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Photographic montages of
the Saturnian system
A spherical half-illuminated cratered body can be seen in the foreground. Behind and partially covered by it is another spherical illuminated orange body with rings. Two small bodies are to the right and above from the ringed body, while other two small bodies are to the left and below. A small orange body is in the top-right corner.
Dione in the forefront, Saturn rising behind, Tethys and Mimas fading in the distance to the right, Enceladus and Rhea off Saturn's rings to the left, and Titan in its distant orbit at the top.
In the foreground there are six round fully illuminated bodies and some small irregular objects. A large half-illuminated body is shown in the background with circular cloud bands around the partially darkened north pole visible.
Largest moons at scale: Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus and some small moons in the foreground, and Titan in the center in the background

The moons of Saturn are numerous and diverse, ranging from tiny moonlets less than 1,000 metres (0.62 mi) across to the enormous Titan, which is larger than the planet Mercury. Saturn has 62 moons with confirmed orbits, 53 of which have names, and only 13 of which have diameters larger than 50 kilometres (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, making the Saturnian system the most diverse in the Solar System. 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 possess prograde orbits not greatly inclined to 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 save one, are irregular satellites, whose orbits are much farther from Saturn, have high inclinations, and are mixed between prograde and retrograde. These moons were likely 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 nineteenth century.

The rings of Saturn are made up of icy objects ranging in size from microscopic to hundreds of metres, each of which is on its own orbit about the planet. 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.

Contents

Discovery and naming

A large bright circle in the center is surrounded by small circles.
Saturn and the moons Iapetus, Titan, Dione, Hyperion, and Rhea
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Ground based 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 mm objective lens[3] on a 50 power refracting telescope that he designed himself.[4] Tethys, Dione, Rhea and Iapetus (the "Sidera Lodoicea") were discovered in 1671–1672 by Giovanni Domenico Cassini.[5] William Herschel discovered Mimas and Enceladus in 1789.[5] Hyperion was discovered 1848 by W.C. Bond, G.P. Bond[6] and William Lassell.[7]

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.[8] In 1966 the tenth satellite of Saturn was discovered by Audouin Dollfus during a ring plane crossing event.[9] 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.[9] 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.[10] In 1980 three additional saturnian moons were discovered from the ground and later confirmed by Voyagers. They are Trojan moons of Dione (Helene) and Tethys (Telesto and Calypso).[10]

Observations by spacecraft

Circular complex rings of Saturn are seen at the low angle. The rings look like two grayish bands running parallel to each other from the left to right and connecting at the far right. Half illuminated Titan and Dione are visible slightly below the rings in the foreground. Two bright dots: one at the lower edge of rings and another above the rings can be seen. They are Prometheus and Telepso.
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.[10] In addition, Epimetheus was confirmed as distinct from Janus. In 1990, Pan was discovered in archival Voyager images.[10]

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.[11] On November 16, 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 (its confirmation was announced on May 6, 2005).[12] On July 18, 2007 Anthe was announced.[13] On March 6, 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.[14] On March 3, 2009, Aegaeon, a moonlet within the G Ring, was announced.[15] On July 26, 2009, S/2009 S 1, the first moonlet within the B Ring, was found and announced on November 2.[2]

Irregular moons

Study of Saturn's moons has also been aided by advances in telescopy especially by introduction of Charge-coupled devices (CCD). For the entire 20th century, Phoebe stood alone among Saturn's known moons in its highly irregular orbit. Beginning in 2000, however, three dozen additional irregular moons have been discovered using ground-based telescopes.[16] 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 and highly inclined orbits to both the equator of Saturn and ecliptic.[17] They are probably fragments of larger bodies captured by Saturn's gravitational pull.[17][16] On May 3, 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons.[18][19] On June 30, 2006 astronomers using the Subaru 8.2 m telescope reported the discovery of further nine irregular moons.[20] On April 13, 2007 Tarqeq (S/2007 S 1) was announced. On May 1, 2007 S/2007 S 2 and S/2007 S 3 were announced.[21]

Naming

The modern names for saturnian moons were suggested by John Herschel in 1847.[5] He proposed to name them after mythological figures associated with the Roman god of agriculture and harvest, Saturn (equated to the Greek Kronos).[5] In particular, the then known seven satellites were named after titans and titanesses—brothers and sisters of Saturn.[8] In 1848 Lassell proposed that the eighth satellite of Saturn was named Hyperion after one of titans.[7] When in 20-th century the names of titans were exhausted, the moons were named after different characters of the Roman-Greek mythology or giants from other mythologies.[22] All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and after Norse ice giants.[23]

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 Jovian moons until spelling differences were made permanent by the IAU: Calypso and asteroid 53 Kalypso; and Helene and asteroid 101 Helena.

Sizes

A pie chart
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, with one moon, Titan, comprising 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]

Saturn's major satellites, compared to Earth's Moon
Name
Diameter
(km)[25]
Mass
(kg)[26]
Orbital radius
(km)[27]
Orbital period
(days)[27]
Mimas 400
(12% Moon)
0.4×1020
(0.05% Moon)
185,000
(50% Moon)
0.9
(3% Moon)
Enceladus 500
(14% Moon)
1.1×1020
(0.2% Moon)
238,000
(60% Moon)
1.4
(5% Moon)
Tethys 1,070
(30% Moon)
6.2×1020
(0.8% Moon)
295,000
(80% Moon)
1.9
(7% Moon)
Dione 1,120
(32% Moon)
11×1020
(1.5% Moon)
377,000
(100% Moon)
2.7
(10% Moon)
Rhea 1,530
(44% Moon)
23×1020
(3% Moon)
527,000
(140% Moon)
4.5
(20% Moon)
Titan 5,150
(148% Moon)
(75% Mars)
1,350×1020
(180% Moon)
1,222,000
(320% Moon)
16
(60% Moon)
Iapetus 1,470
(42% Moon)
18×1020
(2.5% Moon)
3,560,000
(930% Moon)
79
(290% Moon)

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.[28] The innermost moons and most regular satellites all have orbital inclinations ranging from less than a degree to about 1.5 degrees (the exception is Iapetus, which has an inclination of 7.57 degrees) and small orbital eccentricities .[27] 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.[23]

Ring moonlets

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, likely due to the density of the B Ring.[29]

In 2006, four tiny "moonlets" were found in Cassini images of the A Ring.[30] 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.[30] 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.[31] The moonlets themselves are tiny, ranging from about 40–500 meters in diameter, and are too small to be seen directly.[32] 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 thousand kilometers wide, which is less than 1% the width of Saturn's rings.[32] This region is relatively free from the disturbances related to resonances with lager satellites,[32] 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.[31] 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.[32]

Similar moonlets may reside in the F Ring.[32] 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 co-rotation resonance with Mimas.[15] 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), Pandora (F Ring).[11][15] 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.[28]

Co-orbitals

Janus and Epimetheus are called co-orbital moons.[10] They are of roughly equal size, with Janus being slightly larger than Epimetheus.[28] 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,[26] 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.[25] Mimas has a large impact crater one third its diameter situated on the moon's leading hemisphere called Herschel.[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]
A circular part of a grayish surface, which is intersected from the top-left to the bottom-right by four wide sinuous groves. Smaller and shorter grooves can be seen between them running either parallel to the large grooves or criss-crossing them. There is a rough terrain in the top-left corner.
Tiger stripes on Enceladus
  • Enceladus is one of the smallest moons of Saturn that has spherical shape—only Mimas is smaller.[25] Yet Enceladus is the only Saturn's moon except much larger Titan and, possibly Dione, that is currently endogenously active.[37] The surface of the moon is morphologically diverse—it includes ancient heavily cratered terrain as well as younger smooth areas that contain only 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 to be unusually warm and cut by a system of fractures about 130 km long called "tiger stripes", some of which emit jets of the water vapor and dust.[37] These jets form a large plume off the south pole of the moon, which replenishes the Saturn's E ring[37] and serves as the main source of the ions in the magnetosphere of Saturn.[38] The gas and dust are released with a rate of more than 100 kg/s. The source of the energy for this cryovolcanism is thought to be a mean motion resonance with Dione.[37] Enceladus is the smallest known body in the Solar System that is geologically active today, and it may have liquid water underneath the south-polar surface.[37] The pure ice on the surface makes Enceladus one of the brightest known objects in the solar system with the geometrical albedo reaching than 140%.[37]
  • Tethys is the third largest of Saturn's inner moons.[26] Its most prominent features are a large (diameter 400 km) impact crater named Odysseus situated on moon's 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 less 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.[25]
  • 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 endogenously active Enceladus.[25] While the majority of the Dione's surface belongs to the old heavily cratered terrain, this moon is also covered with an extensive network of troughs and lineaments indicating that in the past the tectonic activity was global.[39] The troughs and lineaments are especially prominent on the trailing hemisphere, where a 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 forming a fraction of the moon's surface.[40] They were probably tectonically resurfaced relatively later in the geological history of the Dione. At least at the two locations within smooth plains the strange landforms (depressions) resembling oblong impact craters have been identified. Both of them 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 the fact that according to the Cassini magnetometric measurements 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 Saturn's system. A trojan body orbits at either 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).[11] Helene is by far the largest trojan moon,[25] while Polydeuces is the smallest and has the most erratic orbit.[35]

Outer large moons

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

A spherical body is almost fully illuminated. Its grayish surface is covered by numerous circular craters. The terminator is located near the upper-right limb. A large crater can be seen near the limb in the upper-left part of the body. Another smaller bright crater can be seen in the center. It is surrounded by a large bright patch having the shape of a five-pointed star.
A relatively young crater with prominent butterfly-shaped ejecta on Rhea's leading hemisphere
  • Rhea is second-largest of Saturn's moons.[25] In 2005 the Cassini Orbiter detected a depletion of electrons in the plasma wake of Rhea, which forms when the co-rotating plasma of the Saturn's magnetosphere passes by the moon.[14] The depletion is most likely caused by the presence of dust-sized particles concentrated in a few faint equatorial rings.[14] 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.[14] Otherwise Rhea has rather a typical heavily cratered surface,[36] with the exceptions of a few large Dione-type fractures (whispy 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 of them called Tirawa is roughly comparable to the Odysseus basin on Tethys.[36] There is a 48 km-diameter impact crater at 112°W that is prominent because of an extended system of bright rays.[42] This crater is nick-named "The Splat", and 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 5151 km diameter, is the second largest moon in the Solar system.[26] Out of all the large moons, Titan is the only one with a dense—surface pressure of 1.5 atm—cold atmosphere, which is primarily made of nitrogen with a small fraction of methane.[44] The dense atmosphere frequently produces bright white convective clouds, especially over the south pole region.[44] The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only few impact craters and is likely very young.[44] It contains a pattern of light and dark regions, flow channels and possibly cryovolcanoes.[44][45] Some dark regions are covered by longitudinal dune fields shaped by tidal winds, where sand is made of frozen water or hydrocarbons.[46] 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.[47] The largest lake called Kraken Mare is larger than Caspian Sea.[48] 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.[45]
  • 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 three revolutions around Saturn, Hyperion makes exactly four.[26] With an average diameter of about 270 km Hyperion is smaller and lighter than Mimas.[49] 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.[49] The average density of about 0.55 g/cm3[49] indicates that the porosity exceeds 40% even for a purely icy composition. The surface of Hyperion is covered with numerous impact craters—those with diameters 2–10 km are especially abundant.[49] 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.[49] This makes rotational behavior of Hyperion essentially unpredictable.[50]
A part of a spherical body illuminated from the above and behind. The convex limb runs from the lower-left to the upper-right corner. The black outer space is in the upper-left corner. The terminator is near the bottom. The surface of the body is covered with numerous craters. A large ridge runs in the center from the top to bottom.
Equatorial ridge on Iapetus
  • Iapetus is the third-largest of Saturn's moons.[25] 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.[27] 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.[51] 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.[51] 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.[51] 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.[51] No evidence of any endogenic activity has been discovered.[51] The source of 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—Phoebe ring.[52] Scientists believe that the ring originates from dust and ice particles kicked up by 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 the them as they drift in the direction of Saturn leaving a dark coating on its leading hemisphere.[52]

Irregular moons

Diagram illustrating the orbits of the irregular satellites of Saturn.1 The eccentricity of the orbits is represented by the segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis. The satellites above the axis are prograde, those beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's radius (~65 Gm for Saturn). The prograde Inuit and Gallic groups and the retrograde Norse group are clearly identifiable (from top to bottom).
1 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.

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.[16] The albedo of the irregular moons is not known for sure, but is usually assumed to be quite low—around 6% (albedo of Phoebe) or less.[17] The irregulars generally have featureless visible and near infrared spectra dominated by water absorption bands.[16] They are neutral or moderately red in color—similar to C-type, P-type, or D-type asteroids,[23] though they are much less red than Kuiper Belt objects.[16]

Inuit group

The Inuit group includes five prograde outer moons that are similar enough in their distances from Saturn (186–297 radii of Saturn), their orbital inclinations (45–50°) and their colors that they can be considered a group.[23][17] The moons are Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq.[23]

Gallic group

The Gallic group are four prograde outer moons that are similar enough in their distance from Saturn (207–302 radii of Saturn), their orbital inclination (35–40°) and their color that they can be considered a group.[23][17] They are Albiorix, Bebhionn, Erriapus, and Tarvos.[23] Tarvos, as of 2009, is the most distant of Saturn's moons with a prograde orbit.

Norse group

The Norse (or Phoebe) group consists of 29 retrograde outer moons.[23][17] 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.[23] After Phoebe, Ymir is the largest of the known retrograde irregular moons, with an estimated diameter of only 18 km. Norse group may itself consist of several smaller subgroups.[23]

Phoebe, at 214 km in diameter, is by far the largest of Saturn's irregular satellites.[16] It has a retrograde orbit and rotates on its axis every 9.3 hours.[53] Phoebe was the first moon of Saturn to be studied in detail by the Cassini Orbiter in June 2004; during this encounter Cassini was able to map nearly 90% of the moon's surface. Phoebe appears to have a nearly spherical shape and relatively high density of about 1.6 g/cm3.[16] Cassini images revealed a dark surface scared 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.[16] Phoebe is believed to be a captured Kuiper Belt object or Centaur.[16] It also serves as a source of material for the largest known ring of Saturn, which darkens the leading hemisphere of Iapetus (see above).[52]

Table of moons

The Saturnian moons are listed here by orbital period (or semi-major axis), from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in bold, while the irregular moons are listed in red, orange and gray background.
Key

Major icy moons

Titan

Inuit group

Gallic group

Norse group


Order
Label
[note 2]
Name
Pronunciation (key) Image Diameter (km)[note 3]
Mass
( × 1018 kg)[note 4]
Semi-major axis (km)[note 5]
Orbital period (d)[note 6][note 5]
Inclination (°) [note 7][note 5]
Eccentricity [note 5]
Position Discovery
year
[22]
Discoverer
[22]
0 S/2009 S 1 ≈ 0.3 ? ≈ 117,000 ≈ 0.47 ? outer B Ring 2009 Cassini–Huygens[2]
0 (moonlets) A noisy image showing a few bright dots marked by circles. 0.04 to 0.5 <0.0000001 ≈ 130,000 Three 1000-km bands within A Ring 2006 Cassini–Huygens
1 XVIII Pan ˈpæn A bright fuzzy band (rings of Saturn) is running from the left to right. In the center a bright irregularity shaped body is superimposed on its upper edge. A narrow grayish band, which is a part of the main band, partially covers the body. 28.4 ± 2.6
(35×32×21)
0.00495 ± 0.00075 133,584 +0.57505 0.001° 0.000035 in Encke Division 1990 M. Showalter
2 XXXV Daphnis ˈdæfnɨs Two bright bands run from the left to right. In the narrow gap between them (Keeler gap), which has wavy edges, a small oblong object can be seen. 7.8 ± 1.6
(9×9×6)
0.000084 ± 0.000012 136,505 +0.59408 ≈ 0° ≈ 0 in Keeler Gap 2005 Cassini–Huygens
3 XV Atlas ˈætləs An irregularly shaped body is half illuminated from the right. The terminator runs from the top to bottom. The body, which looks like a cone viewed from the vertex, is elongated in the direction perpendicular to the image. 30.2 ± 2.8
(42×36×18)
0.0066 ± 0.00006 137,670 +0.60169 0.003° 0.0012 outer A Ring shepherd 1980 Voyager 2
4 XVI Prometheus proʊˈmiːθiːəs An irregularly shaped oblong body is fully illuminated. It is elongated in the direction from the right to left. Its surface is covered by craters. There is valley at the top. 86.2 ± 5.4
(123×79×61)
0.1566 ± 0.0019 139,380 +0.61299 0.008° 0.0022 inner F Ring shepherd 1980 Voyager 2
5 XVII Pandora pænˈdoʊrə An irregularly shaped body is half illuminated from the bottom. The terminator runs from the left to right. The surface is covered by numerous craters. 80.6 ± 4.4
(103×80×64)
0.1356 ± 0.0022 141,720 +0.62850 0.050° 0.0042 outer F Ring Shepherd 1980 Voyager 2
6a XI Epimetheus ˌɛpɨˈmiːθiːəs A fully illuminated irregular body, which has a shape remotely resembling a cube. One vertex with a large crater is at the right side of the image pointing towards the light source. The body's surface consists of ridges and valleys and is covered by craters. 113.4 ± 3.8
(116×117×106)
0.5304 ± 0.00193 151,422 +0.69433 0.335° 0.0098 co-orbital 1977 J. Fountain, and S. Larson
6b X Janus ˈdʒeɪnəs An irregular body, whose outline looks like an approximate circle in this image. It is illuminated from the bottom-left. The terminator runs from the top-left to bottom-right. The surface is covered by craters. 179.2 ± 4
(195×194×152)
1.912 ± 0.005 151,472 +0.69466 0.165° 0.0068 co-orbital 1966 A. Dollfus
8 LIII Aegaeon iːˈdʒiːən There images of a ring's segment are stacked together from the right to left. They shows motion of a moon along the ring. ≈ 0.5 ~0.0000001 167,500 +0.80812 0.001° 0.0002 G-ring moonlet 2008 Cassini–Huygens
9 I Mimas ˈmaɪməs A spherical body is half illuminated from the left. The terminator runs from the top to bottom in the vicinity of the right limb. A large crater with a central peak sits on the terminator slightly to the right and above the center of the body. It makes the body look like the Death Star. There are numerous smaller craters. 396.4 ± 1.0
(415×394×381)
37.493 ± 0.031 185,404 +0.942422 1.566° 0.0202   1789 W. Herschel
10 XXXII Methone mɨˈθoʊniː A dot in the glare of Saturn 3.2 ± 1.2 ~0.00002 194,440 +1.00957 0.007° 0.0001 Alkyonides 2004 Cassini–Huygens
11 XLIX Anthe ˈænθiː An animated image showing as a dot (right) moves around Saturn (left) outside the main rings (in the middle), which are viewed from a relatively low angle. ≈ 2 ~0.000007 197,700 +1.03650 0.1° 0.001 Alkyonides 2007 Cassini–Huygens
12 XXXIII Pallene pəˈliːniː A dot in the glare of Saturn 4.4 ± 0.6
(5×4×4)
~0.00005 212,280 +1.15375 0.181° 0.0040 Alkyonides 2004 Cassini–Huygens
13 II Enceladus ɛnˈsɛlədəs A spherical body is half illuminated from the left. The terminator runs from the top to bottom in the vicinity of the right limb. In the center and at the top there are heavily cratered areas. The areas to the left and at the bottom have few craters and are intersected by lots of sinuous greenish grooves. The four prominent grooves at the bottom are Tiger stripes. 504.2 ± 0.4
(513×503×497)
108.022 ± 0.101 237,950 +1.370218 0.010° 0.0047 Generates the E ring 1789 W. Herschel
14 III Tethys ˈtiːθɨs A spherical heavily cratered body is illuminated from the bottom. The terminator runs from the left to right in the vicinity of the top limb. There is a wide curved graben running from the center of the body to the bottom. It is Ithaca Chasma. 1,066 ± 2.8
(1081×1062×1055)
617.049 ± 0.132 294,619 +1.887802 0.168° 0.0001   1684 G. Cassini
14a XIII Telesto tɨˈlɛstoʊ A potato shaped body is illuminated from the right. The terminator runs from the top to bottom. There is a large crater at the bottom near the terminator. The body is elongated from the right to left. 24.8 ± 0.8
(31×24×21)
~0.00941 294,619 +1.887802 1.158° 0.000 leading Tethys trojan 1980 B. Smith, H. Reitsema, S. Larson, and J. Fountain
14b XIV Calypso kəˈlɪpsoʊ An oblong reddish body is seen in this low resolution image. 21.2 ± 1.4
(30×23×14)
~0.0063 294,619 +1.887802 1.473° 0.000 trailing Tethys trojan 1980 D. Pascu, P. Seidelmann, W. Baum, and D. Currie
17 IV Dione daɪˈoʊniː A spherical body is half illuminated from the right. The terminator is running from the top to bottom slightly to the left off the center. The central part of the body is smooth and has only a few craters. A heavily cratered terrain is near the right limb. A part of a large crater is intersected by the terminator in the lower-left corner. To the left of it there is a long crack running parallel to the terminator. 1,123.4 ± 1.8
(1128×1122×1121)
1,095.452 ± 0.168 377,396 +2.736915 0.002° 0.0022   1684 G. Cassini
17a XII Helene ˈhɛlɨniː An irregularly shaped body illuminated from the left. Its surface is covered by numerous impact craters. 33 ± 1.2
(39×37×25)
~0.02446 377,396 +2.736915 0.212° 0.0022 leading Dione trojan 1980 P. Laques and J. Lecacheux
17b XXXIV Polydeuces ˌpɒliˈdjuːsiːz An small oblong body is barley resolved in this image. 2.6 ± 0.8
(3×2×1)
~0.00003 377,396 +2.736915 0.177° 0.0192 trailing Dione trojan 2004 Cassini–Huygens
20 V Rhea ˈriːə A spherical body is almost fully illuminated. The terminator is running near the top edge. The surface is covered by numerous craters. Two partially overlapping large craters can be seen above the center. One that is younger is above and to the right from the older one. 1,528.6 ± 4.4
(1534×1525×1526)
2,306.518 ± 0.353 527,108 +4.518212 0.327° 0.001258   1672 G. Cassini
21 VI Titan ˈtaɪtən An orange spherical body is half illuminated from the right. The terminator is running from the top to bottom slightly to the left off the center. Both limb and terminator are fuzzy due to light scattering in the atmosphere. 5,151 134,520 ± 20 1,221,930 +15.94542 0.3485° 0.0288   1655 C. Huygens
22 VII Hyperion haɪˈpiːriən An irregularly shaped oblong body is illuminated from the left. The terminator is near the right limb. The body is elongated in the top-bottom direction. The surface is punctured by numerous impact craters, which make it look like a sponge or cheese. 266 ± 8
(328×260×214)
5.584 ± 0.068 1,481,010 +21.27661 0.568° 0.123006 in 4:3 resonance with Titan 1848 W. Bond
G. Bond
W. Lassell
23 VIII Iapetus aɪˈæpɨtəs A walnut shaped body illuminated from the bottom-left. The terminator runs from the top to right along the top-right limb. An equatorial ridge runs from the left to right and is convex in the direction of the bottom-left. Above and below it there are dark areas. Above the upper dark area and below the lower one there are bright poles. There numerous craters. Three among them are very large: one sits on the limb at the right another is in the center above the ridge. The third is below the ridge near the left limb. 1,471.2 ± 6.0
(1495×1498×1425)
1,805.635 ± 0.375 3,560,820 +79.3215 7.570° 0.028613   1671 G. Cassini
24 XXIV Kiviuq ˈkɪvioʊk ≈ 16 ~0.00279 11,294,800 +448.16 49.087° 0.3288 Inuit group 2000 B. Gladman, J. Kavelaars, et al.
25 XXII Ijiraq ˈiː.ɨrɒk ≈ 12 ~0.00118 11,355,316 +451.77 50.212° 0.3161 Inuit group 2000 B. Gladman, J. Kavelaars, et al.
26 IX ♣†Phoebe ˈfiːbiː An approximately spherical heavily cratered body is illuminated from the bottom-right. The terminator runs near the left and top limbs. There is huge crater at the top, which affects the shape, and another slightly smaller at the bottom. 214.4 ± 12.4
(230×220×210)
8.292 ± 0.010 12,869,700 −545.09 173.047° 0.156242 Norse group 1899 W. Pickering
27 XX Paaliaq ˈpɑːliɒk ≈ 22 ~0.00725 15,103,400 +692.98 46.151° 0.3631 Inuit group 2000 B. Gladman, J. Kavelaars, et al.
28 XXVII Skathi ˈskɒði ≈ 8 ~0.00035 15,672,500 −732.52 149.084° 0.246 Norse (Skathi) Group 2000 B. Gladman, J. Kavelaars, et al.
29 XXVI Albiorix ˌælbiˈɒrɪks ≈ 32 ~0.0223 16,266,700 +774.58 38.042° 0.477 Gallic group 2000 M. Holman
30   S/2007 S 2 ≈ 6 ~0.00015 16,560,000 −792.96 176.68° 0.2418 Norse group 2007 S. Sheppard, D. Jewitt, J. Kleyna, B. Marsden
31 XXXVII Bebhionn ˈbɛviːn ≈ 6 ~0.00015 17,153,520 +838.77 40.484° 0.333 Gallic group 2004 S. Sheppard, D. Jewitt, J. Kleyna
32 XXVIII Erriapus ˌɛriˈæpəs ≈ 10 ~0.00068 17,236,900 +844.89 38.109° 0.4724 Gallic group 2000 B. Gladman, J. Kavelaars, et al.
33 XLVII Skoll ˈskɒl, ˈskɜːl ≈ 6 ~0.00015 17,473,800 −862.37 155.624° 0.418 Norse (Skathi) group 2006 S. Sheppard, D. Jewitt, J. Kleyna
34 XXIX Siarnaq ˈsiːɑrnək ≈ 40 ~0.0435 17,776,600 +884.88 45.798° 0.24961 Inuit group 2000 B. Gladman, J. Kavelaars, et al.
35 LII Tarqeq ˈtɑrkeɪk ≈ 7 ~0.00023 17,910,600 +894.86 49.904° 0.1081 Inuit group 2007 S. Sheppard, D. Jewitt, J. Kleyna
36   S/2004 S 13 ≈ 6 ~0.00015 18,056,300 −905.85 167.379° 0.261 Norse group 2004 S. Sheppard, D. Jewitt, J. Kleyna
37 LI Greip ˈɡreɪp ≈ 6 ~0.00015 18,065,700 −906.56 172.666° 0.3735 Norse group 2006 S. Sheppard, D. Jewitt, J. Kleyna
38 XLIV Hyrrokkin hɪˈrɒkɨn ≈ 8 ~0.00035 18,168,300 −914.29 153.272° 0.3604 Norse (Skathi) group 2006 S. Sheppard, D. Jewitt, J. Kleyna
39 L Jarnsaxa jɑrnˈsæksə ≈ 6 ~0.00015 18,556,900 −943.78 162.861° 0.1918 Norse group 2006 S. Sheppard, D. Jewitt, J. Kleyna
40 XXI Tarvos ˈtɑrvɵs ≈ 15 ~0.0023 18,562,800 +944.23 34.679° 0.5305 Gallic group 2000 B. Gladman, J. Kavelaars, et al.
41 XXV Mundilfari ˌmʊndəlˈvɛri ≈ 7 ~0.00023 18,725,800 −956.70 169.378° 0.198 Norse group 2000 B. Gladman, J. Kavelaars, et al.
42   S/2006 S 1 ≈ 6 ~0.00015 18,930,200 −972.41 154.232° 0.1303 Norse (Skathi) group 2006 S. Sheppard, D.C. Jewitt, J. Kleyna
43   S/2004 S 17 ≈ 4 ~0.00005 19,099,200 −985.45 166.881° 0.226 Norse group 2004 S. Sheppard, D. Jewitt, J. Kleyna
44 XXXVIII Bergelmir bɛrˈjɛlmɪr ≈ 6 ~0.00015 19,104,000 −985.83 157.384° 0.152 Norse (Skathi) group 2004 S. Sheppard, D. Jewitt, J. Kleyna
45 XXXI Narvi ˈnɑrvi ≈ 7 ~0.00023 19,395,200 −1,008.45 137.292° 0.320 Norse (Narvi) group 2003 S. Sheppard, D. Jewitt, J. Kleyna
46 XXIII Suttungr ˈsʊtʊŋɡər ≈ 7 ~0.00023 19,579,000 −1,022.82 174.321° 0.131 Norse group 2000 B. Gladman, J. Kavelaars, et al.
47 XLIII Hati ˈhɑːti ≈ 6 ~0.00015 19,709,300 −1,033.05 163.131° 0.291 Norse group 2004 S. Sheppard, D. Jewitt, J. Kleyna
48   S/2004 S 12 ≈ 5 ~0.00009 19,905,900 −1,048.54 164.042° 0.396 Norse group 2004 S. Sheppard, D. Jewitt, J. Kleyna
49 XL Farbauti fɑrˈbaʊti ≈ 5 ~0.00009 19,984,800 −1,054.78 158.361° 0.209 Norse (Skathi) group 2004 S. Sheppard, D. Jewitt, J. Kleyna
50 XXX Thrymr ˈθrɪmər ≈ 7 ~0.00023 20,278,100 −1,078.09 174.524° 0.453 Norse group 2000 B. Gladman, J. Kavelaars, et al.
51 XXXVI Aegir ˈaɪ.ər ≈ 6 ~0.00015 20,482,900 −1,094.46 167.425° 0.237 Norse group 2004 S. Sheppard, D. Jewitt, J. Kleyna
52   S/2007 S 3 ≈ 5 ~0.00009 20,518,500 ≈ −1,100 177.22° 0.130 Norse group 2007 S. Sheppard, D. Jewitt, J. Kleyna
53 XXXIX Bestla ˈbɛstlə ≈ 7 ~0.00023 20,570,000 −1,101.45 147.395° 0.77 Norse (Narvi) group 2004 S. Sheppard, D. Jewitt, J. Kleyna
54   S/2004 S 7 ≈ 6 ~0.00015 20,576,700 −1,101.99 165.596° 0.5299 Norse group 2004 S. Sheppard, D. Jewitt, J. Kleyna
55   S/2006 S 3 ≈ 6 ~0.00015 21,076,300 −1,142.37 150.817° 0.4710 Norse (Skathi) group 2006 S. Sheppard, D. Jewitt, J. Kleyna
56 XLI Fenrir ˈfɛnrɪr ≈ 4 ~0.00005 21,930,644 −1,212.53 162.832° 0.131 Norse group 2004 S. Sheppard, D. Jewitt, J. Kleyna
57 XLVIII Surtur ˈsʊərtər ≈ 6 ~0.00015 22,288,916 −1,242.36 166.918° 0.3680 Norse group 2006 S. Sheppard, D. Jewitt, J. Kleyna
58 XLV Kari ˈkɑːri ≈ 7 ~0.00023 22,321,200 −1,245.06 148.384° 0.3405 Norse (Skathi) group 2006 S. Sheppard, D. Jewitt, J. Kleyna
59 XIX Ymir ˈɪmɪr ≈ 18 ~0.00397 22,429,673 −1,254.15 172.143° 0.3349 Norse group 2000 B. Gladman, J. Kavelaars, et al.
60 XLVI Loge ˈlɔɪ.eɪ ≈ 6 ~0.00015 22,984,322 −1,300.95 166.539° 0.1390 Norse group 2006 S. Sheppard, D. Jewitt, J. Kleyna
61 XLII Fornjot ˈfɔrnjɒt ≈ 6 ~0.00015 24,504,879 −1,432.16 167.886° 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.[11]

Name Image Diameter (km) Semi-major
axis (km)[35]
Orbital
period (d)[35]
Position Discovery year
S/2004 S 6 A bright narrow band runs from the top to bottom. To the right of it in the diffuse halo the is a bright small object. ≈ 3–5 ≈ 140,130 +0.61801 uncertain objects around the F-Ring 2004
S/2004 S 3/S 4[note 8] A segment of the ring with bright overexposed Saturn in the top-left corner. Near the right edge of the ring there is a bright dot. ≈ 3−5 ≈ 140,300 ≈ +0.619 2004

Hypothetical moons

Two moons were claimed to be discovered by different astronomers but never seen again. Since they both were said to orbit between Titan and Hyperion, it is possible that they are in fact identical, but most likely, none of these moons exist in reality.

External links

Notes

  1. ^ The mass of the rings is about the mass of Mimas,[24] while the combined mass of Janus, Hyperion and Phoebe—the most massive of the remaining moons—is about one third of that. The total mass of the rings and small moons is around 5.5 × 1019 kg.
  2. ^ A confirmed moon is given a permanent designation by the IAU consisting of a name and a Roman numeral.[22] The nine moons that were known before 1900 (of which Phoebe is the only irregular) are numbered in order of their distance from Saturn; the rest are numbered in the order by which they received their permanent designations. Nine small moons of the Norse group and S/2009 S 1 have not yet received a permanent designation.
  3. ^ The diameters and dimensions of the inner moons from Pan to Janus, Methone, Ahthe, Pallene and trojan moons of Tethys and Dione were taken from Porco et al., 2007, Table 1.[28] Diameters and dimensions of Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus are from Thomas eta al., 2007, Table 1.[25] The values for Phoebe are from Giese, 2006.[53] The diameter and dimensions for Hyperion are from Thomas, 1995[50] and Thomas, 2007, Table 1.[49] The approximate sizes of the irregular satellites are from the website of Scott Sheppard.[54]
  4. ^ Masses of the large moons were taken from Jacobson, 2006.[26] Masses of some small inner moons were taken from Porco, 2007.[28] Masses of other small moons were calculated assuming a density of 1.3 g/cm3.
  5. ^ a b c d The orbital parameters were taken from Spitale, et al. 2006,[35] IAU-MPC Natural Satellites Ephemeris Service,[55] and NASA/NSSDC.[27]
  6. ^ Negative orbital periods indicate a retrograde orbit around Saturn (opposite to the planet's rotation).
  7. ^ To Saturn's equator
  8. ^ S/2004 S4 was most likely a transient clump—it has not been recovered since the first sighting.

References

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  3. ^ Robert Nemiroff and Jerry Bonnell (2005-03-25). "Huygens Discovers Luna Saturni". Astronomy Picture of the Day. http://apod.nasa.gov/apod/ap050325.html. Retrieved 2010-03-04. 
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  9. ^ a b Fountain, John W; Larson, Stephen M (1977). "A New Satellite of Saturn?". Science 197 (4306): 915–917. doi:10.1126/science.197.4306.915. PMID 17730174. http://adsabs.harvard.edu/abs/1977Sci...197..915F. 
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