Moons of Saturn

Artist's concepts of the Saturnian ring–moon system
A spherical yellow-brownish body (Saturn) can be seen on the left. It is viewed at an oblique angle with respect to its equatorial plane. Around Saturn there are rings and small ring moons. Further to the right large round moons are shown in order of their distance.
Saturn, its rings and major icy moons—from Mimas to Rhea
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.
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, [1] ranging from tiny moonlets less than 1 kilometer 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 kilometers, as well as dense rings with complex orbital motions of their own. [2] [3] [4] Seven Saturnian moons are large enough to be ellipsoidal in shape, though only two of those, Titan and Rhea, are currently in hydrostatic equilibrium. Particularly notable among Saturn's moons are Titan, the second- largest moon (after Jupiter's Ganymede) in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes found nowhere else in the solar system; [5] and Enceladus since its chemical composition is similar to that of comets. [6] In particular, Enceladus emits jets of gas and dust which could indicate presence of liquid water under its south pole region and could potentially harbor a global ocean under its surface. [7]

Twenty-four of Saturn's moons are regular satellites; they have prograde orbits not greatly inclined to Saturn's equatorial plane. [8] 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 38, all small except one, are irregular satellites, whose orbits are much farther from Saturn, have high inclinations, and are mixed between prograde and retrograde. These moons are probably captured minor planets, or debris from the breakup of such bodies after they were captured, creating collisional families. The irregular satellites have been classified by their orbital characteristics into the Inuit, Norse, and Gallic groups, and their names are chosen from the corresponding mythologies. The largest of the irregular moons is Phoebe, the ninth moon of Saturn, discovered at the end of the 19th century.

The rings of Saturn are made up of objects ranging in size from microscopic to moonlets hundreds of meters across, each in its own orbit around Saturn. [9] 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. [10]

History

A large bright circle in the center is surrounded by small circles.
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 [11] on a refracting telescope of his own design. [12] Tethys, Dione, Rhea and Iapetus (the " Sidera Lodoicea") were discovered between 1671 and 1684 by Giovanni Domenico Cassini. [13] Mimas and Enceladus were discovered in 1789 by William Herschel. [13] Hyperion was discovered in 1848 by W.C. Bond, G.P. Bond [14] and William Lassell. [15]

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

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 by the Cassini spacecraft: Huge Titan and Dione at the bottom, small Prometheus (under the rings) and tiny Telesto above center.
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. [18] In addition, Epimetheus was confirmed as distinct from Janus. In 1990, Pan was discovered in archival Voyager images. [18]

The Cassini mission, [1] 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. [19] 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. [20] In 2007 Anthe was announced. [21] 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. [22] In March 2009, Aegaeon, a moonlet within the G Ring, was announced. [23] 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. [24] ( related image)

Outer moons

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 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. [25] 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. [26] They are probably fragments of larger bodies captured by Saturn's gravitational pull. [25] [26] In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons, [27] [28] in 2006, astronomers using the Subaru 8.2 m telescope reported the discovery of nine more irregular moons, [29] 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. [30]

Some of the 62 known satellites of Saturn are considered lost because they have not been observed since their discovery and hence their orbits are not well-known enough to pinpoint their current locations. Work has been done to recover many of them in surveys from 2009 onwards, but seven – S/2007 S 2, S/2004 S 13, S/2006 S 1, S/2007 S 3, S/2004 S 17, S/2004 S 12, and S/2004 S 7 – still remain lost today. [31] [32]

Naming

The modern names for Saturnian moons were suggested by John Herschel in 1847. [13] He proposed to name them after mythological figures associated with the Roman god of agriculture and harvest, Saturn (equated to the Greek Cronus). [13] In particular, the then known seven satellites were named after Titans, Titanesses and Giants—brothers and sisters of Cronus. [16] In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. [15] 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. [33] All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and after Norse ice giants. [34]

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.

Other Languages
العربية: أقمار زحل
azərbaycanca: Saturnun peykləri
беларуская (тарашкевіца)‎: Спадарожнікі Сатурна
čeština: Měsíce Saturnu
한국어: 토성의 위성
Bahasa Indonesia: Satelit Saturnus
Bahasa Melayu: Bulan Zuhal
Mìng-dĕ̤ng-ngṳ̄: Tū-sĭng gì ôi-sĭng
Nederlands: Manen van Saturnus
norsk nynorsk: Månane til Saturn
Simple English: List of Saturn's moons
slovenčina: Mesiace Saturna
Soomaaliga: Dayaxa raage
srpskohrvatski / српскохрватски: Saturnovi prirodni sateliti
українська: Супутники Сатурна