Moons of Saturn

Last updated

Artist's concepts of the Saturnian ring–moon system
Saturn's Rings PIA03550.jpg
Saturn, its rings and major icy moons—from Mimas to Rhea
Moons of Saturn 2007.jpg
Images of several moons of Saturn. From left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top right) and irregularly shaped Hyperion (bottom right). Some small moons are also shown. All to scale.

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 83 moons with confirmed orbits that are not embedded in its rings [1] – 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. [2] [3] [4] Seven Saturnian moons are large enough to have collapsed into a relaxed, ellipsoidal shape, though only one or two of those, Titan and possiblyRhea, are currently in hydrostatic equilibrium. Particularly notable among Saturn's moons are Titan, the second-largest moon in the Solar System (after Jupiter's Ganymede), with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes, [5] Enceladus, which emits jets of gas and dust from its south-polar region, [6] and Iapetus, with its contrasting black and white hemispheres.

Contents

Twenty-four of Saturn's moons are regular satellites; they have prograde orbits not greatly inclined to Saturn's equatorial plane. [7] 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 fifty-nine, 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, with two exceptions. One of these is Phoebe (part of the Norse group but named for a Greek Titaness), the ninth moon of Saturn and largest irregular, discovered at the end of the 19th century; the other is Bebhionn, which, though in the Gallic group, is named after an Irish goddess.

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. [8] 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. [9]

There are still 30 unnamed moons (as of November 2021). If named, they will receive names from Gallic, Norse and Inuit mythology based on the orbital groups of the moons. [10] [11]

Discovery

Saturn (overexposed) and the moons Iapetus, Titan, Dione, Hyperion, and Rhea viewed through a 12.5-inch telescope Iapetus-2010Mar04.jpg
Saturn (overexposed) and the moons Iapetus, Titan, Dione, Hyperion, and Rhea viewed through a 12.5-inch 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 [12] on a refracting telescope of his own design. [13] Tethys, Dione, Rhea and Iapetus (the "Sidera Lodoicea") were discovered between 1671 and 1684 by Giovanni Domenico Cassini. [14] Mimas and Enceladus were discovered in 1789 by William Herschel. [14] Hyperion was discovered in 1848 by W.C. Bond, G.P. Bond [15] and William Lassell. [16]

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. [17] In 1966 the tenth satellite of Saturn was discovered by Audouin Dollfus, when the rings were observed edge-on near an equinox. [18] 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. [18] 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. [19] 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). [19]

Observations by spacecraft

Cassini - four Saturn Moons.jpg
Four moons of Saturn can be seen on this image by the Cassini spacecraft: The larger Titan and Dione at the bottom, small Prometheus (under the rings) and small Telesto above center.
PIA12797-Saturn&FiveMoons-20110729-FullColor-20180730.jpg
Five moons in another Cassini image: Rhea bisected in the far-right foreground, Mimas behind it, bright Enceladus above and beyond the rings, Pandora eclipsed by the F Ring, and Janus off to the left.

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. [19] In addition, Epimetheus was confirmed as distinct from Janus. In 1990, Pan was discovered in archival Voyager images. [19]

The Cassini mission, [20] 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. [21] 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, had been visually confirmed at the time. [22] In 2007 Anthe was announced. [23] 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. [24] In March 2009, Aegaeon, a moonlet within the G Ring, was announced. [25] In July of the same year, S/2009 S 1, the first moonlet within the B Ring, was observed. [4] In April 2014, the possible beginning of a new moon, within the A Ring, was reported. [26] (related image)

Outer moons

Quadruple Saturn-moon transit captured by the Hubble Space Telescope Quadruple Saturn moon transit.jpg
Quadruple Saturn–moon transit captured by the Hubble Space Telescope

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 20th century, Phoebe stood alone among Saturn's known moons with its highly irregular orbit. Then in 2000, three dozen additional irregular moons have been discovered using ground-based telescopes. [27] 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. [28] They are probably fragments of larger bodies captured by Saturn's gravitational pull. [27] [28] In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons, [29] [30] in 2006, astronomers using the Subaru 8.2 m telescope reported the discovery of nine more irregular moons, [31] in April 2007, Tarqeq (S/2007 S 1) was announced and in May of the same year S/2007 S 2 and S/2007 S 3 were reported. [32] 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. [11] [33] Yet another was reported in 2021.

Some of the 83 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. [34] [35] Work has been done to recover many of them in surveys from 2009 onwards, but four – S/2004 S 13, S/2004 S 17, S/2004 S 7, and S/2007 S 3 – still remain lost today. [36]

The number of moons known for each of the four outer planets up to October 2019. Saturn currently has 83 known satellites. Outer planet moons.svg
The number of moons known for each of the four outer planets up to October 2019. Saturn currently has 83 known satellites.

Naming

The modern names for Saturnian moons were suggested by John Herschel in 1847. [14] He proposed to name them after mythological figures associated with the Roman titan of time, Saturn (equated to the Greek Cronus). [14] In particular, the then known seven satellites were named after Titans, Titanesses and Giants—brothers and sisters of Cronus. [17] In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. [16] 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. [37] All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and after Norse ice giants. [38]

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

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 (ellipsoidal) moons constitute roughly 4% of the mass, and the remaining 75 small moons, together with the rings, comprise only 0.04%. [lower-alpha 1]

Saturn's major satellites, compared to the Moon
Name
Diameter
(km) [39]
Mass
(kg) [40]
Orbital radius
(km) [41]
Orbital period
(days) [41]
Mimas 396
(12% Moon)
4×1019
(0.05% Moon)
185,539
(48% Moon)
0.9
(3% Moon)
Enceladus 504
(14% Moon)
1.1×1020
(0.2% Moon)
237,948
(62% Moon)
1.4
(5% Moon)
Tethys 1,062
(30% Moon)
6.2×1020
(0.8% Moon)
294,619
(77% Moon)
1.9
(7% Moon)
Dione 1,123
(32% Moon)
1.1×1021
(1.5% Moon)
377,396
(98% Moon)
2.7
(10% Moon)
Rhea 1,527
(44% Moon)
2.3×1021
(3% Moon)
527,108
(137% Moon)
4.5
(20% Moon)
Titan 5,149
(148% Moon)
(75% Mars)
1.35×1023
(180% Moon)
1,221,870
(318% Moon)
16
(60% Moon)
Iapetus 1,470
(42% Moon)
1.8×1021
(2.5% Moon)
3,560,820
(926% Moon)
79
(290% Moon)

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. [42] The innermost moons and most regular satellites all have mean 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. [33] 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. [38]

Ring moonlets

PIA18352-SaturnRings&Moons-Enceladus&Rhea-20150924.jpg
Saturn's F Ring along with the moons, Enceladus and Rhea
Aegaeon (2008 S1).jpg
Sequence of Cassini images of Aegaeon embedded within the bright arc of Saturn's G Ring

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. [4] 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. [43]

In 2006, four tiny moonlets were found in Cassini images of the A Ring. [44] 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. [44] 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. [45] The moonlets themselves are tiny, ranging from about 40 to 500 meters in diameter, and are too small to be seen directly. [9]

Possible beginning of a new moon of Saturn imaged on 15 April 2014 PIA18078-PossibleBeginning-NewMoonOfPlanetSaturn-20130415.jpg
Possible beginning of a new moon of Saturn imaged on 15 April 2014

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. [9] This region is relatively free from the disturbances caused by resonances with larger satellites, [9] 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. [45] 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. [9] In April 2014, NASA scientists reported the possible consolidation of a new moon within the A Ring, implying that Saturn's present moons may have formed in a similar process in the past when Saturn's ring system was much more massive. [26]

Similar moonlets may reside in the F Ring. [9] 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. [46]

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. [25] 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. [47]

Ring shepherds

Shepherd moon Daphnis in the Keeler gap PIA08319 Daphnis in Keeler Gap.jpg
Shepherd moon Daphnis in the Keeler gap
Shepherd moons Atlas, Daphnis and Pan (enhanced color). They bear distinct equatorial ridges that appear to have formed from material accreted from Saturn's rings. PIA21449-SaturnMoons-Atlas-Daphnis-Pan-20170628color.jpg
Shepherd moons Atlas, Daphnis and Pan (enhanced color). They bear distinct equatorial ridges that appear to have formed from material accreted from Saturn's rings.

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). [21] [25] 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. [42]

Co-orbitals

Janus and Epimetheus are called co-orbital moons. [19] They are of roughly equal size, with Janus being slightly larger than Epimetheus. [42] 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, their gravitational interaction causes them to swap orbits every four years. [48]

Inner large

South pole map of tiger stripes on Enceladus Enceladus south pole SE15.png
South pole map of tiger stripes on Enceladus
Saturn's rings and moons
PIA18357-SaturnMoons-TethysEnceladusMimas-CassiniHuygens-20151203.jpg
Saturn's moons from bottom to top: Mimas, Enceladus, and Tethys
PIA18355-SaturnMoon-Tethys-20151123.jpg
Tethys and the rings of Saturn
Dione and Saturn.jpg
Color view of Dione in front of Saturn

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

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. Anthe and Methone have very faint ring arcs along their orbits, whereas Pallene has a faint complete ring. [55] Of these three moons, only Methone has been photographed at close range, showing it to be egg-shaped with very few or no craters. [56]

Trojan

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 (leading) and Calypso (trailing), and Dione also has two, Helene (leading) and Polydeuces (trailing). [21] Helene is by far the largest trojan moon, [49] while Polydeuces is the smallest and has the most chaotic orbit. [48] These moons are coated with dusty material that has smoothed out their surfaces. [57]

Outer large

Saturn's outer moons
PIA08148 (Rhea-Splat).jpg
Inktomi or "The Splat", a relatively young crater with prominent butterfly-shaped ejecta on Rhea's leading hemisphere
Ringside with Titan and Dione.jpg
Titan in front of Dione and the rings of Saturn
PIA17193-SaturnMoon-Hyperion-20150531.jpg
Cassini image of Hyperion
Iapetus equatorial ridge.jpg
Equatorial ridge on Iapetus

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

Irregular

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. Saturn's irregular satellites.png
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.
Orbits and positions of Saturn's irregular moons as of 1 January 2021. Prograde orbits are colored blue while retrograde orbits are colored red. Saturn irregular moon orbits Jan 2021 cropped.png
Orbits and positions of Saturn's irregular moons as of 1 January 2021. Prograde orbits are colored blue while retrograde orbits are colored red.

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. [27] 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. [28] The irregulars generally have featureless visible and near infrared spectra dominated by water absorption bands. [27] They are neutral or moderately red in color—similar to C-type, P-type, or D-type asteroids, [38] though they are much less red than Kuiper belt objects. [27] [lower-alpha 3]

Inuit

The Inuit group includes eight 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. [28] [38] The moons are Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq, [38] along with three unnamed moons Saturn LX, S/2004 S 31, and S/2019 S 1. The largest among them is Siarnaq with an estimated size of about 40 km.

Gallic

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. [28] [38] They are Albiorix, Bebhionn, Erriapus, and Tarvos. [38] 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

The Norse (or Phoebe) group consists of 46 retrograde outer moons. [28] [38] They are Aegir, Bergelmir, Bestla, Farbauti, Fenrir, Fornjot, Greip, Hati, Hyrrokkin, Jarnsaxa, Kari, Loge, Mundilfari, Narvi, Phoebe, Skathi, Skoll, Surtur, Suttungr, Thrymr, Ymir, [38] 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. [38]

  • Phoebe, at 213±1.4 km in diameter, is by far the largest of Saturn's irregular satellites. [27] It has a retrograde orbit and rotates on its axis every 9.3 hours. [79] 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. [27] 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. [27] Phoebe is believed to be a captured centaur that originated in the Kuiper belt. [27] It also serves as a source of material for the largest known ring of Saturn, which darkens the leading hemisphere of Iapetus (see above). [76]

List

Orbital diagram of the orbital inclination and orbital distances for Saturn's rings and moon system at various scales. Notable moons, moon groups, and rings are individually labeled. Open the image for full resolution. Saturnmoonsdiagram.png
Orbital diagram of the orbital inclination and orbital distances for Saturn's rings and moon system at various scales. Notable moons, moon groups, and rings are individually labeled. Open the image for full resolution.

Confirmed

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 and marked with a blue background, while the irregular moons are listed in red, orange and gray background. The orbits and mean distances of the irregular moons are strongly variable over short timescales due to frequent planetary and solar perturbations; [80] therefore, the orbit epochs of all irregular moons are based on the same Julian date of 2459200.5, or 17 December 2020. [81]

Key
 
Small inner moons

Titan

Other round moons

Inuit group

Gallic group

Norse group
Order
[lower-alpha 4]
Label
[lower-alpha 5]
NamePronunciationImage Abs.
magn.
Diameter
(km) [lower-alpha 6]
Mass
(×1015 kg) [lower-alpha 7]
Semi-major
axis

(km) [lower-alpha 8]
Orbital period (d) [lower-alpha 8] [lower-alpha 9] Inclination
(°)
[lower-alpha 8] [lower-alpha 10]
Eccentricity Position Discovery
year
[37]
Discoverer [37]
1 S/2009 S 1
PIA11665 moonlet in B Ring cropped.jpg
20?0.3<0.00011170000.470000.0000.0000outer B Ring2009 Cassini [4]
(moonlets)
First moonlets PIA07792 (closeup).jpg
0.04 to 0.4<0.00011300000.550000.0000.0000Three 1000 km bands within A Ring2006 Cassini
2XVIII Pan /ˈpæn/
Pan by Cassini, March 2017.jpg
9.128.2
(34×31×20)
4.95133584+0.575050.0000.0000in Encke Division1990 Showalter
3XXXV Daphnis /ˈdæfnəs/
Daphnis (Saturn's Moon).jpg
12.07.6
(8.6×8.2×6.4)
0.084136505+0.594080.0040.0000in Keeler Gap2005 Cassini
4XV Atlas /ˈætləs/
Atlas 2017-04-12 raw preview.jpg
10.730.2
(41×35×19)
6.6137670+0.601690.0030.0012outer A Ring shepherd1980 Voyager 1
5XVI Prometheus /prˈmθiəs/
Prometheus 12-26-09a.jpg
6.586.2
(136×79×59)
159.5139380+0.612990.0080.0022inner F Ring shepherd1980 Voyager 1
6XVII Pandora /pænˈdɔːrə/
Pandora PIA07632.jpg
6.681.4
(104×81×64)
137.1141720+0.628500.0500.0042outer F Ring shepherd1980 Voyager 1
7aXI Epimetheus /ɛpəˈmθiəs/
PIA09813 Epimetheus S. polar region.jpg
5.6116.2
(130×114×106)
526.6151422+0.694330.3350.0098co-orbital with Janus1977Fountain & Larson
7bX Janus /ˈnəs/ PIA12714 Janus crop.jpg 4.7179.0
(203×185×153)
1897.5151472+0.694660.1650.0068co-orbital with Epimetheus1966 Dollfus
9LIII Aegaeon /ˈɒn/
N1643264379 1.jpg
18.70.66
(1.4×0.5×0.4)
0.0001167500+0.808120.0010.0004G Ring moonlet2008 Cassini
10I Mimas /ˈmməs/
Mimas Cassini.jpg
2.7396.4
(416×393×381)
37493185404+0.942421.5660.0202 1789 Herschel
11XXXII Methone /məˈθn/
Methone PIA14633.jpg
13.82.90.02194440+1.009570.0070.0001 Alkyonides 2004 Cassini
12XLIX Anthe /ˈænθ/
Anthe crop.jpg
14.81.80.0015197700+1.050890.1000.0011Alkyonides2007 Cassini
13XXXIII Pallene /pəˈln/
Pallene N1665945513 1.jpg
12.94.44
(5.8×4.2×3.7)
0.05212280+1.153750.1810.0040Alkyonides2004 Cassini
14II Enceladus /ɛnˈsɛlədəs/
Enceladus - July 15 2005 (40942341431).jpg
1.8504.2
(513×503×497)
108022237950+1.370220.0100.0047Generates the E ring1789 Herschel
15III Tethys /ˈtθəs/
PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg
0.31062.2
(1077×1057×1053)
617449294619+1.887800.1680.0001 1684 Cassini
15aXIII Telesto /təˈlɛst/
Telesto cassini closeup.jpg
8.724.8
(33×24×20)
9.41294619+1.887801.1580.0000leading Tethys trojan (L4)1980 Smith et al.
15bXIV Calypso /kəˈlɪps/
N00151485 Calypso crop.jpg
8.721.4
(30×23×14)
6.3294619+1.887801.4730.0000trailing Tethys trojan (L5)1980Pascu et al.
18IV Dione /dˈn/
Dione in natural light.jpg
0.41122.8
(1128×1123×1119)
1095452377396+2.736920.0020.0022 1684 Cassini
18aXII Helene /ˈhɛlən/
PIA12758 Helene crop.jpg
7.335.2
(43×38×26)
24.5377396+2.736920.1990.0022leading Dione trojan (L4)1980Laques & Lecacheux
18bXXXIV Polydeuces /pɒliˈdjsz/
Polydeuces.jpg
13.52.6
(3×2×1)
0.03377396+2.736920.1770.0192trailing Dione trojan (L5)2004 Cassini
21V Rhea /ˈrə/
PIA07763 Rhea full globe5.jpg
−0.21527.6
(1530×1526×1525)
2306518527108+4.518210.3270.0013 1672 Cassini
22VI Titan /ˈttən/
Titan globe.jpg
−1.35149.46
(5149×5149×5150)
1345200001221930+15.94540.3490.0288 1655 Huygens
23VII Hyperion /hˈpɪəriən/
Hyperion true.jpg
4.8270.0
(360×266×205)
5619.91481010+21.27660.5680.1230in 4:3 resonance with Titan1848 Bond & Lassell
24VIII Iapetus /ˈæpətəs/
Iapetus 706 1419 1.jpg
1.71468.6
(1491×1491×1424)
18056353560820+79.321515.4700.0286 1671 Cassini
25 S/2019 S 1 15.360.1511221100+443.7844.3790.6229 Inuit group 2021 (2019)Gladman et al.
26XXIVKiviuq /ˈkɪviək/
Kiviuq-CFHT.gif
12.7172.7911307500+448.9148.9300.1521 Inuit group 2000Gladman et al.
27XXIIIjiraq /ˈɪrɒk/
Ijiraq-discovery-CFHT.gif
13.2131.1811348500+451.3649.5100.3758Inuit group2000Gladman et al.
28IXPhoebe /ˈfbi/
Phoebe cassini full.jpg
6.6213.0
(219×217×204)
8292.012905900−547.39172.9980.1604 Norse group 1899 Pickering
29XXPaaliaq /ˈpɑːliɒk/
Paaliaq-CFHT.gif
11.9257.2515012800+686.7543.6880.4826Inuit group2000Gladman et al.
30XXVIISkathi /ˈskɑːði/
Skathi-discovery-CFHT.gif
14.380.3515563600−724.90149.7490.2755Norse (Skathi) Group2000Gladman et al.
31 S/2004 S 37 15.940.0515822400−743.06163.0460.5265Norse group2019 (2004)Sheppard et al.
32 S/2007 S 2 15.760.1515971500−753.58176.6760.2465Norse group2007Sheppard et al.
33XXVIAlbiorix /ˌælbiˈɒrɪks/
Albiorix WISE-W4.jpg
11.128.622.316222700+771.4334.5590.5807 Gallic group 2000 Holman
34XXXVIIBebhionn /ˈbvɪn/
Bebhionn-cassini.png
15.060.1516900900+820.3140.1240.3813Gallic group2004Sheppard et al.
35 LX S/2004 S 29 15.840.0517202800+842.3946.2000.4269Inuit group2019 (2004)Sheppard et al.
36XLVIISkoll /ˈskɒl/ 15.450.1517438300−859.75157.1430.4402Norse group2006Sheppard et al.
37 S/2004 S 31 15.640.0517449700+860.5847.7820.2525Inuit group2019 (2004)Sheppard et al.
38XXVIIIErriapus /ɛriˈæpəs/
Erriapus-discovery-CFHT.gif
13.7100.6817705500+879.5841.7200.4520Gallic group2000Gladman et al.
39LIITarqeq /ˈtɑːrkk/
Tarqeq-cassini.png
14.870.2317724200+880.9750.1590.1373Inuit group2007Sheppard et al.
40XXIXSiarnaq /ˈsɑːrnək/
Siarnaq-discovery-CFHT.gif
10.639.343.517937000+883.8743.7990.5293Inuit group2000Gladman et al.
41XXITarvos /ˈtɑːrvəs/
Tarvos discovery.gif
12.8152.318243800+920.0037.8180.4799Gallic group2000Gladman et al.
42XLIVHyrrokkin /hɪˈrɒkən/
Hyrrokkin-cassini.png
14.380.3518348800−927.95153.5390.3582Norse group2004Sheppard et al.
43LIGreip /ˈɡrp/
Greip-cassini.png
15.450.1518379000−930.24172.8800.3331Norse group2006Sheppard et al.
44XXVMundilfari /mʊndəlˈværi/
Mundilfari-discovery-CFHT.gif
14.570.2318470800−937.22169.6630.1787Norse group2000Gladman et al.
45(lost) S/2004 S 13 15.660.1518594700
(18511200±782400) [83]
−946.67
(−940.39) [83]
167.041
(167.438±0.070) [83]
0.2900
(0.2774±0.0305) [83]
Norse group2004Sheppard et al.
46 S/2006 S 1 15.650.1518839700−965.44154.9040.0972Norse group2006Sheppard et al.
47(lost) S/2007 S 3 15.750.0919143500
(20432100±290200) [84]
−988.89
(−1092.10) [84]
177.065
(177.209±0.110) [84]
0.1671
(0.1287±0.0150) [84]
Norse group2007Sheppard et al.
48XXIIISuttungr /ˈsʊtʊŋɡər/
Suttungr-discovery-CFHT.gif
14.570.2319166800−990.69174.0910.1445Norse group2000Gladman et al.
49 LIV S/2004 S 20 15.840.0519188100−992.34162.7820.1976Norse group2019 (2004)Sheppard et al.
50LJarnsaxa /jɑːrnˈsæksə/ 15.660.1519197900−993.11163.7800.2148Norse group2006Sheppard et al.
51XXXINarvi /ˈnɑːrvi/
Narvi.jpg
14.470.2319226600−995.33136.8030.2990Norse group2003Sheppard et al.
52XXXVIIIBergelmir /bɛərˈjɛlmɪər/
Bergelmir.png
15.250.1519290200−1000.28156.9190.1399Norse group2004Sheppard et al.
53XLIIIHati /ˈhɑːti/
Hati-cassini.png
15.350.1519435300−1011.59164.0000.3295Norse group2004Sheppard et al.
54(lost) S/2004 S 17 16.040.0519574300
(19079700±679200) [85]
−1022.45
(−984.05) [85]
167.936
(166.870±0.350) [85]
0.1916
(0.2268±0.0438) [85]
Norse group2004Sheppard et al.
55 S/2004 S 12 15.750.0919736400
(19999800±121000) [86]
−1035.18
(−1055.57) [86]
163.797
(164.073±0.130) [86]
0.3929
(0.3933±0.0222) [86]
Norse group2004Sheppard et al.
56 LIX S/2004 S 27 15.360.1519982800−1054.63167.7070.1364Norse group2019 (2004)Sheppard et al.
57XLFarbauti /fɑːrˈbti/ 15.750.0920101600−1064.04157.4840.1756Norse group2004Sheppard et al.
58XXXThrymr /ˈθrɪmər/
Thrymr-discovery-CFHT.gif
14.380.2320236700−1074.79174.1770.4332Norse group2000Gladman et al.
59XXXIXBestla /ˈbɛstlə/
Bestla-cassini.png
14.670.2320432100−1090.39143.9550.7072Norse (Skathi) group2004Sheppard et al.
60(lost) S/2004 S 7 15.260.1520576700
(20680600±371000) [87]
−1091.09
(−1110.36) [87]
164.331
(165.614±0.140) [87]
0.4998
(0.5552±0.0195) [87]
Norse group2004Sheppard et al.
61XXXVIAegir /ˈ.ɪər/ 15.560.1520598900−1103.78166.6710.2379Norse group2004Sheppard et al.
62 LXI S/2004 S 30 16.130.0320733300−1114.59157.1210.0859Norse group2019 (2004)Sheppard et al.
63 LV S/2004 S 22 16.130.0320737100−1114.90177.2850.2369Norse group2019 (2004)Sheppard et al.
64 LVII S/2004 S 25 15.940.0520814800−1121.17172.9380.4362Norse group2019 (2004)Sheppard et al.
65 LXII S/2004 S 32 15.640.0520963400−1133.20159.2610.2594Norse group2019 (2004)Sheppard et al.
66 LVI S/2004 S 23 15.640.0521444300−1172.42176.8540.4133Norse group2019 (2004)Sheppard et al.
67 S/2006 S 3 15.660.1521607300−1185.81152.9580.4533Norse group2006Sheppard et al.
68 LXV S/2004 S 35 15.560.1521864500−1207.04176.7710.2030Norse group2019 (2004)Sheppard et al.
69XLVKari /ˈkɑːri/
Kari-cassini.png
14.860.2321988000−1217.28149.1880.3745Norse (Skathi) group2006Sheppard et al.
70 S/2004 S 28 15.840.0522134400−1229.46170.6170.1249Norse group2019 (2004)Sheppard et al.
71XLVILoge /ˈlɔɪ./
Loge N00177425.jpg
15.350.1522563800−1265.42166.8110.1584Norse group2006Sheppard et al.
72 LXVI S/2004 S 38 15.940.0522616000−1269.81154.6540.4084Norse group2019 (2004)Sheppard et al.
73XLIFenrir /ˈfɛnrɪər/ 15.940.0522753400−1281.39162.8740.0949Norse group2004Sheppard et al.
74XIXYmir /ˈmɪər/
Ymir-CFHT.gif
12.3193.9722841900−1288.88172.1570.3431Norse group2000Gladman et al.
75XLVIIISurtur /ˈsɜːrtər/ 15.860.1523065900−1307.89166.0080.3591Norse group2006Sheppard et al.
76 LXIII S/2004 S 33 15.940.0523087600−1309.73159.8660.4113Norse group2019 (2004)Sheppard et al.
77 S/2004 S 24 16.030.0323326300+1330.1036.2400.0400Gallic group? [lower-alpha 11] 2019 (2004)Sheppard et al.
78 S/2004 S 21 16.330.0323356200−1332.66156.2000.3156Norse group2019 (2004)Sheppard et al.
79 S/2004 S 39 16.330.0323463800−1341.87166.5780.0979Norse group2019 (2004)Sheppard et al.
80 S/2004 S 36 16.130.0323576500−1351.56155.1880.7139Norse group [lower-alpha 12] 2019 (2004)Sheppard et al.
81XLIIFornjot /ˈfɔːrnjɒt/
Fornjot-cassini.png
14.960.1524451700−1427.51167.8470.1613Norse group2004Sheppard et al.
82 LXIV S/2004 S 34 16.130.0324865000−1463.85166.0390.2015Norse group2019 (2004)Sheppard et al.
83 LVIII S/2004 S 26 15.840.0526701600−1628.99171.5750.1726Norse group2019 (2004)Sheppard et al.

Unconfirmed

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. [21]

NameImageDiameter (km)Semi-major
axis (km) [48]
Orbital
period (d) [48]
PositionDiscovery yearStatus
S/2004 S 3 and S 4 [lower-alpha 13] S2004 S 3 - PIA06115.png 3–5140300+0.619uncertain objects around the F Ring2004Were undetected in thorough imaging of the region in November 2004, making their existence improbable
S/2004 S 6 S2004 S6.jpg 3–5140130+0.618012004Consistently detected into 2005, may be surrounded by fine dust and have a very small physical core

Hypothetical

Two moons were claimed to be discovered by different astronomers but never seen again. Both moons were said to orbit between Titan and Hyperion. [88]

Temporary

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 (Leonard) is calculated to have made a close approach of 978000±65000 km (608000±40000 mi to Saturn on 8 May 1936, closer than the orbit of Titan to the planet, with an orbital eccentricity of only 1.098±0.007. 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. [89]

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, [90] or that two large moons fused to form Titan, with the collision scattering icy debris that formed the mid-sized moons. [91] 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. [66] 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. [92]

See also

Notes

  1. The mass of the rings is about the mass of Mimas, [8] whereas 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. Inktomi was once known as "The Splat". [62]
  3. The photometric color may be used as a proxy for the chemical composition of satellites' surfaces.
  4. Order refers to the position among other moons with respect to their average distance from Saturn.
  5. A confirmed moon is given a permanent designation by the IAU consisting of a name and a Roman numeral. [37] The eight moons that were known before 1850 are numbered in order of their distance from Saturn; the rest are numbered in the order by which they received their permanent designations. Many small moons have not yet received a permanent designation.
  6. The diameters and dimensions of the inner moons from Pan through Janus, Methone, Pallene, Telepso, Calypso, Helene, Hyperion and Phoebe were taken from Thomas 2010, Table 3. [39] Diameters and dimensions of Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus are from Thomas 2010, Table 1. [39] The approximate sizes of other satellites are from the website of Scott Sheppard. [33]
  7. Masses of the large moons were taken from Jacobson, 2006. [40] Masses of Pan, Daphnis, Atlas, Prometheus, Pandora, Epimetheus, Janus, Hyperion and Phoebe were taken from Thomas, 2010, Table 3. [39] Masses of other small moons were calculated assuming a density of 1.3 g/cm3.
  8. 1 2 3 The orbital parameters were taken from Spitale, et al. 2006, [48] IAU-MPC Natural Satellites Ephemeris Service, [82] and NASA/NSSDC. [41]
  9. Negative orbital periods indicate a retrograde orbit around Saturn (opposite to the planet's rotation).
  10. To Saturn's equator for the regular satellites, and to the ecliptic for the irregular satellites
  11. Only known prograde outer satellite, inclination similar to other satellites of the Gallic group
  12. Probably a captured asteroid due to its unusually high eccentricity, though orbit is similar to the Norse group
  13. S/2004 S 4 was most likely a transient clump—it has not been recovered since the first sighting. [21]

Related Research Articles

Saturn Sixth planet from the Sun and second largest planet in the Solar System

Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; however, with its larger volume, Saturn is over 95 times more massive. Saturn is named after the Roman god of wealth and agriculture. Its astronomical symbol (♄) has been traced back to the Greek Oxyrhynchus Papyri, where it can be seen to be a Greek kappa-rho with a cross-bar, as an abbreviation for Κρονος (Cronos), the Greek name for the planet. It later came to look like a lower-case Greek eta, with the cross added at the top in the 16th century.

Natural satellite Astronomical body that orbits a planet

A natural satellite is in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small solar system body. While natural satellites are often colloquially referred to as moons, there is only the Moon of Earth.

Rhea (moon) Moon of Saturn

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. It was discovered in 1672 by Giovanni Domenico Cassini.

<i>Cassini–Huygens</i> Space research mission sent to the Saturnian system

The Cassini–Huygens space-research mission, commonly called Cassini, involved a collaboration among NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its rings and natural satellites. The Flagship-class robotic spacecraft comprised both NASA's Cassini space probe and ESA's Huygens lander, which landed on Saturn's largest moon, Titan. Cassini was the fourth space probe to visit Saturn and the first to enter its orbit. The two craft took their names from the astronomers Giovanni Cassini and Christiaan Huygens.

Tethys (moon) Moon of Saturn

Tethys, or Saturn III, is a mid-sized moon of Saturn about 1,060 km (660 mi) across. It was discovered by G. D. Cassini in 1684 and is named after the titan Tethys of Greek mythology.

Iapetus (moon) Moon of Saturn

Iapetus is the third-largest natural satellite of Saturn and the eleventh-largest in the Solar System. Discoveries by the Cassini mission in 2007 revealed several unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon and a distinctive color pattern.

Hyperion (moon) Moon of Saturn

Hyperion, also known as Saturn VII (7), is a moon of Saturn discovered by William Cranch Bond, his son George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance. It was the first non-round moon to be discovered.

Calypso (moon) Moon of Saturn

Calypso is a moon of Saturn. It was discovered in 1980, from ground-based observations, by Dan Pascu, P. Kenneth Seidelmann, William A. Baum, and Douglas G. Currie, and was provisionally designated S/1980 S 25. Several other apparitions of it were recorded in the following months: S/1980 S 29, S/1980 S 30, S/1980 S 32, and S/1981 S 2. In 1983 it was officially named after Calypso of Greek mythology. It is also designated Saturn XIV or Tethys C.

Dione (moon) Moon of Saturn

Dione is a moon of Saturn. It was discovered by Italian astronomer Giovanni Domenico Cassini in 1684. It is named after the Titaness Dione of Greek mythology. It is also designated Saturn IV.

Enceladus Natural satellite (moon) orbiting Saturn

Enceladus is the sixth-largest moon of Saturn. It is about 500 kilometers (310 mi) in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon only reaches −198 °C (−324 °F), far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains.

Mimas (moon) Moon of Saturn

Mimas, also designated Saturn I, is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology.

Phoebe (moon) Moon of Saturn

Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km (132 mi). It was discovered by William Henry Pickering on March 18, 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru, by DeLisle Stewart. It was the first satellite to be discovered photographically.

Methone (moon) Moon of Saturn

Methone is a small, egg-shaped moon of Saturn that orbits out past Saturn's ring system, between the orbits of Mimas and Enceladus. It was discovered in 2004, though it wasn't until 2012 that it was imaged in detail by the Cassini spacecraft.

Rings of Saturn Planar assemblage of icy particles orbiting Saturn

The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history, newer data from Cassini suggested they formed relatively late.

Polydeuces (moon) Moon of Saturn

Polydeuces, or Saturn XXXIV, is a small natural satellite of Saturn that is co-orbital with the moon Dione and librates around its trailing Lagrangian point (L5). Its diameter is estimated to be 2–3 km. Dione's other co-orbital moon is Helene, which is bigger and located at the leading L4 point.

Daphnis (moon) Moon of Saturn

Daphnis is an inner satellite of Saturn. It is also known as Saturn XXXV; its provisional designation was S/2005 S 1. Daphnis is about 8 kilometres in diameter, and orbits the planet in the Keeler Gap within the A ring.

Tiger stripes (Enceladus)

The tiger stripes of Enceladus consist of four sub-parallel, linear depressions in the south polar region of the Saturnian moon. First observed on May 20, 2005 by the Cassini spacecraft's Imaging Science Sub-system (ISS) camera, the features are most notable in lower resolution images by their brightness contrast from the surrounding terrain. Higher resolution observations were obtained by Cassini's various instruments during a close flyby of Enceladus on July 14, 2005. These observations revealed the tiger stripes to be low ridges with a central fracture. Observations from the Composite Infrared Spectrometer (CIRS) instrument showed the tiger stripes to have elevated surface temperatures, indicative of present-day cryovolcanism on Enceladus centered on the tiger stripes.

Saturns moons in fiction

Several of Saturn's natural satellites have figured prominently in works of science fiction.

Planetary-mass moon Moons comparable in size to small planets

A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Two moons in the Solar System are larger than the planet Mercury : Ganymede and Titan, and seven are larger and more massive than the dwarf planet Pluto.

The following outline is provided as an overview of and topical guide to Saturn:

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