Moons of Saturn


The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to enormous Titan, which is larger than the planet Mercury. Saturn has 82 moons with confirmed orbits that are not embedded in its rings – of which only 13 have diameters greater than 50 kilometers – as well as dense rings that contain millions of embedded moonlets and innumerable smaller ring particles. Seven Saturnian moons are large enough to have collapsed into a relaxed, ellipsoidal shape, though only one or two of those, Titan and possibly Rhea, are currently in hydrostatic equilibrium. Particularly notable among Saturn's moons are Titan, the second-largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes, Enceladus, which emits jets of gas and dust from its south-polar region, and Iapetus, with its contrasting black and white hemispheres.
Twenty-four of Saturn's moons are regular satellites; they have prograde orbits not greatly inclined to Saturn's equatorial plane. They include the seven major satellites, four small moons that exist in a trojan orbit with larger moons, two mutually co-orbital moons and two moons that 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 58, with mean diameters ranging from 4 to 213 km, 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 moonlets hundreds of meters across, each in its own orbit around Saturn. Thus a precise number of Saturnian moons cannot be given, because 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. Over 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.
There are still 29 moons yet to be named, using names from Gallic, Norse and Inuit mythology based on the orbital groups of the moons. Twenty of these moons are in line to receive permanent designations, with seventeen Norse, two Inuit, and one Gallic name expected.

Discovery

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 objective lens on a refracting telescope of his own design. Tethys, Dione, Rhea and Iapetus were discovered between 1671 and 1684 by Giovanni Domenico Cassini. Mimas and Enceladus were discovered in 1789 by William Herschel. Hyperion was discovered in 1848 by W.C. Bond, G.P. Bond and William Lassell.
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. In 1966 the tenth satellite of Saturn was discovered by Audouin Dollfus, when the rings were observed edge-on near an equinox. 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. 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. In 1980, three additional Saturnian moons were discovered from the ground and later confirmed by the Voyager probes. They are trojan moons of Dione and Tethys.

Observations by spacecraft

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. In addition, Epimetheus was confirmed as distinct from Janus. In 1990, Pan was discovered in archival Voyager images.
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 trojan moon of Dione - Polydeuces. It also observed three suspected but unconfirmed moons in the F Ring. In 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, had been visually confirmed at the time. In 2007 Anthe was announced. 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. In, Aegaeon, a moonlet within the G Ring, was announced. In July of the same year, S/2009 S 1, the first moonlet within the B Ring, was observed. In April 2014, the possible beginning of a new moon, within the A Ring, was reported.

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. 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. They are probably fragments of larger bodies captured by Saturn's gravitational pull. In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons, in 2006, astronomers using the Subaru 8.2 m telescope reported the discovery of nine more irregular moons, in, Tarqeq was announced and in May of the same year S/2007 S 2 and S/2007 S 3 were reported. In 2019, twenty new irregular satellites of Saturn were reported, resulting in Saturn overtaking Jupiter as the planet with the most known moons for the first time since 2000.
Some of the 82 known satellites of Saturn are considered lost because they have not been observed since their discovery and hence their orbits are not known well enough to pinpoint their current locations. Work has been done to recover many of them in surveys from 2009 onwards, but five – S/2004 S 13, S/2004 S 17, S/2004 S 12, S/2004 S 7, and S/2007 S 3 – still remain lost today.

Naming

The modern names for Saturnian moons were suggested by John Herschel in 1847. He proposed to name them after mythological figures associated with the Roman titan of time, Saturn. In particular, the then known seven satellites were named after Titans, Titanesses and Giants—brothers and sisters of Cronus. In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. 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. All the irregular moons are named after Inuit and Gallic gods and after Norse ice giants.
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 : Calypso and asteroid 53 Kalypso; and Helene and asteroid 101 Helena.

Sizes

Saturn's satellite system is very lopsided: one moon, Titan, comprises more than 96% of the mass in orbit around the planet. The six other planemo moons constitute roughly 4% of the mass, and the remaining 75 small moons, together with the rings, comprise only 0.04%.

Orbital groups

Although the boundaries may be somewhat vague, 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. The innermost moons and most regular satellites all have mean orbital inclinations ranging from less than a degree to about 1.5 degrees and small orbital eccentricities. 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.

Ring moonlets

During late July 2009, a moonlet, S/2009 S 1, was discovered in the B Ring, 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, it does not induce a 'propeller' feature, probably due to the density of the B Ring.
along with the moons, Enceladus and Rhea.
In 2006, four tiny moonlets were found in Cassini images of the A Ring. 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. 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. The moonlets themselves are tiny, ranging from about 40 to 500 meters in diameter, and are too small to be seen directly. In 2007, the discovery of 150 more moonlets revealed that they 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. This region is relatively free from the disturbances caused by resonances with larger satellites, 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. 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.
Similar moonlets may reside in the F Ring. 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.
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. 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.
In April 2014, NASA scientists reported the possible consolidation of a new moon within the A Ring.

Ring shepherds

s 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, Daphnis, Atlas, Prometheus and Pandora. These moons together with co-orbitals 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.

Co-orbitals

and Epimetheus are called co-orbital moons. They are of roughly equal size, with Janus being slightly larger than Epimetheus. 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.

Inner large moons

The innermost large moons of Saturn orbit within its tenuous E Ring, along with three smaller moons of the Alkyonides group.
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. Anthe and Methone have very faint ring arcs along their orbits, whereas Pallene has a faint complete ring. Of these three moons, only Methone has been photographed at close range, showing it to be egg-shaped with very few or no craters.

Trojan moons

Trojan moons are a unique feature only known from 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 and Calypso, and Dione also has two, Helene and Polydeuces. Helene is by far the largest trojan moon, while Polydeuces is the smallest and has the most chaotic orbit. These moons are coated with dusty material that has smoothed out their surfaces.

Outer large moons

These moons all orbit beyond the E Ring. They are:
s 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. 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% or less. The irregulars generally have featureless visible and near infrared spectra dominated by water absorption bands. They are neutral or moderately red in color—similar to C-type, P-type, or D-type asteroids, though they are much less red than Kuiper belt objects.

Inuit group

The Inuit group includes seven prograde outer moons that are similar enough in their distances from the planet, their orbital inclinations and their colors that they can be considered a group. The moons are Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq, along with two unnamed moons S/2004 S 29 and S/2004 S 31. 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, their orbital inclination and their color that they can be considered a group. They are Albiorix, Bebhionn, Erriapus, and Tarvos. The largest among these moons is Albiorix with an estimated size of about 32 km. There is an additional satellite S/2004 S 24 that could belong to this group, but more observations are needed to confirm or disprove its categorization. S/2004 S 24 has the most distant prograde orbit of Saturn's known satellites.

Norse group

The Norse group consists of 46 retrograde outer moons. They are Aegir, Bergelmir, Bestla, Farbauti, Fenrir, Fornjot, Greip, Hati, Hyrrokkin, Jarnsaxa, Kari, Loge, Mundilfari, Narvi, Phoebe, Skathi, Skoll, Surtur, Suttungr, Thrymr, Ymir, and twenty-five unnamed satellites. 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.

Confirmed moons

The Saturnian moons are listed here by orbital period, 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.



Order
Label
NamePronunciationImageAbs.
magn.		Diameter
Mass
Semi-major
axis
Orbital period InclinationEccentricityPositionDiscovery
year		Discoverer
S/2009 S 1—outer B Ring2009Cassini
—Three 1000 km bands within A Ring2006Cassini
Panin Encke Division1990Showalter
Daphnisin Keeler Gap2005Cassini
Atlasouter A Ring shepherd1980Voyager 1
Prometheusinner F Ring shepherd1980Voyager 1
Pandoraouter F Ring shepherd1980Voyager 1
Epimetheusco-orbital with Janus1977Fountain & Larson
Janusco-orbital with Epimetheus1966Dollfus
AegaeonG Ring moonlet2008Cassini
10 †Mimas 1789Herschel
11 MethoneAlkyonides2004Cassini
12 AntheAlkyonides2007Cassini
13 PalleneAlkyonides2004Cassini
14 †EnceladusGenerates the E ring1789Herschel
15 †Tethys 1684Cassini
Telestoleading Tethys trojan1980Smith et al.
Calypsotrailing Tethys trojan1980Pascu et al.
18 †Dione 1684Cassini
Heleneleading Dione trojan1980Laques & Lecacheux
Polydeucestrailing Dione trojan2004Cassini
21 †Rhea 1672Cassini
22 ♠Titan 1655Huygens
23 †Hyperionin 4:3 resonance with Titan1848Bond & Lassell
24 †Iapetus 1671Cassini
25 ‡KiviuqInuit group2000Gladman et al.
26 ‡IjiraqInuit group2000Gladman et al.
27 ♣†PhoebeNorse group1899Pickering
28 ‡PaaliaqInuit group2000Gladman et al.
29 ♣SkathiNorse Group2000Gladman et al.
30 ♣S/2004 S 37—Norse group2019Sheppard et al.
31 ♣S/2007 S 2—Norse group2007Sheppard et al.
32 ♦AlbiorixGallic group2000Holman
33 ♦BebhionnGallic group2004Sheppard et al.
34 ‡S/2004 S 29—Inuit group2019Sheppard et al.
35 ♦ErriapusGallic group2000Gladman et al.
36 ‡S/2004 S 31—Inuit group2019Sheppard et al.
37 ♣SkollNorse group2006Sheppard et al.
38 ‡SiarnaqInuit group2000Gladman et al.
39 ‡TarqeqInuit group2007Sheppard et al.
40 ♣S/2004 S 13—


Norse group2004Sheppard et al.
41 ♣HyrrokkinNorse group2006Sheppard et al.
42 ♦TarvosGallic group2000Gladman et al.
43 ♣MundilfariNorse group2000Gladman et al.
44 ♣S/2006 S 1—Norse group2006Sheppard et al.
45 ♣GreipNorse group2006Sheppard et al.
46 ♣JarnsaxaNorse group2006Sheppard et al.
47 ♣BergelmirNorse group2004Sheppard et al.
48 ♣S/2004 S 17—


Norse group2004Sheppard et al.
49 ♣NarviNorse group2003Sheppard et al.
50 ♣S/2004 S 20—Norse group2019Sheppard et al.
51 ♣SuttungrNorse group2000Gladman et al.
52 ♣HatiNorse group2004Sheppard et al.
53 ♣S/2004 S 12—


Norse group2004Sheppard et al.
54 ♣FarbautiNorse group2004Sheppard et al.
55 ♣S/2004 S 27—Norse group2019Sheppard et al.
56 ♣BestlaNorse group2004Sheppard et al.
57 ♣S/2007 S 3—


Norse group2007Sheppard et al.
58 ♣AegirNorse group2004Sheppard et al.
59 ♣S/2004 S 7—


Norse group2004Sheppard et al.
60 ♣S/2004 S 22—Norse group2019Sheppard et al.
61 ♣ThrymrNorse group2000Gladman et al.
62 ♣S/2004 S 30—Norse group2019Sheppard et al.
63 ♣S/2004 S 23—Norse group2019Sheppard et al.
64 ♣S/2004 S 25—Norse group2019Sheppard et al.
65 ♣S/2004 S 32—Norse group2019Sheppard et al.
66 ♣S/2006 S 3—Norse group2006Sheppard et al.
67 ♣S/2004 S 38—Norse group2019Sheppard et al.
68 ♣S/2004 S 28—Norse group2019Sheppard et al.
69 ♣KariNorse group2006Sheppard et al.
70 ♣S/2004 S 35Norse group2019Sheppard et al.
71 ♣FenrirNorse group2004Sheppard et al.
72 ♣S/2004 S 21—Norse group2019Sheppard et al.
73 S/2004 S 24—Gallic group?2019Sheppard et al.
74 ♣S/2004 S 36—Norse group2019Sheppard et al.
75 ♣LogeNorse group2006Sheppard et al.
76 ♣SurturNorse group2006Sheppard et al.
77 ♣S/2004 S 39—Norse group2019Sheppard et al.
78 ♣YmirNorse group2000Gladman et al.
79 ♣S/2004 S 33—Norse group2019Sheppard et al.
80 ♣S/2004 S 34—Norse group2019Sheppard et al.
81 ♣FornjotNorse group2004Sheppard et al.
82 ♣S/2004 S 26—Norse group2019Sheppard et al.

Unconfirmed moons

The following objects 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.
NameImageDiameter Semi-major
axis 		Orbital
period 		PositionDiscovery yearStatus
S/2004 S 3 and S 4≈ 3–5≈ ≈ +uncertain objects around the F Ring2004Were undetected in thorough imaging of the region in November 2004, making their existence improbable
S/2004 S 6≈ 3–5≈ +uncertain objects around the F Ring2004Consistently detected into 2005, may be surrounded by fine dust and have a very small physical core

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.
Much like Jupiter, asteroids and comets will infrequently make close approaches to Saturn, even more infrequently becoming captured into orbit of the planet. The comet P/2020 F1 is calculated to have made a close approach of km ( mi to Saturn on 8 May 1936, closer than the orbit of Titan to the planet, with an orbital eccentricity of only. The comet may have been orbiting Saturn prior to this as a temporary satellite, but difficulty modelling the non-gravitational forces makes whether or not it was indeed a temporary satellite uncertain.
Other comets and asteroids may have temporarily orbited Saturn at some point, but none are presently known to have.

Formation

It is thought that the Saturnian system of Titan, mid-sized moons, and rings developed from a set-up closer to the Galilean moons of Jupiter, though the details are unclear. It has been proposed either that a second Titan-sized moon broke up, producing the rings and inner mid-sized moons, or that two large moons fused to form Titan, with the collision scattering icy debris that formed the mid-sized moons. On June 23, 2014, NASA claimed to have strong evidence that nitrogen in the atmosphere of Titan came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn in earlier times. Studies based on Enceladus's tidal-based geologic activity and the lack of evidence of extensive past resonances in Tethys, Dione, and Rhea's orbits suggest that the moons inward of Titan may be only 100 million years old.