Rings of Neptune - AbsoluteAstronomy.com

The rings of Neptune consist primarily of five principal rings

Planetary ring

A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

predicted in 1984 by André Brahic and imaged in 1989 by the Voyager 2

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

spacecraft. At their densest, they are comparable to the less dense portions of Saturn's main rings

Rings of Saturn

The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...

such as the C ring and the Cassini Division, but much of Neptune's ring system is quite tenuous, faint and dusty

Cosmic dust

Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

, more closely resembling the rings of Jupiter

Rings of Jupiter

The planet Jupiter has a system of rings, known as the rings of Jupiter or the Jovian ring system. It was the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed in 1979 by the Voyager 1 space probe and thoroughly investigated in the 1990s...

. Neptune

Neptune

Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

's rings are named after astronomers who contributed important work on the planet: Galle

Johann Gottfried Galle

Johann Gottfried Galle was a German astronomer 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 planet Neptune, and know what he was looking at...

, Le Verrier, Lassell

William Lassell

William Lassell FRS was an English merchant and astronomer.Born in Bolton and educated in Rochdale after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy...

, Arago

François Arago

François Jean Dominique Arago , known simply as François Arago , was a French mathematician, physicist, astronomer and politician.-Early life and work:...

, and Adams

John Couch Adams

John Couch Adams was a British mathematician and astronomer. Adams was born in Laneast, near Launceston, Cornwall, and died in Cambridge. The Cornish name Couch is pronounced "cooch"....

. Neptune also has a faint unnamed ring coincident with the orbit of the moon Galatea

Galatea (moon)

Galatea , also known as Neptune VI, is the fourth closest inner satellite of Neptune. It is named after Galatea, one of the Nereids of Greek legend.Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe...

. Three other moons orbit between the rings: Naiad

Naiad (moon)

Naiad , also known as Neptune III, is the innermost satellite of Neptune, named after the Naiads of Greek legend.Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe. The last moon to be discovered during the flyby, it was designated...

, Thalassa

Thalassa (moon)

Thalassa , also known as Neptune IV, is the second innermost satellite of Neptune. Thalassa was named after sea goddesss Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea"....

and Despina

Despina (moon)

Despina , also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon and Demeter....

.

The rings of Neptune are made of extremely dark material, likely organic

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

compounds processed by radiation

Radiation

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

, similar to that found in the rings of Uranus. The proportion of dust in the rings (between 20% and 70%) is high, while their optical depth

Optical depth

Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

is low to moderate, at less than 0.1. Uniquely, the Adams ring includes five distinct arcs, named Fraternité, Égalité 1 and 2, Liberté, and Courage. The arcs occupy a narrow range of orbital longitude

Longitude of the periapsis

In astrodynamics, the longitude of the periapsis of an orbiting body is the longitude at which the periapsis would occur if the body's inclination were zero. For motion of a planet around the sun, this position could be called longitude of perihelion...

s and are remarkably stable, having changed only slightly since their initial detection in 1980. How the arcs are stabilized is still under debate. However, their stability is probably related to the resonant

Orbital resonance

In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

interaction between the Adams ring and its inner shepherd moon, Galatea.

Discovery and observations

The first mention of rings around Neptune dates back to 1846 when William Lassell

William Lassell

, the discoverer of Neptune's largest moon Triton

Triton (moon)

Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

, thought he had seen a ring around the planet. However, his claim was never confirmed and it is likely that it was an observational artifact. The first reliable detection of a ring was made in 1968 by stellar occultation

Occultation

An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

, although that result would go unnoticed until 1977 when the rings of Uranus

Rings of Uranus

The planet Uranus has a system of rings intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Douglas J. Mink...

were discovered. Soon after the Uranus discovery, a team from Villanova University

Villanova University

Villanova University is a private university located in Radnor Township, a suburb northwest of Philadelphia, Pennsylvania, in the United States...

led by Harold J. Reitsema began searching for rings around Neptune. On 24 May 1981, they detected a dip in a star's brightness during one occultation; however, the manner in which the star dimmed did not suggest a ring. Later, after the Voyager fly-by, it was found that the occultation was due to the small Neptunian moon Larissa

Larissa (moon)

Larissa , also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon in Greek mythology and eponymous nymph of the city in Thessaly.- Discovery :...

, a highly unusual event.

In the 1980s, significant occultations were much rarer for Neptune than for Uranus, which lay near the Milky Way

Milky Way

The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

at the time and was thus moving against a denser field of stars. Neptune's next occultation, on 12 September 1983, resulted in a possible detection of a ring. However, ground-based results were inconclusive. Over the next six years, approximately 50 other occultations were observed with only about one-third of them yielding positive results. Something (probably incomplete arcs) definitely existed around Neptune, but the features of the ring system remained a mystery. The Voyager 2

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

spacecraft made the definitive discovery of the Neptunian rings during its fly-by of Neptune in 1989. It confirmed that occasional occultation events observed before were indeed caused by the arcs within the Adams ring (see below). After the Voyager fly-by the previous terrestrial occultation observations were reanalyzed yielding features of the ring's arcs as they were in 1980s, which matched those found by Voyager almost perfectly.

Since Voyager's fly-by, the brightest rings (Adams and Le Verrier) have been imaged with the Hubble Space Telescope

Hubble Space Telescope

The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

and Earth-based telescopes, owing to advances in resolution and light-gathering power. They are visible, slightly above background noise levels, at methane

Methane

Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

-absorbing wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s in which the glare from Neptune is significantly reduced. The fainter rings are still far below the visibility threshold.

General properties

Neptune possesses five distinct rings named, in order of increasing distance from the planet, Galle, Le Verrier, Lassell, Arago and Adams. In addition to these well-defined rings, Neptune may also possess an extremely faint sheet of material stretching inward from the Le Verrier to the Galle ring, and possibly farther in toward the planet. Three of the Neptunian rings are narrow, with widths of about 100 km or less; in contrast, the Galle and Lassell rings are broad—their widths are between 2,000 and 5,000 km. The Adams ring consists of five bright arcs embedded in a fainter continuous ring. Proceeding counterclockwise, the arcs are: Fraternité, Égalité 1 and 2, Liberté, and Courage. The first three names come from "liberty, equality, fraternity

Liberté, égalité, fraternité

Liberté, égalité, fraternité, French for "Liberty, equality, fraternity ", is the national motto of France, and is a typical example of a tripartite motto. Although it finds its origins in the French Revolution, it was then only one motto among others and was not institutionalized until the Third...

", the motto of the French Revolution

French Revolution

The French Revolution , sometimes distinguished as the 'Great French Revolution' , was a period of radical social and political upheaval in France and Europe. The absolute monarchy that had ruled France for centuries collapsed in three years...

. The terminology was suggested by their original discoverers, who had found them during stellar occultations in 1984 and 1985. Four small Neptunian moons have orbits inside the ring system: Naiad

Naiad (moon)

and Thalassa

Thalassa (moon)

orbit in the gap between the Galle and Le Verrier rings; Despina

Despina (moon)

Despina , also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon and Demeter....

is just inward of the Le Verrier ring; and Galatea

Galatea (moon)

lies slightly inward of the Adams ring, embedded in an unnamed faint, narrow ringlet.

The Neptunian rings contain a large quantity of micrometer

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

-sized dust

Cosmic dust

: the dust fraction by cross-section area is between 20% and 70%. In this respect they are similar to the rings of Jupiter

Rings of Jupiter

, in which the dust fraction is 50%–100%, and are very different from the rings of Saturn

Rings of Saturn

and Uranus

Rings of Uranus

, which contain little dust (less than 0.1%). The particles in Neptune's rings are made from a dark material; probably a mixture of ice with radiation

Radiation

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

-processed organics

Organic compound

. The rings are reddish in color, and their geometrical (0.05) and Bond

Bond albedo

The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...

(0.01–0.02) albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

s are similar to those of the Uranian rings' particles and the inner Neptunian moons. The rings are generally optically thin (transparent); their normal optical depth

Optical depth

Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

s do not exceed 0.1. As a whole, the Neptunian rings resemble those of Jupiter; both systems consist of faint, narrow, dusty ringlets and even fainter broad dusty rings.

The rings of Neptune, like those of Uranus, are thought to be relatively young; their age is probably significantly less than that of the Solar System

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. Also, like those of Uranus, Neptune's rings probably resulted from the collisional fragmentation of onetime inner moons. Such events create moonlet belts, which act as the sources of dust for the rings. In this respect the rings of Neptune are similar to faint dusty bands observed by Voyager 2 between the main rings of Uranus.

Inner rings

The innermost ring of Neptune is called the Galle ring after Johann Gottfried Galle

Johann Gottfried Galle

, the first person to see Neptune through a telescope (1846). It is about 2,000 km wide and orbits 41,000–43,000 km from the planet. It is a faint ring with an average normal optical depth of around 10⁻⁴, and with an equivalent depth of 0.15 km. The fraction of dust in this ring is estimated from 40% to 70%.

The next ring is named the Le Verrier ring after Urbain Le Verrier, who predicted Neptune's position in 1846. With an orbital radius of about 53,200 km, it is narrow, with a width of about 113 km. Its normal optical depth is 0.0062 ± 0.0015, which corresponds to an equivalent depth of 0.7 ± 0.2 km. The dust fraction in the Le Verrier ring ranges from 40% to 70%. The small moon Despina

Despina (moon)

Despina , also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon and Demeter....

, which orbits just inside of it at 52,526 km, may play a role in the ring's confinement by acting as a shepherd.

The Lassell ring, also known as the plateau, is the broadest ring in the Neptunian system. It is the namesake of William Lassell

William Lassell

, the English astronomer who discovered Neptune's largest moon, Triton

Triton (moon)

. This ring is a faint sheet of material occupying the space between the Le Verrier ring at about 53,200 km and the Arago ring at 57,200 km. Its average normal optical depth is around 10⁻⁴, which corresponds to an equivalent depth of 0.4 km. The ring's dust fraction is in the range from 20% to 40%.

There is a small peak of brightness near the outer edge of the Lassell ring, located at 57,200 km from Neptune and less than 100 km wide, which some planetary scientists call the Arago ring after François Arago

François Arago

François Jean Dominique Arago , known simply as François Arago , was a French mathematician, physicist, astronomer and politician.-Early life and work:...

, a French mathematician, physicist, astronomer and politician. However, many publications do not mention the Arago ring at all.

Adams ring

The outer Adams ring, with an orbital radius of about 63,930 km, is the best studied of Neptune's rings. It is named after John Couch Adams

John Couch Adams

John Couch Adams was a British mathematician and astronomer. Adams was born in Laneast, near Launceston, Cornwall, and died in Cambridge. The Cornish name Couch is pronounced "cooch"....

, who predicted the position of Neptune independently of Le Verrier. This ring is narrow, slightly eccentric and inclined, with total width of about 35 km (15–50 km), and its normal optical depth is around 0.011 ± 0.003 outside the arcs, which corresponds to the equivalent depth of about 0.4 km. The fraction of dust in this ring is from 20% to 40%—lower than in other narrow rings. Neptune's small moon Galatea

Galatea (moon)

, which orbits just inside of the Adams ring at 61,953 km, acts like a shepherd, keeping ring particles inside a narrow range of orbital radii through a 42:43 outer Lindblad resonance

Orbital resonance

. Galatea's gravitational influence creates 42 radial wiggles in the Adams ring with an amplitude of about 30 km, which have been used to infer Galatea's mass

Mass

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

Arcs

The brightest parts of the Adams ring, the ring arcs, were the first elements of Neptune's ring system to be discovered. The arcs are discrete regions within the ring in which the particles that comprise it are mysteriously clustered together. The Adams ring is known to comprise five short arcs, which occupy a relatively narrow range of longitude

Longitude

Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

s from 247° to 294°. In 1986 they were located between longitudes of:

247–257° (Fraternité),
261–264° (Égalité 1),
265–266° (Égalité 2),
276–280° (Liberté),
284.5–285.5° (Courage).

The brightest and longest arc was Fraternité; the faintest was Courage. The normal optical depths of the arcs are estimated to lie in the range 0.03–0.09 (0.034 ± 0.005 for the leading edge of Liberté arc as measured by stellar occultation); the radial widths are approximately the same as those of the continuous ring—about 30 km. The equivalent depths of arcs vary in the range 1.25–2.15 km (0.77 ± 0.13 km for the leading edge of Liberté arc). The fraction of dust in the arcs is from 40% to 70%. The arcs in the Adams ring are somewhat similar to the arc in Saturn's G ring.

The highest resolution Voyager 2 images revealed a pronounced clumpiness in the arcs, with a typical separation between visible clumps of 0.1° to 0.2°, which corresponds to 100–200 km along the ring. Because the clumps were not resolved, they may or may not include larger bodies, but are certainly associated with concentrations of microscopic dust as evidenced by their enhanced brightness when backlit by the Sun.

The arcs are quite stable structures. They were detected by ground based stellar occultations in the 1980s, by Voyager 2 in 1989 and by Hubble Space Telescope and ground based telescopes in 1997–2005 and remained at approximately the same orbital longitudes. However some changes have been noticed. The overall brightness of arcs decreased since 1986. The Courage arc jumped forward by 8° to 294° (it probably jumped over to the next stable co-rotation resonance position) while the Liberté arc had almost disappeared by 2003. The Fraternité and Égalité (1 and 2) arcs have demonstrated irregular variations in their relative brightness. Their observed dynamics is probably related to the exchange of dust between them. Courage, a very faint arc found during the Voyager flyby, was seen to flare in brightness in 1998, while more recently it was back to its usual dimness. Visible light observations show that the total amount of material in the arcs has remained approximately constant, but they are dimmer in the infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

light wavelengths where previous observations were taken.

Confinement

The arcs in the Adams ring remain unexplained. Their existence is a puzzle because basic orbital dynamics imply that they should spread out into a uniform ring over a matter of years. Several theories about the arcs' confinement have been suggested, the most widely publicized of which holds that Galatea confines the arcs via its 42:43 co-rotational inclination resonance (CIR). The resonance creates 84 stable sites along the ring's orbit, each 4° long, with arcs residing in the adjacent sites. However measurements of the rings' mean motion with Hubble and Keck telescopes in 1998 led to the conclusion that the rings are not in CIR with Galatea.

A later model suggested that confinement resulted from a co-rotational eccentricity resonance (CER). The model takes into account the finite mass of the Adams ring, which is necessary to move the resonance closer to the ring. A byproduct of this theory is a mass estimate for the Adams ring—about 0.002 of the mass of Galatea. A third theory proposed in 1986 requires an additional moon orbiting inside the ring; the arcs in this case are trapped in its stable Lagrangian point

Lagrangian point

The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

s. However Voyager 2s observations placed strict constraints on the size and mass of any undiscovered moons, making such a theory unlikely. Some other more complicated theories hold that a number of moonlets are trapped in co-rotational resonances with Galatea, providing confinement of the arcs and simultaneously serving as sources of the dust.

Exploration

The rings were investigated in detail during the Voyager 2 spacecraft's flyby of Neptune in August 1989. They were studied with optical imaging, and through observations of occultations in ultraviolet and visible light. Voyager 2 observed the rings in different geometries relative to the Sun, producing images of back-scattered

Backscatter

In physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...

, forward-scattered

Forward scatter

In telecommunication and astronomy, forward scatter is the deflection—by diffraction, nonhomogeneous refraction, or nonspecular reflection by particulate matter of dimensions that are large with respect to the wavelength in question but small with respect to the beam diameter—of a portion of an...

and side-scattered light. Analysis of these images allowed derivation of the phase function (dependence of the ring's reflectivity on the angle between the observer and Sun), and geometrical and Bond albedo

Bond albedo

of ring particles. Analysis of Voyager's images also led to discovery of six inner moons of Neptune, including the Adams ring shepherd Galatea

Galatea (moon)

Properties

Ring name	Radius (km)	Width (km)	Eq. depth (km)	N. Opt. depth	Dust fraction,%	Ecc.	Incl.(°)	Notes
Galle (N42)	40,900–42,900	2,000	0.15	~ 10⁻⁴	40–70	?	?	Broad faint ring
Le Verrier (N53)	53,200 ± 20	113	0.7 ± 0.2	6.2 ± 1.5	40–70	?	?	Narrow ring
Lassell	53,200–57,200	4,000	0.4	~ 10⁻⁴	20–40	?	?	Lassell ring is a faint sheet of material stretching from Le Verrier to Arago
Arago	57,200	<100	?	?	?	?	?
Adams (N63)	62,932 ± 2	15–50	0.4 1.25–2.15 (in arcs)	0.011 ± 0.003 0.03–0.09 (in arcs)	20–40 40–70 (in arcs)	4.7 ± 0.2	0.0617 ± 0.0043	Five bright arcs

A question mark means that the parameter is not known.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.