement showed that the surface is made of water ice, carbon dioxide, phyllosilicates, organics and possibly iron bearing minerals.^[18] Phoebe is believed to be a captured Kuiper Belt object or Centaur.^[18] It also serves as a source of material for the largest known ring of Saturn, which darkens the leading hemisphere of Iapetus (see above).^[54]

Table of moons

Order	Label [note 4]	Name	Pronunciation (key)	Image	Diameter (km)[note 5]	Mass ( × 1018 kg)[note 6]	Semi-major axis (km)[note 7]	Orbital period (d)[note 7][note 8]	Inclination (°) [note 7][note 9]	Eccentricity	Position	Discovery year[24]	Discoverer [24]
00 !0		S/2009 S 1	-		00003 !≈ 0.3	000000000001 !<0.0000001	00117 !≈ 117,000	000047 !≈ 0.47	0 !≈ 0°	0 !≈ 0	outer B Ring	2009	Cassini–Huygens[2]
0 !0		(moonlets)			00004 !0.04 to 0.4 (Earhart)	000000000001 !<0.0000001	00130 !≈ 130,000	000055 !≈ 0.55	0 !≈ 0°	0 !≈ 0	Three 1000-km bands within A Ring	2006	Cassini–Huygens
01 !1	18 !XVIII	Pan	ˈpæn		0028 !28.4 ± 2.6 (35×32×21)	0000000049 !0.00495±0.00075	00133 !133,584	0000575 !+0.57505	000001 !0.001°	000035 !0.000035	in Encke Division	1990	M. Showalter
02 !2	35 !XXXV	Daphnis	ˈdæfnɨs		00078 !7.8 ± 1.6 (9×9×6)	00000000008 !0.000084±0.000012	00136 !136,505	0000594 !+0.59408	000000 !≈ 0°	000000 !≈ 0	in Keeler Gap	2005	Cassini–Huygens
03 !3	15 !XV	Atlas	ˈætləs		0030 !30.2 ± 2.8 (42×36×18)	0000000066 !0.0066 ± 0.00006	00137 !137,670	0000601 !+0.60169	000003 !0.003°	0012 !0.0012	outer A Ring shepherd	1980	Voyager 2
04 !4	16 !XVI	Prometheus	proʊˈmiːθiːəs		0086 !86.2 ± 5.4 (123×79×61)	0000001566 !0.1566 ± 0.0019	00139 !139,380	0000612 !+0.61299	000008 !0.008°	0022 !0.0022	inner F Ring shepherd	1980	Voyager 2
05 !5	17 !XVII	Pandora	pænˈdɔərə		0081 !80.6 ± 4.4 (103×80×64)	0000001356 !0.1356 ± 0.0022	00141 !141,720	0000628 !+0.62850	000050 !0.050°	0042 !0.0042	outer F Ring Shepherd	1980	Voyager 2
06 !6a	11 !XI	Epimetheus	ˌɛpɨˈmiːθiːəs		0113 !113.4 ± 3.8 (116×117×106)	0000005304 !0.53040±0.00193	00151 !151,422	0000694 !+0.69433	000335 !0.335°	0098 !0.0098	co-orbital	1977	J. Fountain, and S. Larson
07 !6b	10 !X	Janus	ˈdʒeɪnəs		0179 !179.2 ± 4 (195×194×152)	000001912 !1.912 ± 0.005	001514 !151,472	0000694 !+0.69466	000165 !0.165°	0068 !0.0068	co-orbital	1966	A. Dollfus
08 !8	53 !LIII	Aegaeon	iːˈdʒiːən		00005 !≈ 0.5	000000000001 !~0.0000001	001675 !167,500	0000808 !+0.80812	000001 !0.001°	0002 !0.0002	G Ring moonlet	2008	Cassini–Huygens
09 !9	01 !I	†Mimas	ˈmaɪməs		0397 !396.4 ± 1.0 (415×394×381)	000037493 !37.493 ± 0.031	00185 !185,404	0000942 !+0.942422	001566 !1.566°	0202 !0.0202		1789	W. Herschel
10	32 !XXXII	Methone	mɨˈθoʊniː		00032 !3.2 ± 1.2	00000000002 !~0.00002	00194 !194,440	0001009 !+1.00957	000007 !0.007°	0001 !0.0001	Alkyonides	2004	Cassini–Huygens
11	49 !XLIX	Anthe	ˈænθiː		0002 !≈ 2	000000000007 !~0.000007	00197 !197,700	0001036 !+1.03650	000100 !0.1°	001 !0.001	Alkyonides	2007	Cassini–Huygens
12	33 !XXXIII	Pallene	pəˈliːniː		00044 !4.4 ± 0.6 (5×4×4)	00000000005 !~0.00005	00212 !212,280	0001153 !+1.15375	000181 !0.181°	0040 !0.0040	Alkyonides	2004	Cassini–Huygens
13	02 !II	†Enceladus	ɛnˈsɛlədəs		0504 !504.2 ± 0.4 (513×503×497)	000108022 !108.022 ± 0.101	00237 !237,950	0001370 !+1.370218	000010 !0.010°	0047 !0.0047	Generates the E ring	1789	W. Herschel
14	03 !III	†Tethys	ˈtiːθɨs		1066 !1,066 ± 2.8 (1081×1062×1055)	000617449 !617.049 ± 0.132	00294 !294,619	0001887 !+1.887802	000168 !0.168°	0001 !0.0001		1684	G. Cassini
15 !14a	13 !XIII	Telesto	tɨˈlɛstoʊ		0025 !24.8 ± 0.8 (31×24×21)	00000000941 !~0.00941	00294 !294,619	0001887 !+1.887802	001158 !1.158°	0 !0.000	leading Tethys Trojan	1980	B. Smith, H. Reitsema, S. Larson, and J. Fountain
16 !14b	14 !XIV	Calypso	kəˈlɪpsoʊ		0021 !21.2 ± 1.4 (30×23×14)	0000000063 !~0.0063	00294 !294,619	0001887 !+1.887802	001473 !1.473°	0 !0.000	trailing Tethys Trojan	1980	D. Pascu, P. Seidelmann, W. Baum, and D. Currie
17	04 !IV	†Dione	daɪˈoʊniː		1123 !1,123.4 ± 1.8 (1128×1122×1121)	001095452 !1,095.452 ± 0.168	00377 !377,396	0002736 !+2.736915	000002 !0.002°	0022 !0.0022		1684	G. Cassini
18 !17a	12 !XII	Helene	ˈhɛlɨniː		0033 !33 ± 1.2 (39×37×25)	00000002446 !~0.02446	00377 !377,396	0002736 !+2.736915	000212 !0.212°	0022 !0.0022	leading Dione Trojan	1980	P. Laques and J. Lecacheux
19 !17b	34 !XXXIV	Polydeuces	ˌpɒliˈdjuːsiːz		00026 !2.6 ± 0.8 (3×2×1)	00000000003 !~0.00003	00377 !377,396	0002736 !+2.736915	000177 !0.177°	0192 !0.0192	trailing Dione Trojan	2004	Cassini–Huygens
20	05 !V	†Rhea	ˈriːə		1529 !1,528.6 ± 4.4 (1534×1525×1526)	002306518 !2,306.518 ± 0.353	00527 !527,108	000451 !+4.518212	000327 !0.327°	001258 !0.001258		1672	G. Cassini
21	06 !VI	♠Titan	ˈtaɪtən		51151 !5,151	13452 !134,520 ± 20	0122 !1,221,930	001594 !+15.94542	0003485 !0.3485°	0288 !0.0288		1655	C. Huygens
22	07 !VII	†Hyperion	haɪˈpɪəriən		0266 !266 ± 8 (328×260×214)	000005584 !5.584 ± 0.068	0148 !1,481,010	002127 !+21.27661	000568 !0.568°	123006 !0.123006	in 4:3 resonance with Titan	1848	W. Bond G. Bond W. Lassell
23	08 !VIII	†Iapetus	aɪˈæpɨtəs		1472 !1,471.2 ± 6.0 (1495×1498×1425)	001805635 !1,805.635 ± 0.375	0356 !3,560,820	007932 !+79.3215	007570 !7.570°	028613 !0.028613		1671	G. Cassini
24	24 !XXIV	‡Kiviuq	ˈkɪvioʊk		0016 !≈ 16	00000000279 !~0.00279	112 !11,294,800	0448 !+448.16	0490 !49.087°	3288 !0.3288	Inuit group	2000	B. Gladman, J. Kavelaars, et al.
25	22 !XXII	‡Ijiraq	ˈiː.ɨrɒk		0012 !≈ 12	00000000118 !~0.00118	113 !11,355,316	0451 !+451.77	0502 !50.212°	3161 !0.3161	Inuit group	2000	B. Gladman, J. Kavelaars, et al.
26	09 !IX	♣†Phoebe	ˈfiːbiː		0214 !214.4 ± 12.4 (230×220×210)	000008292 !8.292 ± 0.010	12869 !12,869,700	0545 !−545.09	1730 !173.047°	156242 !0.156242	Norse group	1899	W. Pickering
27	20 !XX	‡Paaliaq	ˈpɑːliɒk		0022 !≈ 22	00000000725 !~0.00725	15103 !15,103,400	0692 !+692.98	0461 !46.151°	3631 !0.3631	Inuit group	2000	B. Gladman, J. Kavelaars, et al.
28	27 !XXVII	♣Skathi	ˈskɒði		0008 !≈ 8	00000000035 !~0.00035	15672 !15,672,500	0732 !−732.52	1490 !149.084°	246 !0.246	Norse (Skathi) Group	2000	B. Gladman, J. Kavelaars, et al.
29	26 !XXVI	♦Albiorix	ˌælbiˈɒrɪks		0032 !≈ 32	0000000223 !~0.0223	16266 !16,266,700	0774 !+774.58	0380 !38.042°	477 !0.477	Gallic group	2000	M. Holman
30		♣S/2007 S 2	—		0006 !≈ 6	00000000015 !~0.00015	16650 !16,560,000	0792 !−792.96	1766 !176.68°	2418 !0.2418	Norse group	2007	S. Sheppard, D. Jewitt, J. Kleyna, B. Marsden
31	37 !XXXVII	♦Bebhionn	bɛˈviːn, ˈvɪvi.ɒn		0006 !≈ 6	00000000015 !~0.00015	17153 !17,153,520	0838 !+838.77	0404 !40.484°	333 !0.333	Gallic group	2004	S. Sheppard, D. Jewitt, J. Kleyna
32	28 !XXVIII	♦Erriapus	ˌɛriˈæpəs		0010 !≈ 10	00000000068 !~0.00068	17236 !17,236,900	0844 !+844.89	0381 !38.109°	4724 !0.4724	Gallic group	2000	B. Gladman, J. Kavelaars, et al.
33	47 !XLVII	♣Skoll	ˈskɒl, ˈskɜːl		0006 !≈ 6	00000000015 !~0.00015	17473 !17,473,800	0862 !−862.37	1556 !155.624°	418 !0.418	Norse (Skathi) group	2006	S. Sheppard, D. Jewitt, J. Kleyna
34	29 !XXIX	‡Siarnaq	ˈsiːɑrnək		0040 !≈ 40	0000000435 !~0.0435	17776 !17,776,600	0884 !+884.88	0457 !45.798°	24961 !0.24961	Inuit group	2000	B. Gladman, J. Kavelaars, et al.
35	52 !LII	‡Tarqeq	ˈtɑrkeɪk		0007 !≈ 7	00000000023 !~0.00023	17911 !17,910,600	0894 !+894.86	0499 !49.904°	1081 !0.1081	Inuit group	2007	S. Sheppard, D. Jewitt, J. Kleyna
36		♣S/2004 S 13	—		0006 !≈ 6	00000000015 !~0.00015	18056 !18,056,300	0905 !−905.85	1673 !167.379°	261 !0.261	Norse group	2004	S. Sheppard, D. Jewitt, J. Kleyna
37	51 !LI	♣Greip	ˈɡreɪp		0006 !≈ 6	00000000015 !~0.00015	18065 !18,065,700	0906 !−906.56	1726 !172.666°	3735 !0.3735	Norse group	2006	S. Sheppard, D. Jewitt, J. Kleyna
38	44 !XLIV	♣Hyrrokkin	hɪˈrɒkɨn		0008 !≈ 8	00000000035 !~0.00035	18168 !18,168,300	0914 !−914.29	1532 !153.272°	3604 !0.3604	Norse (Skathi) group	2006	S. Sheppard, D. Jewitt, J. Kleyna
39	50 !L	♣Jarnsaxa	jɑrnˈsæksə		0006 !≈ 6	00000000015 !~0.00015	18556 !18,556,900	0943 !−943.78	1628 !162.861°	1918 !0.1918	Norse group	2006	S. Sheppard, D. Jewitt, J. Kleyna
40	21 !XXI	♦Tarvos	ˈtɑrvɵs		0015 !≈ 15	0000000023 !~0.0023	18562 !18,562,800	0944 !+944.23	0346 !34.679°	5305 !0.5305	Gallic group	2000	B. Gladman, J. Kavelaars, et al.
41	25 !XXV	♣Mundilfari	ˌmʊndəlˈvɛri		0007 !≈ 7	00000000023 !~0.00023	18725 !18,725,800	0956 !−956.70	1693 !169.378°	198 !0.198	Norse group	2000	B. Gladman, J. Kavelaars, et al.
42		♣S/2006 S 1	—		0006 !≈ 6	00000000015 !~0.00015	18930 !18,930,200	0972 !−972.41	1542 !154.232°	1303 !0.1303	Norse (Skathi) group	2006	S. Sheppard, D.C. Jewitt, J. Kleyna
43		♣S/2004 S 17	—		0004 !≈ 4	00000000005 !~0.00005	19099 !19,099,200	0985 !−985.45	1668 !166.881°	226 !0.226	Norse group	2004	S. Sheppard, D. Jewitt, J. Kleyna
44	38 !XXXVIII	♣Bergelmir	bɛərˈjɛlmɪər		0006 !≈ 6	00000000015 !~0.00015	19104 !19,104,000	0986 !−985.83	1573 !157.384°	152 !0.152	Norse (Skathi) group	2004	S. Sheppard, D. Jewitt, J. Kleyna
45	31 !XXXI	♣Narvi	ˈnɑrvi		0007 !≈ 7	00000000023 !~0.00023	19395 !19,395,200	1008 !−1,008.45	1372 !137.292°	320 !0.320	Norse (Narvi) group	2003	S. Sheppard, D. Jewitt, J. Kleyna
46	23 !XXIII	♣Suttungr	ˈsʊtʊŋɡər		0007 !≈ 7	00000000023 !~0.00023	19579 !19,579,000	1022 !−1,022.82	1743 !174.321°	131 !0.131	Norse group	2000	B. Gladman, J. Kavelaars, et al.
47	43 !XLIII	♣Hati	ˈhɑːti		0006 !≈ 6	00000000015 !~0.00015	19709 !19,709,300	1033 !−1,033.05	1631 !163.131°	291 !0.291	Norse group	2004	S. Sheppard, D. Jewitt, J. Kleyna
48		♣S/2004 S 12	—		0005 !≈ 5	00000000009 !~0.00009	19905 !19,905,900	1048 !−1,048.54	1640 !164.042°	396 !0.396	Norse group	2004	S. Sheppard, D. Jewitt, J. Kleyna
49	40 !XL	♣Farbauti	fɑrˈbaʊti		0005 !≈ 5	00000000009 !~0.00009	19984 !19,984,800	1054 !−1,054.78	1583 !158.361°	209 !0.209	Norse (Skathi) group	2004	S. Sheppard, D. Jewitt, J. Kleyna
50	30 !XXX	♣Thrymr	ˈθrɪmər		0007 !≈ 7	00000000023 !~0.00023	20278 !20,278,100	1078 !−1,078.09	1745 !174.524°	453 !0.453	Norse group	2000	B. Gladman, J. Kavelaars, et al.
51	36 !XXXVI	♣Aegir	ˈaɪ.ɪər		0006 !≈ 6	00000000015 !~0.00015	20482 !20,482,900	1094 !−1,094.46	1674 !167.425°	237 !0.237	Norse group	2004	S. Sheppard, D. Jewitt, J. Kleyna
52		♣S/2007 S 3	—		0005 !≈ 5	00000000009 !~0.00009	20518 !20,518,500	1100 !≈ −1,100	1772 !177.22°	130 !0.130	Norse group	2007	S. Sheppard, D. Jewitt, J. Kleyna
53	39 !XXXIX	♣Bestla	ˈbɛstlə		0007 !≈ 7	00000000023 !~0.00023	20570 !20,570,000	1101 !−1,101.45	1473 !147.395°	77 !0.77	Norse (Narvi) group	2004	S. Sheppard, D. Jewitt, J. Kleyna
54		♣S/2004 S 7	—		0006 !≈ 6	00000000015 !~0.00015	20576 !20,576,700	1102 !−1,101.99	1655 !165.596°	5299 !0.5299	Norse group	2004	S. Sheppard, D. Jewitt, J. Kleyna
55		♣S/2006 S 3	—		0006 !≈ 6	00000000015 !~0.00015	21076 !21,076,300	1142 !−1,142.37	1508 !150.817°	4710 !0.4710	Norse (Skathi) group	2006	S. Sheppard, D. Jewitt, J. Kleyna
56	41 !XLI	♣Fenrir	ˈfɛnrɪər		0004 !≈ 4	00000000005 !~0.00005	21930 !21,930,644	1212 !−1,212.53	1628 !162.832°	131 !0.131	Norse group	2004	S. Sheppard, D. Jewitt, J. Kleyna
57	48 !XLVIII	♣Surtur	ˈsʊərtər		0006 !≈ 6	00000000015 !~0.00015	22288 !22,288,916	1242 !−1,242.36	1669 !166.918°	3680 !0.3680	Norse group	2006	S. Sheppard, D. Jewitt, J. Kleyna
58	45 !XLV	♣Kari	ˈkɑri		0007 !≈ 7	00000000023 !~0.00023	22321 !22,321,200	1245 !−1,245.06	1483 !148.384°	3405 !0.3405	Norse (Skathi) group	2006	S. Sheppard, D. Jewitt, J. Kleyna
59	19 !XIX	♣Ymir	ˈɪmɪər		0018 !≈ 18	00000000397 !~0.00397	22429 !22,429,673	1254 !−1,254.15	1721 !172.143°	3349 !0.3349	Norse group	2000	B. Gladman, J. Kavelaars, et al.
60	46 !XLVI	♣Loge	ˈlɔɪ.eɪ		0006 !≈ 6	00000000015 !~0.00015	22984 !22,984,322	1300 !−1,300.95	1665 !166.539°	139 !0.1390	Norse group	2006	S. Sheppard, D. Jewitt, J. Kleyna
61	42 !XLII	♣Fornjot	ˈfɔrnjɒt		0006 !≈ 6	00000000015 !~0.00015	24504 !24,504,879	1432 !−1,432.16	1678 !167.886°	186 !0.186	Norse group	2004	S. Sheppard, D. Jewitt, J. Kleyna

Unconfirmed moons

The following objects (observed by Cassini) have not been confirmed as solid bodies. It is not yet clear if these are real satellites or merely persistent clumps within the F Ring.^[13]

Name	Image	Diameter (km)	Semi-major axis (km)[35]	Orbital period (d)[35]	Position	Discovery year
S/2004 S 6		≈ 3–5	≈ 140,130	+0.61801	uncertain objects around the F Ring	2004
S/2004 S 3/S 4[note 10]		≈ 3−5	≈ 140,300	≈ +0.619		2004

Hypothetical moons

Two moons were claimed to be discovered by different astronomers but never seen again. Both moons were said to orbit between Titan and Hyperion.^[58]

• Chiron which was supposedly sighted by Hermann Goldschmidt in 1861, but never observed by anyone else.^[58]
• Themis was allegedly discovered in 1905 by astronomer William Pickering, but never seen again. Nevertheless it was included in numerous almanacs and astronomy books until the 1960s.^[58]

Notes

References

External links

• "Simulation showing the position of Saturn's Moon". http://orinetz.com/planet/animatesystem.php?ephid=Q07IAL5QATR7V073RO44XQVPA00001. Retrieved 26 May 2010. 
• "Saturn's Rings". NASA's Solar System Exploration. http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn&Display=Rings. Retrieved 26 May 2010. 
• "Saturn's Moons". Astronomy Cast episode #61, includes full transcript. http://www.astronomycast.com/astronomy/episode-61-saturns-moons/. Retrieved 26 May 2010. 
• Carolyn Porco. Fly me to the moons of Saturn. http://www.youtube.com/watch?v=xxXa9pxwzoY. Retrieved 26 May 2010. 

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