ring system
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as ...
of any
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
in the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. They consist of countless small particles, ranging in size from
micrometers
The micrometre (American and British English spelling differences#-re, -er, international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American and British English spelling differences# ...
to
meters
The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its prefi ...
, that
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
around
Saturn
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 has only one-eighth the average density of Earth; h ...
. 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.
Although reflection from the rings increases Saturn's
brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. The perception is not linear to luminance, ...
, they are not visible from Earth with unaided vision. In 1610, the year after
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
turned a
telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
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
Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of ...
was the first person to describe them as a disk surrounding Saturn. The concept that Saturn's rings are made up of a series of tiny ringlets can be traced to
Pierre-Simon Laplace
Pierre-Simon, marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French scholar and polymath whose work was important to the development of engineering, mathematics, statistics, physics, astronomy, and philosophy. He summarized ...
, although true gaps are few – it is more correct to think of the rings as an annular disk with
concentric
In geometry, two or more objects are said to be concentric, coaxal, or coaxial when they share the same center or axis. Circles, regular polygons and regular polyhedra, and spheres may be concentric to one another (sharing the same center point ...
local
maxima and minima
In mathematical analysis, the maxima and minima (the respective plurals of maximum and minimum) of a function, known collectively as extrema (the plural of extremum), are the largest and smallest value of the function, either within a given ran ...
in density and brightness. 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 resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
s with the
moons of Saturn
The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to enormous Titan (moon), Titan, which is larger than the planet Mercury (planet), Mercury. Saturn has 83 natural satellite, moons with confirmed ...
. 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 G Ring may refer to:
* Rings of Saturn#G Ring, a planetary ring system around Saturn.
* G-ring
In commutative algebra, a G-ring or Grothendieck ring is a Noetherian ring such that the map of any of its local rings to the completion is regular ( ...
.
Well beyond the main rings is the
Phoebe ring
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 entirel ...
, 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
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 has only one-eighth the average density of Earth; h ...
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.
History
Early observations
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
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
The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the Sun path, apparent path of the Sun across the celestial sphere over the course of the year. ...
, and the middle one (Saturn itself) is about three times the size of the lateral ones." 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." He mused, "Has Saturn swallowed his children?" — referring to the myth of the
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
Saturn
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 has only one-eighth the average density of Earth; h ...
devouring his offspring to forestall the prophecy of them overthrowing him. He was further confused when the rings again became visible in 1613.
Early
astronomer
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...
s used
anagram
An anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once. For example, the word ''anagram'' itself can be rearranged into ''nag a ram'', also the word ...
s as a form of
commitment scheme
A commitment scheme is a cryptographic primitive that allows one to commit to a chosen value (or chosen statement) while keeping it hidden to others, with the ability to reveal the committed value later.Oded Goldreich (2001). Foundations of Crypt ...
to lay claim to new discoveries before their results were ready for publication. Galileo used the anagram "'" for ''Altissimum planetam tergeminum observavi'' ("I have observed the most distant planet to have a triple form") for discovering the rings of Saturn.
In 1657
Christopher Wren
Sir Christopher Wren PRS FRS (; – ) was one of the most highly acclaimed English architects in history, as well as an anatomist, astronomer, geometer, and mathematician-physicist. He was accorded responsibility for rebuilding 52 churches ...
became Professor of Astronomy at Gresham College, London. He had been making observations of the planet Saturn from around 1652 with the aim of explaining its appearance. His hypothesis was written up in ''De corpore saturni,'' in which he came close to suggesting the planet had a ring. However, Wren was unsure whether the ring was independent of the planet, or physically attached to it. Before Wren's theory was published
Christiaan Huygens
Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of ...
presented his theory of the rings of Saturn. Immediately Wren recognised this as a better hypothesis than his own and ''De corpore saturni'' was never published.
Robert Hooke
Robert Hooke FRS (; 18 July 16353 March 1703) was an English polymath active as a scientist, natural philosopher and architect, who is credited to be one of two scientists to discover microorganisms in 1665 using a compound microscope that ...
was another early observer of the rings of Saturn, and noted the casting of shadows on the rings.
Huygens' ring theory and later developments
Huygens
Huygens (also Huijgens, Huigens, Huijgen/Huygen, or Huigen) is a Dutch patronymic surname, meaning "son of Hugo". Most references to "Huygens" are to the polymath Christiaan Huygens. Notable people with the surname include:
* Jan Huygen (1563– ...
began grinding lenses with his brother Constantijn in 1655 and was able to observe Saturn with greater detail using a 43× power refracting telescope that he designed himself. He was the first to suggest that
Saturn
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 has only one-eighth the average density of Earth; h ...
was surrounded by a ring detached from the planet, and famously published the anagram: "'". Three years later, he revealed it to mean ''Annuto cingitur, tenui, plano, nusquam coherente, ad eclipticam inclinato'' (" aturnis surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic"). He published his ring theory in ''Systema Saturnium'' (1659) which also included his discovery of Saturn's moon,
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
, as well as the first clear outline of the dimensions of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
.
In 1675,
Giovanni Domenico Cassini
Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 September 1712) was an Italian (naturalised French) mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the C ...
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
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 entirel ...
. This division is a region between the
A ring
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 entirel ...
and
B Ring
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 entirel ...
.
In 1787,
Pierre-Simon Laplace
Pierre-Simon, marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French scholar and polymath whose work was important to the development of engineering, mathematics, statistics, physics, astronomy, and philosophy. He summarized ...
proved that a uniform solid ring would be unstable and suggested that the rings were composed of a large number of solid ringlets.
In 1859,
James Clerk Maxwell
James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish mathematician and scientist responsible for the classical theory of electromagnetic radiation, which was the first theory to describe electricity, magnetism and ligh ...
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. Later,
Sofia Kovalevskaya
Sofya Vasilyevna Kovalevskaya (russian: link=no, Софья Васильевна Ковалевская), born Korvin-Krukovskaya ( – 10 February 1891), was a Russian mathematician who made noteworthy contributions to analysis, partial differen ...
also found that Saturn's rings cannot be liquid ring-shaped bodies. Spectroscopic studies of the rings which were carried out independently in 1895 by
James Keeler
James Edward Keeler (September 10, 1857 – August 12, 1900) was an American astronomer. He was an early observer of galaxies using photography, as well as the first to show observationally that the rings of Saturn do not rotate as a solid body. ...
of the
Allegheny Observatory
The Allegheny Observatory is an American astronomical research institution, a part of the Department of Physics and Astronomy at the University of Pittsburgh. The facility is listed on the National Register of Historic Places (ref. # 79002157, ad ...
and by
Aristarkh Belopolsky
Aristarkh Apollonovich Belopolsky (Аристарх Аполлонович Белопольский) (, Moscow – 16 May 1934, Pulkovo, Leningrad) was a Russian astronomer. He was born in Moscow but his father's ancestors are from a Serbian ...
of the
Pulkovo Observatory
The Pulkovo Astronomical Observatory (russian: Пулковская астрономическая обсерватория, Pulkovskaya astronomicheskaya observatoriya), officially named the Central Astronomical Observatory of the Russian Academ ...
showed that Maxwell's analysis was correct.
Four robotic spacecraft have observed Saturn's rings from the vicinity of the planet. ''
Pioneer 11
''Pioneer 11'' (also known as ''Pioneer G'') is a robotic space probe launched by NASA on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays. It was the first probe to encounter ...
''s closest approach to Saturn occurred in September 1979 at a distance of . ''Pioneer 11'' was responsible for the discovery of the F ring. ''
Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
''s closest approach occurred in November 1980 at a distance of . 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. ''
Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
''s closest approach occurred in August 1981 at a distance of . ''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.''Cassini'' spacecraft entered into orbit around Saturn in July 2004. ''Cassini'' images of the rings are the most detailed to-date, and are responsible for the discovery of yet more ringlets.
The rings are named alphabetically in the order they were discovered (A and B in 1675 by
Giovanni Domenico Cassini
Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 September 1712) was an Italian (naturalised French) mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the C ...
, C in 1850 by
William Cranch Bond
William Cranch Bond (September 9, 1789 – January 29, 1859) was an American astronomer, and the first director of Harvard College Observatory.
Upbringing
William Cranch Bond was born in Falmouth, Maine (near Portland) on September 9, 1789. ...
and his son
George Phillips Bond
George Phillips Bond (May 20, 1825February 17, 1865) was an American astronomer. He was the son of William Cranch Bond. Some sources give his year of birth as 1826.
His early interest was in nature and birds, but after his elder brother William ...
, D in 1933 by
Nikolai P. Barabachov
Nikolai or Nikolay is an East Slavic variant of the masculine name Nicholas. It may refer to:
People Royalty
* Nicholas I of Russia (1796–1855), or Nikolay I, Emperor of Russia from 1825 until 1855
* Nicholas II of Russia (1868–1918), or Niko ...
and
B. Semejkin
B is the second letter of the Latin alphabet.
B may also refer to:
Science, technology, and mathematics Astronomy
* Astronomical objects in the List of dark nebulae#Barnard objects, Barnard list of dark nebulae (abbreviation B)
* Latitude (''b ...
, E in 1967 by
Walter A. Feibelman
Walter may refer to:
People
* Walter (name), both a surname and a given name
* Little Walter, American blues harmonica player Marion Walter Jacobs (1930–1968)
* Gunther (wrestler), Austrian professional wrestler and trainer Walter Hahn (born 19 ...
, F in 1979 by ''
Pioneer 11
''Pioneer 11'' (also known as ''Pioneer G'') is a robotic space probe launched by NASA on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays. It was the first probe to encounter ...
'', and G in 1980 by ''
Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
''). 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.
Saturn's axial inclination
Saturn's axial tilt is 26.7°, meaning that widely varying views of the rings, of which the visible ones occupy its equatorial plane, are obtained from Earth at different times. Earth makes passes through the ring plane every 13 to 15 years, about every half Saturn year, and there are about equal chances of either a single or three crossings occurring in each such occasion. The most recent ring plane crossings were on 22 May 1995, 10 August 1995, 11 February 1996 and 4 September 2009; upcoming events will occur on 23 March 2025, 15 October 2038, 1 April 2039 and 9 July 2039. Favorable ring plane crossing viewing opportunities (with Saturn not close to the Sun) only come during triple crossings.
Saturn's
equinox
A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
es, when the Sun passes through the ring plane, are not evenly spaced; on each orbit the Sun is south of the ring plane for 13.7 Earth years, then north of the plane for 15.7 years. Dates for its northern hemisphere autumnal equinoxes include 19 November 1995 and 6 May 2025, with northern vernal equinoxes on 11 August 2009 and 23 January 2039. During the period around an equinox the illumination of most of the rings is greatly reduced, making possible unique observations highlighting features that depart from the ring plane.
Physical characteristics
The dense main rings extend from to away from Saturn's equator, whose radius is (see Major subdivisions). With an estimated local thickness of as little as 10 metres (30') and as much as 1 km (1000 yards), they are composed of 99.9% pure water
ice
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
with a smattering of impurities that may include
tholin
Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon ...
s or
silicate
In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...
s. The main rings are primarily composed of particles smaller than 10 m.
''Cassini'' directly measured the mass of the ring system via their gravitational effect during its final set of orbits that passed between the rings and the cloud tops, yielding a value of 1.54 (± 0.49) × 1019 kg, or 0.41 ± 0.13
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
masses. This is as massive as about half the mass of the Earth's entire
Antarctic
The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...
ice shelf
An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The b ...
, spread across a surface area 80 times larger than that of Earth. The estimate is close to the value of 0.40 Mimas masses derived from ''Cassini'' observations of density waves in the A, B and C rings. It is a small fraction of the total mass of Saturn (about 0.25 ppb). Earlier ''Voyager'' observations of density waves in the A and B rings and an optical depth profile had yielded a mass of about 0.75 Mimas masses, with later observations and computer modeling suggesting that was an underestimate.
Although the largest gaps in the rings, such as the Cassini Division and
Encke Gap
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 entirel ...
, can be seen from Earth, the
Voyager
Voyager may refer to:
Computing and communications
* LG Voyager, a mobile phone model manufactured by LG Electronics
* NCR Voyager, a computer platform produced by NCR Corporation
* Voyager (computer worm), a computer worm affecting Oracle ...
spacecraft discovered that the rings have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise, in several different ways, from the gravitational pull of Saturn's many moons. Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small
shepherd satellite
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known a ...
s (similar to
Prometheus
In Greek mythology, Prometheus (; , , possibly meaning "forethought")Smith"Prometheus". is a Titan god of fire. Prometheus is best known for defying the gods by stealing fire from them and giving it to humanity in the form of technology, know ...
and
Pandora
In Greek mythology, Pandora (Greek: , derived from , ''pān'', i.e. "all" and , ''dōron'', i.e. "gift", thus "the all-endowed", "all-gifted" or "all-giving") was the first human woman created by Hephaestus on the instructions of Zeus. As Hes ...
's maintenance of the F ring). Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out;
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
maintains the Cassini Division in this manner. Still more structure in the rings consists of spiral waves raised by the inner moons' periodic gravitational perturbations at less disruptive resonances.
Data from the ''Cassini'' space probe indicate that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
gas (O2) produced when ultraviolet light from the Sun interacts with water ice in the rings. Chemical reactions between water molecule fragments and further
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
stimulation create and eject, among other things, O2. According to models of this atmosphere, H2 is also present. The O2 and H2 atmospheres are so sparse that if the entire atmosphere were somehow condensed onto the rings, it would be about one atom thick. The rings also have a similarly sparse OH (hydroxide) atmosphere. Like the O2, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s that bombard water molecules ejected by Saturn's moon
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about 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 refl ...
. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.
Saturn shows complex patterns in its brightness. Most of the variability is due to the changing aspect of the rings, and this goes through two cycles every orbit. However, superimposed on this is variability due to the eccentricity of the planet's orbit that causes the planet to display brighter oppositions in the northern hemisphere than it does in the southern.
In 1980, ''Voyager 1'' made a fly-by of Saturn that showed the F ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.
New images of the rings taken around the 11 August 2009 equinox of Saturn by NASA's ''Cassini'' spacecraft have shown that the rings extend significantly out of the nominal ring plane in a few places. This displacement reaches as much as at the border of the
Keeler Gap
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 entire ...
, due to the out-of-plane orbit of
Daphnis
In Greek mythology, Daphnis (; grc, Δάφνις, from , ''daphne'', "Bay Laurel") was a Sicilian shepherd who was said to be the inventor of pastoral poetry.
Family
According to tradition, he was the son of Hermes and a nymph, despite which ...
, the moon that creates the gap.
Formation and evolution of main rings
Estimates of the age of Saturn's rings vary widely, depending on the approach used. They have been considered to possibly be very old, dating to the formation of Saturn itself. However, data from ''Cassini'' suggest they are much younger, having most likely formed within the last 100 million years, and may thus be between 10 million and 100 million years old. This recent origin scenario is based on a new, low mass estimate, modeling of the rings' dynamical evolution, and measurements of the flux of interplanetary dust, which feed into an estimate of the rate of ring darkening over time. Since the rings are continually losing material, they would have been more massive in the past than at present. The mass estimate alone is not very diagnostic, since high mass rings that formed early in the Solar System's history would have evolved by now to a mass close to that measured. Based on current depletion rates, they may disappear in 300 million years.
There are two main theories regarding the origin of Saturn's inner rings. A theory originally proposed by
Édouard Roche
Édouard Albert Roche (; 17 October 1820 – 27 April 1883) was a French astronomer and mathematician, who is best known for his work in the field of celestial mechanics. His name was given to the concepts of the Roche sphere, Roche limit, and Ro ...
in the 19th century, is that the rings were once a moon of Saturn (named Veritas, after a
Roman goddess
Roman mythology is the body of myths of ancient Rome as represented in the literature and visual arts of the Romans. One of a wide variety of genres of Roman folklore, ''Roman mythology'' may also refer to the modern study of these representat ...
who hid in a well). According to the theory the moon's orbit decayed until it was close enough to be ripped apart by
tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
s (see
Roche limit
In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forc ...
). Numerical simulations carried out in 2022 support this theory; the authors of that study proposed the name "
Chrysalis
A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their ...
" for the destroyed moon. A variation on this theory is that this moon disintegrated after being struck by a large
comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
or
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
. The second theory is that the rings were never part of a moon, but are instead left over from the original
nebula
A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...
r material from which Saturn formed.
A more traditional version of the disrupted-moon theory is that the rings are composed of debris from a moon 400 to 600 km (200 to 400 miles) in diameter, slightly larger than
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
. The last time there were collisions large enough to be likely to disrupt a moon that large was during the
Late Heavy Bombardment
The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypoth ...
some four billion years ago.
A more recent variant of this type of theory by R. M. Canup is that the rings could represent part of the remains of the icy mantle of a much larger, Titan-sized, differentiated moon that was stripped of its outer layer as it spiraled into the planet during the formative period when Saturn was still surrounded by a gaseous nebula. This would explain the scarcity of rocky material within the rings. The rings would initially have been much more massive (≈1,000 times) and broader than at present; material in the outer portions of the rings would have coalesced into the moons of Saturn out to Tethys, also explaining the lack of rocky material in the composition of most of these moons. Subsequent collisional or cryovolcanic evolution of Enceladus might then have caused selective loss of ice from this moon, raising its density to its current value of 1.61 g/cm3, compared to values of 1.15 for Mimas and 0.97 for Tethys.
The idea of massive early rings was subsequently extended to explain the formation of Saturn's moons out to Rhea. If the initial massive rings contained chunks of rocky material (>100 km; 60 miles across) as well as ice, these silicate bodies would have accreted more ice and been expelled from the rings, due to gravitational interactions with the rings and tidal interaction with Saturn, into progressively wider orbits. Within the
Roche limit
In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forc ...
, bodies of rocky material are dense enough to accrete additional material, whereas less-dense bodies of ice are not. Once outside the rings, the newly formed moons could have continued to evolve through random mergers. This process may explain the variation in silicate content of Saturn's moons out to Rhea, as well as the trend towards less silicate content closer to Saturn. Rhea would then be the oldest of the moons formed from the primordial rings, with moons closer to Saturn being progressively younger.
The brightness and purity of the water ice in Saturn's rings have also been cited as evidence that the rings are much younger than Saturn, as the infall of meteoric dust would have led to a darkening of the rings. However, new research indicates that the B Ring may be massive enough to have diluted infalling material and thus avoided substantial darkening over the age of the Solar System. Ring material may be recycled as clumps form within the rings and are then disrupted by impacts. This would explain the apparent youth of some of the material within the rings. Evidence suggesting a recent origin of the C ring has been gathered by researchers analyzing data from the Cassini Titan Radar Mapper, which focused on analyzing the proportion of rocky silicates within this ring. If much of this material was contributed by a recently disrupted
centaur
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as being ...
or moon, the age of this ring could be on the order of 100 million years or less. On the other hand, if the material came primarily from micrometeoroid influx, the age would be closer to a billion years.
The ''Cassini'' UVIS team, led by
Larry Esposito
Larry W. Esposito (born April 15, 1951) is an American planetary science, planetary astronomer and a professor at the Laboratory for Atmospheric and Space Physics, University of Colorado Boulder. A 1973 graduate at the Massachusetts Institute of Te ...
, used
stellar occultation
Stellar means anything related to one or more stars (''stella''). The term may also refer to:
Arts, entertainment, and media
* ''Stellar'' (magazine), an Irish lifestyle and fashion magazine
* Stellar Loussier, a character from ''Mobile Suit Gun ...
to discover 13 objects, ranging from 27 metres (89') to 10 km (6 miles) across, within the
F ring
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 entirel ...
. They are translucent, suggesting they are temporary aggregates of ice boulders a few meters across. Esposito believes this to be the basic structure of the Saturnian rings, particles clumping together, then being blasted apart.
Research based on rates of infall into Saturn favors a younger ring system age of hundreds of millions of years. Ring material is continually spiraling down into Saturn; the faster this infall, the shorter the lifetime of the ring system. One mechanism involves gravity pulling electrically charged water ice grains down from the rings along planetary magnetic field lines, a process termed 'ring rain'. This flow rate was inferred to be 432–2870 kg/s using ground-based
Keck telescope
The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state
In the United States, a state is a constituent political entit ...
observations; as a consequence of this process alone, the rings will be gone in ~ million years. While traversing the gap between the rings and planet in September 2017, the ''Cassini'' spacecraft detected an equatorial flow of charge-neutral material from the rings to the planet of 4,800–44,000 kg/s. Assuming this influx rate is stable, adding it to the continuous 'ring rain' process implies the rings may be gone in under 100 million years.
Subdivisions and structures within the rings
The densest parts of the Saturnian ring system are the A and B Rings, which are separated by the Cassini Division (discovered in 1675 by
Giovanni Domenico Cassini
Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 September 1712) was an Italian (naturalised French) mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the C ...
). Along with the C Ring, which was discovered in 1850 and is similar in character to the Cassini Division, these regions constitute the ''main rings''. The main rings are denser and contain larger particles than the tenuous ''dusty rings''. The latter include the D Ring, extending inward to Saturn's cloud tops, the G and E Rings and others beyond the main ring system. These diffuse rings are characterised as "dusty" because of the small size of their particles (often about a
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
); their chemical composition is, like the main rings, almost entirely water ice. The narrow F Ring, just off the outer edge of the A Ring, is more difficult to categorize; parts of it are very dense, but it also contains a great deal of dust-size particles.
Physical parameters of the rings
''Notes:'' (1) Names as designated by the
International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
, unless otherwise noted. Broader separations between named rings are termed ''divisions'', while narrower separations within named rings are called ''gaps''. (2) Data mostly from th Gazetteer of Planetary Nomenclature
and several papers. (3) distance is to centre of gaps, rings and ringlets that are narrower than 1,000 km (600 miles) (4) unofficial name
Major subdivisions
C Ring structures
Cassini Division structures
* Source:
A Ring structures
D Ring
The D Ring is the innermost ring, and is very faint. In 1980, ''
Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
'' detected within this ring three ringlets designated D73, D72 and D68, with D68 being the discrete ringlet nearest to Saturn. Some 25 years later, ''Cassini'' images showed that D72 had become significantly broader and more diffuse, and had moved planetward by 200 km (100 miles).
Present in the D Ring is a finescale structure with waves 30 km (20 miles) apart. First seen in the gap between the C Ring and D73, the structure was found during Saturn's 2009 equinox to extend a radial distance of 19,000 km (12,000 miles) from the D Ring to the inner edge of the B Ring. The waves are interpreted as a spiral pattern of vertical corrugations of 2 to 20 m amplitude; the fact that the period of the waves is decreasing over time (from 60 km; 40 miles in 1995 to 30 km; 20 miles by 2006) allows a deduction that the pattern may have originated in late 1983 with the impact of a cloud of debris (with a mass of ≈1012 kg) from a disrupted comet that tilted the rings out of the equatorial plane. A similar spiral pattern in Jupiter's main ring has been attributed to a perturbation caused by impact of material from
Comet Shoemaker-Levy 9
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
in 1994.
C Ring
The C Ring is a wide but faint ring located inward of the
B Ring
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 entirel ...
. It was discovered in 1850 by
William
William is a male given name of Germanic origin.Hanks, Hardcastle and Hodges, ''Oxford Dictionary of First Names'', Oxford University Press, 2nd edition, , p. 276. It became very popular in the English language after the Norman conquest of Engl ...
Johann Galle
Johann Gottfried Galle (9 June 1812 – 10 July 1910) was a German astronomer from Radis, Germany, at the Berlin Observatory who, on 23 September 1846, with the assistance of student Heinrich Louis d'Arrest, was the first person to view the pl ...
also saw it independently.
William Lassell
William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer.Harland, David M., ''Mission to Saturn: Cassini and the Huygens Probe'', Chichester: Praxis Publishing, 2002.
Its vertical thickness is estimated at 5 metres (16'), its mass at around 1.1 kg, and its
optical depth
In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material.
Thus, the larger the optical depth, the smaller the amount of transmitted radiant power through ...
varies from 0.05 to 0.12. That is, between 5 and 12 percent of light shining perpendicularly through the ring is blocked, so that when seen from above, the ring is close to transparent. The 30-km wavelength spiral corrugations first seen in the D Ring were observed during Saturn's equinox of 2009 to extend throughout the C Ring (see above).
Colombo Gap and Titan Ringlet
The Colombo Gap lies in the inner C Ring. Within the gap lies the bright but narrow Colombo Ringlet, centered at 77,883 km (48,394 miles) from Saturn's center, which is slightly
elliptical
Elliptical may mean:
* having the shape of an ellipse, or more broadly, any oval shape
** in botany, having an elliptic leaf shape
** of aircraft wings, having an elliptical planform
* characterised by ellipsis (the omission of words), or by conc ...
rather than circular. This ringlet is also called the Titan Ringlet as it is governed by an orbital resonance with the moon
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
. At this location within the rings, the length of a ring particle's
apsidal precession
In celestial mechanics, apsidal precession (or apsidal advance) is the precession (gradual rotation) of the line connecting the apsides (line of apsides) of an astronomical body's orbit. The apsides are the orbital points closest (periapsi ...
is equal to the length of Titan's orbital motion, so that the outer end of this eccentric ringlet always points towards Titan.
Maxwell Gap and Ringlet
The Maxwell Gap lies within the outer part of the C Ring. It also contains a dense non-circular ringlet, the Maxwell Ringlet. In many respects this ringlet is similar to the ε ring of Uranus. There are wave-like structures in the middle of both rings. While the wave in the ε ring is thought to be caused by Uranian moon
Cordelia
Cordelia is a feminine given name. It was borne by the tragic heroine of Shakespeare's ''King Lear'' (1606), a character based on the legendary queen Cordelia. The name is of uncertain origin. It is popularly associated with Latin '' cor'' ( geni ...
, no moon has been discovered in the Maxwell gap as of July 2008.
B Ring
The B Ring is the largest, brightest, and most massive of the rings. Its thickness is estimated as 5 to 15 m and its optical depth varies from 0.4 to greater than 5, meaning that >99% of the light passing through some parts of the B Ring is blocked. The B Ring contains a great deal of variation in its density and brightness, nearly all of it unexplained. These are concentric, appearing as narrow ringlets, though the B Ring does not contain any gaps. In places, the outer edge of the B Ring contains vertical structures deviating up to 2.5 km (1½ miles) from the main ring plane, a significant deviation from the vertical thickness of the main A, B and C rings, which is generally only about 10 meters (about 30 feet). Vertical structures can be created by unseen embedded moonlets.
A 2016 study of spiral density waves using stellar occultations indicated that the B Ring's surface density is in the range of 40 to 140 g/cm2, lower than previously believed, and that the ring's optical depth has little correlation with its mass density (a finding previously reported for the A and C rings). The total mass of the B Ring was estimated to be somewhere in the range of 7 to kg. This compares to a mass for
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
of kg.
Spokes
Until 1980, the structure of the rings of Saturn was explained as being caused exclusively by the action of
gravitation
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
al forces. Then images from the Voyager spacecraft showed radial features in the
B Ring
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 entirel ...
, known as ''spokes'', which could not be explained in this manner, as their persistence and rotation around the rings was not consistent with gravitational
orbital mechanics
Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of ...
. The spokes appear dark in
backscatter
In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, a ...
Gallery
Gallery or The Gallery may refer to:
Arts, entertainment, and media
* Art gallery
** Contemporary art gallery
Music
* Gallery (band), an American soft rock band of the 1970s
Albums
* ''Gallery'' (Elaiza album), 2014 album
* ''Gallery'' (Gr ...
); the transition occurs at a phase angle near 60 °. The leading theory regarding the spokes' composition is that they consist of
microscopic
The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens (optics), lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded a ...
dust particles suspended away from the main ring by
electrostatic
Electrostatics is a branch of physics that studies electric charges at rest (static electricity).
Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for amber ...
repulsion, as they rotate almost synchronously with the
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
of Saturn. The precise mechanism generating the spokes is still unknown, although it has been suggested that the electrical disturbances might be caused by either
lightning
Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
bolts in Saturn's
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
or
micrometeoroid
A micrometeoroid is a tiny meteoroid: a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through Earth's atmosphere and reaches Earth's surface.
The term "micrometeoroid ...
impacts on the rings.
The spokes were not observed again until some twenty-five years later, this time by the ''Cassini'' space probe. The spokes were not visible when ''Cassini'' arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe their formation. Nevertheless, the ''Cassini'' imaging team kept looking for spokes in images of the rings, and they were next seen in images taken on 5 September 2005.
The spokes appear to be a
season
A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and pol ...
al phenomenon, disappearing in the Saturnian midwinter and midsummer and reappearing as Saturn comes closer to
equinox
A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
. Suggestions that the spokes may be a seasonal effect, varying with Saturn's 29.7-year orbit, were supported by their gradual reappearance in the later years of the Cassini mission.
Moonlet
In 2009, during equinox, a moonlet embedded in the B ring was discovered from the shadow it cast. It is estimated to be in diameter. The moonlet was given the provisional designation
S/2009 S 1
S/2009 S 1 is a moonlet embedded in the outer part of Saturn's B Ring, orbiting away from the planet. The moonlet was discovered by the ''Cassini'' Imaging Team during the Saturnian equinox event on 26 July 2009, when the ''Cassini'' spacecraft ...
.
Cassini Division
The Cassini Division is a region in width between Saturn's
A Ring
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 entirel ...
and
B Ring
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 entirel ...
. It was discovered in 1675 by
Giovanni Cassini
Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 September 1712) was an Italian (naturalised French) mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the C ...
at the
Paris Observatory
The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histor ...
using a
refracting telescope
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
that had a 2.5-inch
objective lens
In optical engineering, the objective is the optical element that gathers light from the object being observed and Focus (optics), focuses the ray (optics), light rays to produce a real image. Objectives can be a single Lens (optics), lens or mirr ...
with a 20-foot-long
focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foca ...
and a 90x
magnification
Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in siz ...
. From Earth it appears as a thin black gap in the rings. However, ''Voyager'' discovered that the gap is itself populated by ring material bearing much similarity to the C Ring. The division may appear bright in views of the unlit side of the rings, since the relatively low density of material allows more light to be transmitted through the thickness of the rings (see second image in
gallery
Gallery or The Gallery may refer to:
Arts, entertainment, and media
* Art gallery
** Contemporary art gallery
Music
* Gallery (band), an American soft rock band of the 1970s
Albums
* ''Gallery'' (Elaiza album), 2014 album
* ''Gallery'' (Gr ...
).
The inner edge of the Cassini Division is governed by a strong orbital resonance. Ring particles at this location orbit twice for every orbit of the moon
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
. The resonance causes Mimas' pulls on these ring particles to accumulate, destabilizing their orbits and leading to a sharp cutoff in ring density. Many of the other gaps between ringlets within the Cassini Division, however, are unexplained.
Huygens Gap
The Huygens Gap is located at the inner edge of the Cassini Division. It contains the dense, eccentric Huygens Ringlet in the middle. This ringlet exhibits irregular
azimuthal
An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north.
Mathematically, ...
variations of geometrical width and optical depth, which may be caused by the nearby 2:1 resonance with
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
and the influence of the eccentric outer edge of the B-ring. There is an additional narrow ringlet just outside the Huygens Ringlet.
A Ring
The A Ring is the outermost of the large, bright rings. Its inner boundary is the
Cassini Division
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 entirel ...
and its sharp outer boundary is close to the orbit of the small moon
Atlas
An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth.
Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographic ...
. The A Ring is interrupted at a location 22% of the ring width from its outer edge by the
Encke Gap
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 entirel ...
. A narrower gap 2% of the ring width from the outer edge is called the
Keeler Gap
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 entire ...
.
The thickness of the A Ring is estimated to be 10 to 30 m, its surface density from 35 to 40 g/cm2 and its total mass as 4 to kg (just under the mass of Hyperion). Its optical depth varies from 0.4 to 0.9.
Similarly to the B Ring, the A Ring's outer edge is maintained by orbital resonances, albeit in this case a more complicated set. It is primarily acted on by the 7:6 resonance with
Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
and
Epimetheus
In Greek mythology, Epimetheus (; grc-gre, Ἐπιμηθεύς, , afterthought) was the brother of Prometheus (traditionally interpreted as "foresight", literally "fore-thinker"), a pair of Titans who "acted as representatives of mankind". The ...
, with other contributions from the 5:3 resonance with
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
and various resonances with
Prometheus
In Greek mythology, Prometheus (; , , possibly meaning "forethought")Smith"Prometheus". is a Titan god of fire. Prometheus is best known for defying the gods by stealing fire from them and giving it to humanity in the form of technology, know ...
and
Pandora
In Greek mythology, Pandora (Greek: , derived from , ''pān'', i.e. "all" and , ''dōron'', i.e. "gift", thus "the all-endowed", "all-gifted" or "all-giving") was the first human woman created by Hephaestus on the instructions of Zeus. As Hes ...
. Other orbital resonances also excite many spiral density waves in the A Ring (and, to a lesser extent, other rings as well), which account for most of its structure. These waves are described by the same physics that describes the spiral arms of galaxies. Spiral bending waves, also present in the A Ring and also described by the same theory, are vertical corrugations in the ring rather than compression waves.
In April 2014, NASA scientists reported observing the possible formative stage of a new moon near the outer edge of the A Ring.
Encke Gap
The Encke Gap is a 325-km (200 mile) wide gap within the
A ring
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 entirel ...
, centered at a distance of 133,590 km (83,000 miles) from Saturn's center. It is caused by the presence of the small moon Pan, which orbits within it. Images from the ''Cassini'' probe have shown that there are at least three thin, knotted ringlets within the gap.Spiral density waves visible on both sides of it are induced by resonances with nearby
moons
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
exterior to the rings, while Pan induces an additional set of spiraling wakes (see image in
gallery
Gallery or The Gallery may refer to:
Arts, entertainment, and media
* Art gallery
** Contemporary art gallery
Music
* Gallery (band), an American soft rock band of the 1970s
Albums
* ''Gallery'' (Elaiza album), 2014 album
* ''Gallery'' (Gr ...
).
Johann Encke
Johann Franz Encke (; 23 September 179126 August 1865) was a German astronomer. Among his activities, he worked on the calculation of the periods of comets and asteroids, measured the distance from the Earth to the Sun, and made observations ...
himself did not observe this gap; it was named in honour of his ring observations. The gap itself was discovered by
James Edward Keeler
James Edward Keeler (September 10, 1857 – August 12, 1900) was an American astronomer. He was an early observer of galaxies using photography, as well as the first to show observationally that the rings of Saturn do not rotate as a solid body ...
in 1888. The second major gap in the
A ring
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 entirel ...
, discovered by ''Voyager'', was named the
Keeler Gap
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 entire ...
in his honor.
The Encke Gap is a ''gap'' because it is entirely within the A Ring. There was some ambiguity between the terms ''gap'' and ''division'' until the IAU clarified the definitions in 2008; before that, the separation was sometimes called the "Encke Division".
Keeler Gap
The Keeler Gap is a 42-km (26 mile) wide gap in the
A ring
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 entirel ...
, approximately 250 km (150 miles) from the ring's outer edge. The small moon
Daphnis
In Greek mythology, Daphnis (; grc, Δάφνις, from , ''daphne'', "Bay Laurel") was a Sicilian shepherd who was said to be the inventor of pastoral poetry.
Family
According to tradition, he was the son of Hermes and a nymph, despite which ...
, discovered 1 May 2005, orbits within it, keeping it clear. The moon's passage induces waves in the edges of the gap (this is also influenced by its slight orbital eccentricity). Because the orbit of Daphnis is slightly inclined to the ring plane, the waves have a component that is perpendicular to the ring plane, reaching a distance of 1500 m "above" the plane.
The Keeler gap was discovered by ''Voyager'', and named in honor of the astronomer
James Edward Keeler
James Edward Keeler (September 10, 1857 – August 12, 1900) was an American astronomer. He was an early observer of galaxies using photography, as well as the first to show observationally that the rings of Saturn do not rotate as a solid body ...
. Keeler had in turn discovered and named the
Encke Gap
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 entirel ...
in honor of
Johann Encke
Johann Franz Encke (; 23 September 179126 August 1865) was a German astronomer. Among his activities, he worked on the calculation of the periods of comets and asteroids, measured the distance from the Earth to the Sun, and made observations ...
.
Propeller moonlets
In 2006, four tiny "
moonlet
A moonlet, minor moon, minor natural satellite, or minor satellite is a particularly small natural satellite orbiting a planet, dwarf planet, or other minor planet.
Up until 1995, moonlets were only hypothetical components of Saturn's F-ring ...
s" were found in ''Cassini'' images of the A Ring. The moonlets themselves are only about a hundred metres in diameter, too small to be seen directly; what ''Cassini'' sees are the "propeller"-shaped disturbances the moonlets create, which are several km (miles) across. It is estimated that the A Ring contains thousands of such objects. In 2007, the discovery of eight more moonlets revealed that they are largely confined to a 3,000 km (2000 mile) belt, about 130,000 km (80,000 miles) from Saturn's center, and by 2008 over 150 propeller moonlets had been detected. One that has been tracked for several years has been nicknamed ''Bleriot''.
Roche Division
The separation between the
A ring
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 entirel ...
and the
F Ring
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 entirel ...
has been named the Roche Division in honor of the French physicist
Édouard Roche
Édouard Albert Roche (; 17 October 1820 – 27 April 1883) was a French astronomer and mathematician, who is best known for his work in the field of celestial mechanics. His name was given to the concepts of the Roche sphere, Roche limit, and Ro ...
. The Roche Division should not be confused with the
Roche limit
In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forc ...
which is the distance at which a large object is so close to a planet (such as Saturn) that the planet's
tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
s will pull it apart. Lying at the outer edge of the main ring system, the Roche Division is in fact close to Saturn's Roche limit, which is why the rings have been unable to accrete into a moon.
Like the
Cassini Division
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 entirel ...
, the Roche Division is not empty but contains a sheet of material. The character of this material is similar to the tenuous and dusty D, E, and G Rings. Two locations in the Roche Division have a higher concentration of dust than the rest of the region. These were discovered by the ''Cassini'' probe imaging team and were given temporary designations: R/2004 S 1, which lies along the orbit of the moon
Atlas
An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth.
Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographic ...
; and R/2004 S 2, centered at 138,900 km (86,300 miles) from Saturn's center, inward of the orbit of
Prometheus
In Greek mythology, Prometheus (; , , possibly meaning "forethought")Smith"Prometheus". is a Titan god of fire. Prometheus is best known for defying the gods by stealing fire from them and giving it to humanity in the form of technology, know ...
.
F Ring
The F Ring is the outermost discrete ring of Saturn and perhaps the most active ring in the Solar System, with features changing on a timescale of hours. It is located 3,000 km (2000 miles) beyond the outer edge of the
A ring
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 entirel ...
. The ring was discovered in 1979 by the
Pioneer 11
''Pioneer 11'' (also known as ''Pioneer G'') is a robotic space probe launched by NASA on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays. It was the first probe to encounter ...
imaging team. It is very thin, just a few hundred km (miles) in radial extent. While the traditional view has been that it is held together by two
shepherd moons
''Shepherd Moons'' is the third studio album by Irish singer, songwriter and musician Enya, released on 4 November 1991 by WEA. After the unexpected critical and commercial success of her previous album ''Watermark'' (1988), Enya embarked on a w ...
,
Prometheus
In Greek mythology, Prometheus (; , , possibly meaning "forethought")Smith"Prometheus". is a Titan god of fire. Prometheus is best known for defying the gods by stealing fire from them and giving it to humanity in the form of technology, know ...
and
Pandora
In Greek mythology, Pandora (Greek: , derived from , ''pān'', i.e. "all" and , ''dōron'', i.e. "gift", thus "the all-endowed", "all-gifted" or "all-giving") was the first human woman created by Hephaestus on the instructions of Zeus. As Hes ...
, which orbit inside and outside it, recent studies indicate that only Prometheus contributes to the confinement. Numerical simulations suggest the ring was formed when Prometheus and Pandora collided with each other and were partially disrupted.
More recent closeup images from the '' Cassini'' probe show that the F Ring consists of one core ring and a spiral strand around it. They also show that when Prometheus encounters the ring at its
apoapsis
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
, its gravitational attraction creates kinks and knots in the F Ring as the moon 'steals' material from it, leaving a dark channel in the inner part of the ring (see video link and additional F Ring images in
gallery
Gallery or The Gallery may refer to:
Arts, entertainment, and media
* Art gallery
** Contemporary art gallery
Music
* Gallery (band), an American soft rock band of the 1970s
Albums
* ''Gallery'' (Elaiza album), 2014 album
* ''Gallery'' (Gr ...
). Since Prometheus orbits Saturn more rapidly than the material in the F ring, each new channel is carved about 3.2 degrees in front of the previous one.
In 2008, further dynamism was detected, suggesting that small unseen moons orbiting within the F Ring are continually passing through its narrow core because of perturbations from Prometheus. One of the small moons was tentatively identified as S/2004 S 6.
As of 2023, the clumpy structure of the ring "is thought to be caused by the presence of thousands of small parent bodies (1.0 to 0.1 km in size) that collide and produce dense strands of micrometre- to centimetre-sized particles that re-accrete over a few months onto the parent bodies in a steady-state regime."
Outer rings
Janus/Epimetheus Ring
A faint dust ring is present around the region occupied by the orbits of
Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
and
Epimetheus
In Greek mythology, Epimetheus (; grc-gre, Ἐπιμηθεύς, , afterthought) was the brother of Prometheus (traditionally interpreted as "foresight", literally "fore-thinker"), a pair of Titans who "acted as representatives of mankind". The ...
, as revealed by images taken in forward-scattered light by the ''Cassini'' spacecraft in 2006. The ring has a radial extent of about 5,000 km (3000 miles).NASA Planetary Photojourna PIA08328: Moon-Made Rings /ref> Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.
G Ring
The G Ring (see last image in
gallery
Gallery or The Gallery may refer to:
Arts, entertainment, and media
* Art gallery
** Contemporary art gallery
Music
* Gallery (band), an American soft rock band of the 1970s
Albums
* ''Gallery'' (Elaiza album), 2014 album
* ''Gallery'' (Gr ...
) is a very thin, faint ring about halfway between the
F Ring
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 entirel ...
and the beginning of the E Ring, with its inner edge about 15,000 km (10,000 miles) inside the orbit of
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
. It contains a single distinctly brighter arc near its inner edge (similar to the arcs in the
rings of Neptune
The rings of Neptune consist primarily of five principal rings. They were first discovered (as "arcs") by simultaneous observations of a stellar occultation on 22 July 1984 by André Brahic's and William B. Hubbard's teams at La Silla Observato ...
) that extends about one-sixth of its circumference, centered on the half-km (500 yard) diameter moonlet Aegaeon, which is held in place by a 7:6 orbital resonance with Mimas. The arc is believed to be composed of icy particles up to a few m in diameter, with the rest of the G Ring consisting of dust released from within the arc. The radial width of the arc is about 250 km (150 miles), compared to a width of 9,000 km (6000 miles) for the G Ring as a whole. The arc is thought to contain matter equivalent to a small icy moonlet about a hundred m in diameter. Dust released from Aegaeon and other source bodies within the arc by
micrometeoroid
A micrometeoroid is a tiny meteoroid: a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through Earth's atmosphere and reaches Earth's surface.
The term "micrometeoroid ...
impacts drifts outward from the arc because of interaction with Saturn's
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
(whose
plasma
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
corotates with Saturn's
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
, which rotates much more rapidly than the orbital motion of the G Ring). These tiny particles are steadily eroded away by further impacts and dispersed by plasma drag. Over the course of thousands of years the ring gradually loses mass, which is replenished by further impacts on Aegaeon.
Methone Ring Arc
A faint ring arc, first detected in September 2006, covering a longitudinal extent of about 10 degrees is associated with the moon Methone. The material in the arc is believed to represent dust ejected from Methone by micrometeoroid impacts. The confinement of the dust within the arc is attributable to a 14:15 resonance with Mimas (similar to the mechanism of confinement of the arc within the G ring). Under the influence of the same resonance, Methone librates back and forth in its orbit with an amplitude of 5° of longitude.
Anthe Ring Arc
A faint ring arc, first detected in June 2007, covering a longitudinal extent of about 20 degrees is associated with the moon Anthe. The material in the arc is believed to represent dust knocked off Anthe by micrometeoroid impacts. The confinement of the dust within the arc is attributable to a 10:11 resonance with Mimas. Under the influence of the same resonance, Anthe drifts back and forth in its orbit over 14° of longitude.
Pallene Ring
A faint dust ring shares Pallene's orbit, as revealed by images taken in forward-scattered light by the ''Cassini'' spacecraft in 2006. The ring has a radial extent of about 2,500 km (1500 miles). Its source is particles blasted off Pallene's surface by meteoroid impacts, which then form a diffuse ring around its orbital path.
E Ring
The E Ring is the second outermost ring and is extremely wide; it consists of many tiny (micron and sub-micron) particles of water ice with silicates, carbon dioxide and ammonia. The E Ring is distributed between the orbits of
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
and
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
. Unlike the other rings, it is composed of microscopic particles rather than macroscopic ice chunks. In 2005, the source of the E Ring's material was determined to be
cryovolcanic
A cryovolcano (sometimes informally called an ice volcano) is a type of volcano that erupts volatiles such as water, ammonia or methane into an extremely cold environment that is at or below their freezing point. The process of formation is known ...
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about 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 refl ...
. Unlike the main rings, the E Ring is more than 2,000 km (1000 miles) thick and increases with its distance from Enceladus. Tendril-like structures observed within the E Ring can be related to the emissions of the most active south polar jets of Enceladus.
Particles of the E Ring tend to accumulate on moons that orbit within it. The equator of the leading hemisphere of Tethys is tinted slightly blue due to infalling material. The trojan moons Telesto, Calypso, Helene and Polydeuces are particularly affected as their orbits move up and down the ring plane. This results in their surfaces being coated with bright material that smooths out features.
Phoebe ring
In October 2009, the discovery of a tenuous disk of material just interior to the orbit of Phoebe was reported. The disk was aligned edge-on to Earth at the time of discovery. This disk can be loosely described as another ring. Although very large (as seen from Earth, the apparent size of two full moons), the ring is virtually invisible. It was discovered using
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
's
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
Spitzer Space Telescope
The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, f ...
, and was seen over the entire range of the observations, which extended from 128 to 207 times the radius of Saturn, with calculations indicating that it may extend outward up to 300 Saturn radii and inward to the orbit of
Iapetus
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other ...
at 59 Saturn radii. The ring was subsequently studied using the WISE, '' Herschel'' and ''Cassini'' spacecraft; WISE observations show that it extends from at least between 50 and 100 to 270 Saturn radii (the inner edge is lost in the planet's glare). Data obtained with WISE indicate the ring particles are small; those with radii greater than 10 cm comprise 10% or less of the cross-sectional area.
Phoebe orbits the planet at a distance ranging from 180 to 250 radii. The ring has a thickness of about 40 radii. Because the ring's particles are presumed to have originated from impacts (
micrometeoroid
A micrometeoroid is a tiny meteoroid: a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through Earth's atmosphere and reaches Earth's surface.
The term "micrometeoroid ...
and larger) on Phoebe, they should share its
retrograde orbit
Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or ...
, which is opposite to the orbital motion of the next inner moon,
Iapetus
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other ...
. This ring lies in the plane of Saturn's orbit, or roughly the
ecliptic
The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
, and thus is tilted 27 degrees from Saturn's
equatorial plane
The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projectio ...
and the other rings. Phoebe is
inclined
Incline, inclined, inclining, or inclination may refer to:
*Grade (slope), the tilt, steepness, or angle from horizontal of a topographic feature (hillside, meadow, etc.) or constructed element (road, railway, field, etc.)
*Slope, the tilt, steepn ...
by 5° with respect to Saturn's orbit plane (often written as 175°, due to Phoebe's retrograde orbital motion), and its resulting vertical excursions above and below the ring plane agree closely with the ring's observed thickness of 40 Saturn radii.
The existence of the ring was proposed in the 1970s by
Steven Soter
Steven Soter is an astrophysicist currently holding the positions of scientist-in-residence for New York University's Environmental Studies Program and of Research Associate for the Department of Astrophysics at the American Museum of Natural Hi ...
. The discovery was made by Anne J. Verbiscer and Michael F. Skrutskie (of the
University of Virginia
The University of Virginia (UVA) is a Public university#United States, public research university in Charlottesville, Virginia. Founded in 1819 by Thomas Jefferson, the university is ranked among the top academic institutions in the United S ...
) and Douglas P. Hamilton (of the
University of Maryland, College Park
The University of Maryland, College Park (University of Maryland, UMD, or simply Maryland) is a public land-grant research university in College Park, Maryland. Founded in 1856, UMD is the flagship institution of the University System of Mary ...
). The three had studied together at
Cornell University
Cornell University is a private statutory land-grant research university based in Ithaca, New York. It is a member of the Ivy League. Founded in 1865 by Ezra Cornell and Andrew Dickson White, Cornell was founded with the intention to teach an ...
as graduate students.
Ring material migrates inward due to reemission of solar radiation, with a speed inversely proportional to particle size; a 3 cm particle would migrate from the vicinity of Phoebe to that of Iapetus over the age of the Solar System. The material would thus strike the leading hemisphere of Iapetus. Infall of this material causes a slight darkening and reddening of the leading hemisphere of Iapetus (similar to what is seen on the Uranian moons
Oberon
Oberon () is a king of the fairies in medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania, Queen of the Fair ...
and Titania) but does not directly create the dramatic two-tone coloration of that moon. Rather, the infalling material initiates a
positive feedback
Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the ...
sublimation
Sublimation or sublimate may refer to:
* ''Sublimation'' (album), by Canvas Solaris, 2004
* Sublimation (phase transition), directly from the solid to the gas phase
* Sublimation (psychology), a mature type of defense mechanism
* Sublimate of mer ...
from warmer regions, followed by vapor condensation onto cooler regions. This leaves a dark residue of "lag" material covering most of the equatorial region of Iapetus's leading hemisphere, which contrasts with the bright ice deposits covering the polar regions and most of the trailing hemisphere.
Possible ring system around Rhea
Saturn's second largest moon Rhea has been hypothesized to have a tenuous ring system of its own consisting of three narrow bands embedded in a disk of solid particles. These putative rings have not been imaged, but their existence has been inferred from ''Cassini'' observations in November 2005 of a depletion of energetic electrons in Saturn's
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
near Rhea. The Magnetospheric Imaging Instrument (MIMI) observed a gentle gradient punctuated by three sharp drops in plasma flow on each side of the moon in a nearly symmetric pattern. This could be explained if they were absorbed by solid material in the form of an equatorial disk containing denser rings or arcs, with particles perhaps several decimeters to approximately a meter in diameter. A more recent piece of evidence consistent with the presence of Rhean rings is a set of small ultraviolet-bright spots distributed in a line that extends three quarters of the way around the moon's circumference, within 2 degrees of the equator. The spots have been interpreted as the impact points of deorbiting ring material. However, targeted observations by ''Cassini'' of the putative ring plane from several angles have turned up nothing, suggesting that another explanation for these enigmatic features is needed.
Gallery
File:PIA14943-SaturnBehindTheRings-20180813.jpg, Saturn, behind the rings and draped with their shadows, as seen by ''Cassini'' from a distance of 725,000 km (450,000 miles).
File:PIA11660- Mimas' shadow cut off by B ring (trimmed).jpg, ''Cassini'' image mosaic of the unlit side of the outer C Ring (bottom) and inner
B Ring
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 entirel ...
(top) near Saturn's equinox, showing multiple views of the shadow of
Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
. The shadow is attenuated by the denser B ring. The
Maxwell Gap
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 entire ...
is below center.
File:PIA21627 - Janus 2-to-1 spiral density wave in Saturn's inner B Ring.jpg, A
spiral density wave
Density wave theory or the Lin–Shu density wave theory is a theory proposed by Chia-Chiao Lin, C.C. Lin and Frank Shu in the mid-1960s to explain the spiral arm structure of spiral galaxy, spiral galaxies. The Lin–Shu theory introduces the ide ...
in Saturn's inner B Ring which forms at a 2:1
orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
with
Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
. The wavelength decreases as the wave propagates away from the resonance, so the apparent foreshortening in the image is illusory.
File:PIA22418 - Gravity's Rainbow - Saturn's B Ring in color.jpg, Natural color view of the outer C Ring and B Ring.
File:Spokes in Saturn's B Ring.jpg, Dark
B Ring
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 entirel ...
spokes in a low- phase-angle ''Cassini'' image of the rings' unlit side. Left of center, two dark gaps (the larger being the Huygens Gap) and the bright (from this viewing geometry) ringlets to their left comprise the
Cassini Division
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 entirel ...
.
File:Srings.jpg, ''Cassini'' image of the sun-lit side of the rings taken in 2009 at a phase angle of 144°, with bright
B Ring
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 entirel ...
spokes.
File:PIA06099 Enke Gap.jpg, Pan's motion through the
A ring
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 entirel ...
's
Encke Gap
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 entirel ...
induces edge waves and (non-self-propagating)
spiral
In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point.
Helices
Two major definitions of "spiral" in the American Heritage Dictionary are:spiral density waves.
File:Daphnis makes waves - 4x vertical stretch.jpg, Radially stretched (4x) view of the
Keeler Gap
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 entire ...
edge waves induced by
Daphnis
In Greek mythology, Daphnis (; grc, Δάφνις, from , ''daphne'', "Bay Laurel") was a Sicilian shepherd who was said to be the inventor of pastoral poetry.
Family
According to tradition, he was the son of Hermes and a nymph, despite which ...
.
File:Prometheus und Pandora.jpg,
Prometheus
In Greek mythology, Prometheus (; , , possibly meaning "forethought")Smith"Prometheus". is a Titan god of fire. Prometheus is best known for defying the gods by stealing fire from them and giving it to humanity in the form of technology, know ...
(at right) and
Pandora
In Greek mythology, Pandora (Greek: , derived from , ''pān'', i.e. "all" and , ''dōron'', i.e. "gift", thus "the all-endowed", "all-gifted" or "all-giving") was the first human woman created by Hephaestus on the instructions of Zeus. As Hes ...
orbit just inside and outside the
F Ring
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 entirel ...
, but only Prometheus acts as a ring shepherd.
File:PIA12684 F Ring.png,
Prometheus
In Greek mythology, Prometheus (; , , possibly meaning "forethought")Smith"Prometheus". is a Titan god of fire. Prometheus is best known for defying the gods by stealing fire from them and giving it to humanity in the form of technology, know ...
near
apoapsis
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
carving a dark channel in the
F Ring
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 entirel ...
(with older channels to the right). A movie of the process may be viewed at the ''Cassini'' Imaging Team website or
YouTube
YouTube is a global online video platform, online video sharing and social media, social media platform headquartered in San Bruno, California. It was launched on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim. It is owned by ...
.
File:F Ring Dynamism PIA08290.jpg,
F ring
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 entirel ...
dynamism, probably due to perturbing effects of small moonlets orbiting close to or through the ring's core.
File:PIA11635- Slicing the Arc.jpg, Saturn's shadow truncates the backlit
G Ring G Ring may refer to:
* Rings of Saturn#G Ring, a planetary ring system around Saturn.
* G-ring
In commutative algebra, a G-ring or Grothendieck ring is a Noetherian ring such that the map of any of its local rings to the completion is regular ( ...
and its bright inner arc. A video showing the arc's orbital motion may be viewed on YouTube or the ''Cassini'' Imaging Team website.
File:PIA23170-Saturn-Rings-IR-Map-20190613.jpg, Saturn and its A, B and C rings in visible and (inset) infrared light. In the false-color IR view, greater water ice content and larger grain size lead to blue-green color, while greater non-ice content and smaller grain size yield a reddish hue.
See also
*
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
– the first person to observe Saturn's rings, in 1610
*
Christiaan Huygens
Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of ...
– the first to propose that there was a ring surrounding Saturn, in 1655
*
Giovanni Cassini
Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 September 1712) was an Italian (naturalised French) mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the C ...
– discovered the separation between the A and B rings (the Cassini Division), in 1675
*
Édouard Roche
Édouard Albert Roche (; 17 October 1820 – 27 April 1883) was a French astronomer and mathematician, who is best known for his work in the field of celestial mechanics. His name was given to the concepts of the Roche sphere, Roche limit, and Ro ...
– French astronomer who described how a satellite that comes within the
Roche limit
In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forc ...
Mark Showalter
Mark Robert Showalter (born December 5, 1957) is a Senior Research Scientist at the SETI Institute. He is the discoverer of six moons and three planetary rings. He is the Principal Investigator of NASA's Planetary Data System Rings Node, a co-inv ...
Saturn
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 has only one-eighth the average density of Earth; h ...