Saturn's natural satellites

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The Saturnian system (photographic montage)
The Saturnian system (photographic montage)

Saturn has fifty-six confirmed natural satellites, plus three unconfirmed moons.

Contents

[edit] Introduction

Saturn's rings cut across an eerie scene that is ruled by Titan's luminous crescent and globe-encircling haze, broken by the small moon Enceladus, whose icy jets are dimly visible at its south pole. North is up.
Saturn's rings cut across an eerie scene that is ruled by Titan's luminous crescent and globe-encircling haze, broken by the small moon Enceladus, whose icy jets are dimly visible at its south pole. North is up.

Saturn is currently known to have fifty-six moons, many of which were discovered very recently, and three additional un-confirmed, hypothetical moons. However, a precise number of moons can never be given, as there is no objective dividing line between the anonymous orbiting fragments that form Saturn's ring system and the larger objects that have already been named as moons.

Before the advent of telescopic photography, eight moons of Saturn were discovered by direct observation using an optical telescope:

The use of long-exposure photographic plates made it possible to discover additional moons:

  • Phoebe was the first satellite discovered by telescopic photograph in 1899 by W.H. Pickering.
  • In 1966, the satellites Janus and Epimetheus were observed, but not confirmed, and it was not realized that there were two distinct moons sharing an orbit.

The study of the outer planets has since been revolutionized, first by the use of unmanned space probes, and then by advances in telescopy:

  • From 1980, when the first of the Voyager space probes arrived at Saturn, to 1990, analysis of Voyager images revealed eight more moons in the inner Saturnian system. The last discovered was Pan.
  • A survey starting in late 2000 found thirteen new moons orbiting Saturn at a great distance in orbits that suggest they are fragments of larger bodies captured by Saturn's gravitational pull (Nature vol. 412, pp. 163–166).
  • The Cassini mission, which arrived at Saturn in the summer of 2004, discovered three small moons in the inner Saturnian system as well as three suspected but unconfirmed moons in the F Ring. This increased the total to thirty-seven moons, confirmed and unconfirmed.
  • 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, but only one, Daphnis, has been visually confirmed so far (its confirmation was announced on May 6, 2005).[1].
  • On May 3, 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons [2] [3].
  • On June 30, 2006, astronomers using the Subaru 8.2 m telescope announced the discovery of nine more small outer moons [4].

The spurious satellite Chiron, "discovered" in 1861, is now known not to exist. Themis, "discovered" in 1905, also was later proven not to exist.

[edit] Table of known moons

The Saturnian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in light purple. Titan, which is planetary in size, has darker highlighting. The irregular (captured) moons are indicated in grey: light grey for prograde satellites, darker grey for retrograde satellites.

Order Name (spheroidal moons in bold)

(Pronunciation key)

 Image Diameter (km) Semi-major
axis (km)		 Orbital
period (d)		 Inclination (°)
(to Saturn's
equator)		 Position Discovered
1 XVIII Pan IPA: [ˈpʰæn]
30 (35 × 35 × 23) [1] 133,584 [2] +0.57505 [2] 0.001° in Encke Division 1990
2 XXXV Daphnis IPA: [ˈdæf.nɪs]
6 − 8 136,505 [2] +0.59408 [2] ≈ 0° in Keeler Gap 2005
3 XV Atlas IPA: [ˈæt.ləs]
31 (46 × 38 × 19) [1] 137,670 [2] +0.60169 [2] 0.003° outer A Ring shepherd 1980
4 XVI Prometheus IPA: [pʰɹoʊˈmi.θi.əs]
86 (119 × 87 × 61) [1] 139,380 [2] +0.61299 [2] 0.008° inner F Ring shepherd 1980
*   S/2004 S 6 [3]  
~3−5 140,130 [2] +0.61801 [2] *Uncertain objects
around the F-ring		 2004
*   S/2004 S 4 [4]   ~3−5 ~140,100 [5] +0.619 [6] 2004
*   S/2004 S 3 [3]  
~3−5 ~140,300 [7] ~ +0.62 2004
5 XVII Pandora IPA: [pʰænˈdo.ɹə]
81 (103 × 80 × 64) [1] 141,720 [2] +0.62850 [2] 0.050° outer F Ring Shepherd 1980
6 XI Epimetheus IPA: [ˌɛ.pəˈmi.θi.əs] 113 (135 × 108 × 105) [1] 151,422 [2] +0.69433 [2] 0.335° co-orbitals 1980
7 X Janus IPA: [ˈdʒeɪ.nəs]
179 (193 × 173 × 137) [1] 151,472 [2] +0.69466 [2] 0.165° 1966
8 I Mimas IPA: [ˈmaɪ.məs]
397 (415 × 394 × 381) [8] 185,404 [9] +0.942422 [10] 1.566°   1789
9 XXXII Methone IPA: [mɪˈθoʊ.ni]
3 194,440 [2] +1.00957 [2] 0.007°   2004
10 XXXIII Pallene IPA: [pʰəˈli.ni]
4 212,280 [2] +1.15375 [2] 0.181°   2004
11 II Enceladus IPA: [ɛnˈsɛ.lə.dəs]
504 (513 × 503 × 497) [8] 237,950 [9] +1.370218 [10] 0.010° In the thick of E ring 1789
12 III Tethys IPA: [ˈtʰi.θɪs]
1066 (1081 × 1062 × 1055) [8] 294,619 [9] +1.887802 [10] 0.168°   1684
12a XIII Telesto IPA: [tʰɪˈlɛ.stoʊ]
24 (29 × 22 × 20) [1] 1.158° leading Tethys trojan 1980
12b XIV Calypso IPA: [kʰəˈlɪp.soʊ]
21 (30 × 23 × 14) [1] 1.473° trailing Tethys trojan 1980
15 IV Dione IPA: [daɪˈoʊ.ni]
1123 (1128 × 1122 × 1121) [8] 377,396 [9] +2.736915 [10] 0.002°   1684
15a XII Helene IPA: [ˈhɛ.lə.ni]
33 (36 × 32 × 30) 0.212° leading Dione trojan 1980
15b XXXIV Polydeuces IPA: [ˌpʰɑ.ləˈdju.siz]
3.5 [11] 0.177° trailing Dione trojan 2004
18 V Rhea IPA: [ˈɹi.ə]
1529 (1535 × 1525 × 1526) [8] 527,108 [12] +4.518212 [12] 0.327°   1672
19 VI Titan IPA: [ˈtʰaɪ.tən]
5151 1,221,930 [9] +15.94542 1.634°   1655
20 VII Hyperion IPA: [haɪˈpʰɪ.ɹi.ən]
292 (360 × 280 × 225) 1,481,010 [9] +21.27661 0.568°   1848
21 VIII Iapetus IPA: [aɪˈæ.pə.təs]
1472 (1494 × 1498 × 1425) [8] 3,560,820 +79.3215 [6] 7.570°   1671
22 XXIV Kiviuq IPA: [ˈkʰi.vi.uk] ~16 11 294 800 [12] +448.16 [12] 49.087° Inuit group 2000
23 XXII Ijiraq IPA: [ˈi.ji.ɹɑk] ~12 11 355 316 [12] +451.77 [12] 50.212° 2000
24 IX Phoebe IPA: [ˈfi.bi]
220 (230 × 220 × 210) 12 869 700 -545.09[6][13] 173.047° Norse group 1899
25 XX Paaliaq IPA: [ˈpʰɔ.li.ɑk] ~22 15 103 400 [12] +692.98 [12] 46.151° Inuit group 2000
26 XXVII Skathi IPA: [ˈskɑ.ði] ~8 15 672 500 [12] -732.52 [10][13] 149.084° Norse (Skathi) Group 2000
27 XXVI Albiorix IPA: [ælˈbaɪ.ə.ɹɪks] ~32 16 266 700 [12] +774.58 [12] 38.042° Gallic group 2000
28 XXXVII Bebhionn - ~6 17 153 520 [12] +838.77 [12] 40.484° Inuit group 2004
29 XXVIII Erriapo IPA: [ˌɛ.ɹiˈæ.poʊ] ~10 17 236 900 [12] +844.89 [12] 38.109° Gallic group 2000
30 XLVII Skoll - ~6 17 473 800 [9] -862.37 [12] 155.624° Norse group 2006
31 XXIX Siarnaq IPA: [ˈsi.ɑɹ.nɑk] ~40 17 776 600 [12] +884.88 [12] 45.798° Inuit group 2000
32   S/2004 S 13 - ~6 18 056 300 [12] -905.85 [10][13] 167.379° Norse group 2004
33   S/2006 S 4 - ~6 18 065 700 [9] -906.56 [12] 172.666° 2006
34 XLIV Hyrokkin - ~8 18 168 300 [9] -914.29 [12] 153.272° 2006
35   S/2006 S 6 - ~6 18 556 900 [9] -943.78 [12] 162.861° 2006
36 XXI Tarvos IPA: [ˈtʰɑɹ.vɑs] ~15 18 562 800 [12] +944.23 [12] 34.679° Gallic group 2000
37 XXV Mundilfari IPA: [ˈmʊn.dɪlˌfɑ.ɹi] ~7 18 725 800 [12] -956.70 [10][13] 169.378° Norse group 2000
38   S/2006 S 1 - ~6 18 930 200 [9] -972.41 [12] 154.232° 2006
39   S/2004 S 17 - ~4 19 099 200 [12] -985.45 [10][13] 166.881° 2004
40 XXXVIII Bergelmir - ~6 19 104 000 [12] -985.83 [10][13] 157.384° Norse (Skathi) group 2004
41 XXXI Narvi IPA: [ˈnɑɹ.vi] ~7 19 395 200 [12] -1008.45 [10][13] 137.292° Norse group 2003
42 XXIII Suttungr IPA: [ˈsʊ.tʊŋ.gɚ] ~7 19 579 000 [12] -1022.82 [10][13] 174.321° 2000
43 XLIII Hati - ~6 19 709 300 [12] -1033.05 [10][13] 163.131° 2004
44   S/2004 S 12 - ~5 19 905 900 [12] -1048.54 [10][13] 164.042° 2004
45 XL Farbauti - ~5 19 984 800 [12] -1054.78 [10][13] 158.361° Norse (Skathi) group 2004
46 XXX Thrymr IPA: [ˈθrɪ.mɚ] ~7 20 278 100 [12] -1078.09 [10][13] 174.524° Norse group 2000
47 XXXVI Aegir - ~6 20 482 900 [12] -1094.46 [10][13] 167.425° 2004
48 XXXIX Bestla - ~7 20 570 000 [12] -1101.45 [10][13] 147.395° Norse (Skathi) group 2004
49   S/2004 S 7 - ~6 20 576 700 [12] -1101.99 [10][13] 165.596° Norse group 2004
50   S/2006 S 3 - ~6 21 076 300 [9] -1142.37 [12] 150.817° 2006
51 XLI Fenrir - ~4 21 930 644 [12] -1212.53 [10][13] 162.832° 2004
52 XLVIII Surtur - ~6 22 288 916 [9] -1242.36 [12] 166.918° 2006
53 XLV Kari - ~7 22 321 200 [9] -1245.06 [12] 148.384° 2006
54 XIX Ymir IPA: [ˈi.mɪɹ] ~18 22 429 673 [12] -1254.15 [10][13] 172.143° 2000
55 XLVI Loge - ~6 22 984 322 [9] -1300.95 [12] 166.539° 2006
56 XLII Fornjot - ~6 24 504 879 [12] -1432.16 [10][13] 167.886° 2004
  1. ^ a b c d e f g h C.C. Porco et al. (2006). "Physical characteristics and possible accretionary origins for Saturn's small satellites". Bulletin of the American Astronomical Society 37: 768. 
  2. ^ a b c d e f g h i j k l m n o p q r s t J.N. Spitale et al (2006). "The orbits of Saturn's small satellites derived from combined historic and Cassini imaging observations". The Astronomical Journal 132: 692. 
  3. ^ a b It is not yet clear if these are real satellites or merely persistent clumps within the F Ring
  4. ^ S/2004 S4 was most likely a transient clump − it has not been recovered since the first sighting.
  5. ^ PGJ Astronomie webpage (Gilbert Javaux). Note that the F ring is centered at ~140,180 km
  6. ^ a b c Computed from the semi-major axis using the IAU-MPC Natural Satellites Ephemeris Service µ value
  7. ^ See references in S/2004 S 3
  8. ^ a b c d e f Source: Thomas et al. 2006
  9. ^ a b c d e f g h i j k l m n o Computed from the period using the IAU-MPC Natural Satellites Ephemeris Service µ value
  10. ^ a b c d e f g h i j k l m n o p q r s t u Source: NASA
  11. ^ Source: Porco et al. 2005
  12. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ar Source: IAU-MPC Natural Satellites Ephemeris Service
  13. ^ a b c d e f g h i j k l m n o p q r Negative orbital periods indicate a retrograde orbit around Saturn (opposite to the planet's rotation)

[edit] Grouping the moons

Although the borders may be somewhat nebulous, Saturn's moons can be divided into eight groups.

[edit] Ring shepherds

Shepherd satellites are 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, Pandora, in addition to the unconfirmed moons S/2004 S 4, S/2004 S 6 and S/2004 S 3.

[edit] Co-orbitals

Janus and Epimetheus are co-orbital moons. These two moons are of roughly equal size and have orbits with only a few kilometers difference in diameter, close enough that they would collide if they attempted to pass each other. Instead of colliding, however, their gravitational interaction causes them to swap orbits every four years. See Epimetheus' article for a more detailed explanation of this arrangement.

[edit] Inner large moons

The innermost large moons of Saturn orbit within its tenuous E Ring. They are Mimas, Enceladus, Tethys and Dione.

Two recently discovered tiny moons also orbit within this group: Methone and Pallene. So too do the co-orbital moons that form a group of their own (see below).

[edit] Trojan moons

Main article: Trojan moon

Trojan moons are another kind of co-orbital. Like the other co-orbitals, they are a feature unique to the Saturnian system. They are moons that orbit at exactly the same distance from Saturn as another moon, but at such a distance from the other moon that they never collide. Tethys has two tiny co-orbitals Telesto and Calypso, and Dione also has two, Helene and Polydeuces. All four of these moons orbit in the larger moons' L4 or L5 Lagrangian points, one in each point.

[edit] Outer large moons

Saturn's largest moons all orbit beyond its E Ring and can thus be considered a distinct group. They are Rhea, Titan, Hyperion (which is relatively small and very irregular), and Iapetus.

[edit] Irregular satellites

Irregular satellites of Saturn.
Irregular satellites of Saturn.

Irregular satellites are satellites with large-radius, inclined, and sometimes retrograde orbits, believed to have been acquired by the parent planet through a capture process.

[edit] Inuit group

Main article: Saturn's Inuit group of satellites

The Inuit group are five prograde outer moons that are similar enough in their distances from Saturn and their orbital inclinations that they can be considered a group. They are Bebhionn, Ijiraq, Kiviuq, Paaliaq, and Siarnaq.

[edit] Norse group

Main article: Saturn's Norse group of satellites

The Norse group are eighteen retrograde outer moons that are similar enough in their distance from Saturn to be considered a group. They are Aegir, Bergelmir, Bestla, Farbauti, Fenrir, Fornjot, Hati, Hyrokkin, Kari, Loge, Mundilfari, Narvi, Phoebe, Skathi, Skoll, Surtur, Suttungr, Thrymr, Ymir, S/2004 S 7, S/2004 S 12, S/2004 S 17, S/2006 S 1, S/2006 S 3, S/2006 S 4, and S/2006 S 6. All of these moons orbit Saturn in a retrograde direction.

[edit] Gallic group

Main article: Saturn's Gallic group of satellites

The Gallic group are three prograde outer moons that are similar enough in their distance from Saturn and their orbital inclination that they can be considered a group. They are Albiorix, Erriapo and Tarvos.

The diagram illustrates the orbits of the irregular satellites of Saturn discovered so far1. 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, the satellites beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's (gravitational influence) radius (~65 Gm for Saturn). Prograde groups: Inuit and Gallic and the retrograde Norse group are clearly identifiable (from top to bottom).

1Named 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.

[edit] Naming notes

Some asteroids share the same names as moons of Saturn: 55 Pandora, 106 Dione, 577 Rhea, 1809 Prometheus, 1810 Epimetheus, 4450 Pan. See also Name conflicts of solar system objects.

[edit] See also

[edit] References

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Natural satellites of the Solar System
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Other satellite systems Plutonian · Eridian · Asteroid satellites
Largest satellites Ganymede · Titan · Callisto · Io · Moon · Europa · Triton
Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys  · Enceladus · Miranda · Proteus · Mimas
 v • d • e The Solar System
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