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corotational

Rings of Neptune

Neptune has a faint dusty planetary ring system, which was discovered in 1989 by Voyager 2 spacecraft. The Neptunian ring system is simple as compared to the rings of Saturn and Uranus, but it resempbles the ring system of Jupiter. As of 2008 the ring system of Neptune is known to consist of 5 named rings. By the distance from the planet they are: Galle, LeVerrier, Lassell, Arago and Adams rings. These rings were named after astronomers, who contributed heavly into the study of Neptune and its system. There also exists a faint unnamed ring coincident with the orbit of Neptunian moon Galatea. Three other moons orbit between the rings (Naiad, Thalassa and Despina).

The rings of Neptune are made of the extremely dark material—probably, radiation processed organics similar to that found in the rings of Uranus. The fraction of dust in the rings is high—20–70%. The optical depth of the rings is low—it does not exceed a few hundredth. The unusual five ring arcs are present in the outer Adams ring. The arc's names—Fraternite, Egalite 1 and 2, Liberte, and Courage—mean liberty, equality, and brotherhood after the famous motto of the French Republic and French Revolution. They occupy a narrow range in orbital longitudes and are remarkably stable—the arcs were first detected in 1980-th and since then changed only slightly. The mechanism that maintaines stability of the arcs is still undeer debate. However it probably related to the resonant interaction between Adams ring and its inner shepherd moon Galatea.

Discovery and observations

The first mention of the rings around Neptune dates back to 1846. William Lassell, a discoverer of Triton, claimed that he had seen a ring. However his observation has never been confirmed and was likely an observational artifact. The first reliable detection of a ring was made in 1968 by stellar occultation, although it had gone unnoticed until 1977, when the rings of Uranus were discovered. Soon after that discovery a team from Villanova University led by Harold J. Reitsema began searching for the rings around Neptune. On 24 May 1981 they detected a dip in a star brightness during one of the occultations. However, it looked different from what could be expected from a ring. Later, after Voyager fly-by, it was found that they the occulting body was Neptunian small moon Larissa—a highly unlikely event.

In 1980-th the occultations were much rarer for Neptune as compared to Uranus, which moved in the middle of the Milky Way. The next such event happened on 12 September 1983 and resulted in a possible detection of a ring. Until Voyager fly-by in August 1989 around 50 other occultations were observed with only about one third of them yielding positive results. So the ground based results were inconclusive—something (probably incomplete arcs) definitely existed around Neptune, but the parameters of the ring system remained a mystery. The definite discovery of the Neptunian ring system was made only by Voyager 2 spacecraft during its historic fly-by of Neptune in 1989. It confirmed that the occasional occultation events observed before were indeed cause by the arcs of the Adams ring (see below).

Recently, the brightest rings (Adams and LeVerrier) have been imaged with Hubble Space Telescope and Earth-based telescopes, owing to advances in resolution and light-gathering power. They are visible slightly above noise levels in methane absorbing wavelengths at which the glare from Neptune is significantly reduced. The fainter rings are still far below the visibility threshold.

General properties

The ring system of Neptune consists of five distinct rings. In order of the increasing distance from the planet they are: Galle, LeVerrier, Lassell, Arago and Adams rings. In addition to these well-defined rings there may exist an extremely faint sheet of material stretching inward from the LeVerrier to Galle ring and, possibly further to Neptune. Three of the Neptunian rings are narrow—their widths are about 100 km or less. In contrast, Galle and Lassell rings are broad—their widths are 2000–5000 km. The outer Adams ring is azimuthally inhomogeneous. It consists of five bright arcs embedded into a fainter continuous ring. In the direction of the orbital motion (counterclockwise) the arcs are: Fraternite, Egalite 1 and 2, Liberte, and Courage, which mean liberty, equality, and brotherhood after the famous motto of the French Republic and French Revolution. The terminology was suggested by their original discoverers, who had found them during stellar occultations in 1984 and 1985. The orbits of four neptunian small moons lie inside the ring system. Naiad and Thalassa orbit in the gap between Galle and LeVerrier rings; Despina—just inward of LeVerrier ring; and Galatea—slightly inward of the Adams ring. Galatea is embedded into an unnamed narrow and faint ringlet.

The neptunian rings contain a lot of micrometer-sized dust: dust fraction (by cross-section area) is from 20% to 70%. In this respect they are similar to the rings of Jupiter, where dust fraction is 50–100%, and are very different from the rings of Saturn and Uranus, which contain little dust. The particles in the rings of Neptune are made from a dark material—probably, a mixture of ice with radiation processed organics. The rings are a red in color, and their geometrical (0.05) and bond (0.01–0.02) albedo is similar to that of Uranian rings particles and to inner Moons of Neptune neptunian moons. The rings are generally optically thin (transparent)—their normal optical depths do exceed 0.1. As a whole the neptunian rings resemble those of Jupiter; both systems consist of faint narrow dusty ringlets and even more faint broad dusty rings.

The rings of Neptune like the rings of Uranus are thought to be relatively young. Their age is probably smaller than the age of the Solar System. The rings are likely result from the collisional fragmentation of inner moons of Neptune. Such events create moonlet belts, which act as the sources of dust for the rings. In this respect rings of Neptune are similar to faint dusty bands observed in the rings of Uranus.

Inner rings

Galle

Galle ring is the innermost ring of Neptune. It is named after Johann Galle—a co-discoverer of Neptune. The width of this ring is about 2 000 km and orbital radius is 41 000–43 000 km. It is a faint ring with the average normal optical depth of around 10−4, and with the equivalent depth of 0.15 km. The fraction of dust in this ring is from 40 to 70%.

LeVerrier

The orbital radius of this ring is about 53 200 km. It is named after Urbain Le Verrier, who predicted the position of the Neptune in 1846. It is a narrow ring with width of about 113 km. The normal optical depth is around 0.006 2 ± 0.001 5, which corresponds to the equivalent depth of 0.7 ± 0.2 km. The fraction of dust in this ring is from 40 to 70%. Small Neptunian moon Despina, which orbits just inside of it at 52 526 km, may play a role in its confinement acting like a shepherd.

Lassell

Lassell ring also known as plateau is the broadest ring in the Neptunian system. It is named after William Lassell—a English mathematician, physicist, astronomer and politician. It looks like a faint sheet of material occupying the space between LeVerrier ring at about 53 200 km and Arago ring at 57 200 km. The average normal optical depth is around 10−4 which corresponds to the equivalent depth of 0.4 km. The fraction of dust in this ring is in the range from 20 to 40%.

Arago

There is a small peak of brightness near the outer edge of the Lassell ring located at 57 200 km. It is named after François Arago—a discoverer of Neptune. Some planetary scientists call it Arago ring. However in many publications this ring is not mentioned at all. The width of this ring is less than 100 km.

Adams ring

The orbital radius of the Adams ring is about 63 930 km. It is named after John Couch Adams—a co-discoverer of Neptune. It is a narrow, slightly eccentric and inclined ring with total width of about 35 km (15–50 km). 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. Small Neptunian moon Galatea, which orbits just inside of it at 61 953 km, acts like a shepherd keeping ring particles inside a narrow range of orbital radii. The confinement is due to action of the 42:43 outer Lindbland resonance.

The Adams ring has 42 radial wiggles with an amplitude of about 30 km also caused by the gravitational influence of Galatea. They have been used to infer Galatea's mass..

Arcs

The brightness of the Adams ring depends of the longitude strongly. The brightest parts of the ring are called ring arcs. As of 2008 there exist five short arcs, which occupy a relatively narrow range of longitudes from 247° to 294°. In 1986 they were located between 247–257° (Fraternite), 261–264° (Égalité 1), 265–266° (Égalité 2), 276–280° (Liberte), and 284.5–285.5° (Courage) longitudes. The brightest and longest arc was Fraternite; the faintest was Courage. The normal optical depths of arcs are estimated to lie in the range 0.03–0.09 (0.034 ± 0.005 for the leading edge of Liberte arc as measured by Voyager 2 stellar occultation), the radial widths are approximately same as the width of the continuous ring—about 30 km. The fraction of dust in this ring 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 pronounced clumpiness in the arcs, with the typical separation between visible clumps being 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 1980-th, 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. Courage arc jumped forward by 8 to 294° (it appears to have jumped over to the next stable corotation resonance position), while Liberte arc had almost disappeared by 2003. Fraternite and Égalité (1 and 2) arcs have demonstrated irregular variations in the relative brightness. The observed dynamics is probably related to the exchange of the dust between the arcs. Courage, a very faint arc during the Voyager flyby, was seen to flare in brightness in 1998, while more recently it was back to its usual dimness. However, visible light observations show that the total amount of material in the arcs has remained approximately constant, but they are dimmer in the infra-red where previous observations were taken.

Confinement

The existence of arcs was initially a puzzle because basic orbital dynamics implies that they should spread out into a uniform ring in just several years. There exist several theories about the arc’s confinement. The most widely publicized model holds that Galatea confines the arcs by the action of 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 ring’s mean motion done in 1998 with Hubble Space Telescope and from the ground by Keck telescope led to a conclusion that the rings are not in CIR with Galatea.

Later a model was proposed where confinement is provided by co-rotational eccentricity resonance (CER). The model takes into account the finite mass of the Adams ring, which is necessary to move the resonance close to the ring. A byproduct of this theory is a mass estimate for the Adams ring—about 0.002 of the mass of Galatea. There exists the third theory proposed in 1986 that requires an additional moon orbiting inside the ring; the arcs in this case are trapped in its Lagrangian points. However Voyager 2 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 is trapped in corotational resonances with Galatea providing confinement of the arcs and simultaneously serving as sources of the dust. So as of July 2008 the arcs in the Adams remain unexplained.

Exploration

The rings were thoroughly investigated during the Voyager 2 spacecraft's flyby of Neptune in August 1989. Rings were studied by analysing results of otical imaging, ultraviolet and optical occultations. Voyager 2 observed the rings in different geometries relative to the sun, producing images of back-scattered, forward-scattered and side-scattered light. Analysis of these images allowed derivation of the phase function, geometrical and bond albedo of ring particles. Analysis of Voyager's images also led to discovery of 6 inner moons of Neptune, including including Galatea that is a shepherd of Adams ring.

Properties

This table summarizes the properties of the planetary ring system of Neptune.
Ring name Radius (km) Width (km) Eq. depth (km) N. Opt. depth Dust fraction,% Ecc. Incl.(°) Notes
Galle 40 900–42 900 2 000 0.15 ~ 10−4 40–70 ? ? Broad faint ring
LeVerrier 53 200 ± 20 110 0.7 ± 0.2 0.006 2 ± 0.001 5 40–70 ? ? Narrow ring
Lassell 53 200–57 200 4 000 0.4 ~ 10−4 20–40 ? ? Faint ring lying between LeVerrier and Arago rings
Arago 57 200 <100 ? ? ? ? ? Bright outer edge of the Lassell ring
Adams 62 932 ± 2 15–35 0.4

0.77 ± 0.13 (in arcs)

0.011 ± 0.003

0.03–0.09 (in arcs)

20–40

40–70 (in arcs)

0.00047 ± 0.00002 0.0617 ± 0.0043 Contains five bright arcs

Notes

  1. The normal optical depth τ of a ring is the ratio of the total geometrical cross-section of the ring's particles to the square area of the ring. It assumes values from zero to infinity. A light beam passing normally through a ring will be attenuated by the factor e−τ.
  2. The equivalent depth ED of a ring is defined as an integral of the normal optical depth across the ring. In other words ED=∫τdr, where r is radius.
  3. The equivalent depth of Galle and Lassel rings is a product of their width and the normal optical depth.

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References

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