Rings of Saturn

The full set of rings, imaged as Saturn eclipsed the Sun from the vantage of the Cassini orbiter, 1.2 million km distant, on 19 July 2013 (brightness is exaggerated). Earth appears as a dot at 4 o'clock, between the G and E rings.

The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging from μm to m in size,[1] that orbit about 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,[2] new data from Cassini suggest they formed relatively late.[3]

Although reflection from the rings increases Saturn's brightness, they are not visible from Earth with unaided vision. In 1610, the year after Galileo Galilei turned a telescope to the sky, he became the first person to observe Saturn's rings, though he could not see them well enough to discern their true nature. In 1655, Christiaan Huygens was the first person to describe them as a disk surrounding Saturn.[4] Although many people think of Saturn's rings as being made up of a series of tiny ringlets (a concept that goes back to Laplace),[4] true gaps are few. It is more correct to think of the rings as an annular disk with concentric local maxima and minima in density and brightness.[2] On the scale of the clumps within the rings there is much empty space.

The rings have numerous gaps where particle density drops sharply: two opened by known moons embedded within them, and many others at locations of known destabilizing orbital resonances with the moons of Saturn. Other gaps remain unexplained. Stabilizing resonances, on the other hand, are responsible for the longevity of several rings, such as the Titan Ringlet and the G Ring.

Well beyond the main rings is the Phoebe ring, which is presumed to originate from Phoebe and thus to share its retrograde orbital motion. It is aligned with the plane of Saturn's orbit. Saturn has an axial tilt of 27 degrees, so this ring is tilted at an angle of 27 degrees to the more visible rings orbiting above Saturn's equator.

Voyager 2 view of Saturn casting a shadow across its rings. Four satellites, two of their shadows and ring spokes are visible.


Galileo's work

Galileo first observed the rings in 1610.

Galileo Galilei was the first to observe the rings of Saturn in 1610 using his telescope, but was unable to identify them as such. He wrote to the Duke of Tuscany that "The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one (Saturn itself) is about three times the size of the lateral ones."[5] He also described the rings as Saturn's "ears". In 1612 the Earth passed through the plane of the rings and they became invisible. Mystified, Galileo remarked "I do not know what to say in a case so surprising, so unlooked for and so novel."[4] He mused, "Has Saturn swallowed his children?" — referring to the myth of the Titan Saturn devouring his offspring to forestall the prophecy of them overthrowing him.[5] He was further confused when the rings again became visible in 1613.[4]

Early astronomers used anagrams as a form of commitment scheme to lay claim to new discoveries before their results were ready for publication. Galileo used smaismrmilmepoetaleumibunenugttauiras for Altissimum planetam tergeminum observavi ("I have observed the most distant planet to have a triple form") for discovering the rings of Saturn.[6]

Ring theory, observations and exploration

Robert Hooke noted the shadows (a and b) cast by both the globe and the rings on each other in this 1666 drawing of Saturn.

In 1655, Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. Using a 50× power refracting telescope that he designed himself, far superior to those available to Galileo, Huygens observed Saturn and in 1656, like Galileo, had published an anagram saying "aaaaaaacccccdeeeeeghiiiiiiillllmmnnnnnnnnnooooppqrrstttttuuuuu". Upon confirming his observations, three years later he revealed it to mean "Annuto cingitur, tenui, plano, nusquam coherente, ad eclipticam inclinato"; that is, "It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic".[4][7] Robert Hooke was another early observer of the rings of Saturn, and noted the casting of shadows on the rings.[8]

In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division is a 4,800-km-wide region between the A ring and B Ring.[9]

In 1787, Pierre-Simon Laplace proved that a uniform solid ring would be unstable and suggested that the rings were composed of a large number of solid ringlets.[4][10]

In 1859, James Clerk Maxwell demonstrated that a nonuniform solid ring, solid ringlets or a continuous fluid ring would also not be stable, indicating that the ring must be composed of numerous small particles, all independently orbiting Saturn.[10] Later, Sofia Kovalevskaya also found that Saturn's rings cannot be liquid ring-shaped bodies.[11] Spectroscopic studies of the rings carried out in 1895 by James Keeler of Allegheny Observatory and Aristarkh Belopolsky of Pulkovo Observatory showed Maxwell's analysis was correct.

Four robotic spacecraft have observed Saturn's rings from the vicinity of the planet. Pioneer 11's closest approach to Saturn occurred in September 1979 at a distance of 20,900 km.[12] Pioneer 11 was responsible for the discovery of the F ring.[12] Voyager 1's closest approach occurred in November 1980 at a distance of 64,200 km.[13] A failed photopolarimeter prevented Voyager 1 from observing Saturn's rings at the planned resolution; nevertheless, images from the spacecraft provided unprecedented detail of the ring system and revealed the existence of the G ring.[14] Voyager 2's closest approach occurred in August 1981 at a distance of 41,000 km.[13] Voyager 2's working photopolarimeter allowed it to observe the ring system at higher resolution than Voyager 1, and to thereby discover many previously unseen ringlets.[15] Cassini spacecraft entered into orbit around Saturn in July 2004.[16] Cassini's images of the rings are the most detailed to-date, and are responsible for the discovery of yet more ringlets.[17]

The rings are named alphabetically in the order they were discovered.[18] The main rings are, working outward from the planet, C, B and A, with the Cassini Division, the largest gap, separating Rings B and A. Several fainter rings were discovered more recently. The D Ring is exceedingly faint and closest to the planet. The narrow F Ring is just outside the A Ring. Beyond that are two far fainter rings named G and E. The rings show a tremendous amount of structure on all scales, some related to perturbations by Saturn's moons, but much unexplained.[18]

Other Languages
العربية: حلقات زحل
azərbaycanca: Saturnun halqaları
Bân-lâm-gú: Thó͘-chheⁿ khoân
беларуская: Кольцы Сатурна
беларуская (тарашкевіца)‎: Кольцы Сатурна
čeština: Prstence Saturnu
한국어: 토성의 고리
Bahasa Indonesia: Cincin Saturnus
latviešu: Saturna gredzeni
Lëtzebuergesch: Réng vum Saturn
lietuvių: Saturno žiedai
Bahasa Melayu: Gelang Zuhal
日本語: 土星の環
norsk nynorsk: Ringane til Saturn
português: Anéis de Saturno
Seeltersk: Ringe fon Saturn
Simple English: Rings of Saturn
slovenčina: Prstence Saturna
slovenščina: Saturnovi obroči
srpskohrvatski / српскохрватски: Saturnovi prsteni
українська: Кільця Сатурна
Tiếng Việt: Vành đai Sao Thổ
吴语: 土星环
中文: 土星環