Rings of Neptune
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Neptune has a faint planetary ring system consisting of several distinct rings, and the unusual ring arcs present in the outer Adams ring. The ring particles are unusually dark and contain a large proportion of microscopic dust.
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[edit] Discovery and observations
The first indication of rings around Neptune were stellar occultation observations. However, although about 50 were tracked prior to the Voyager 2 flyby, only five in the early 1980s gave any indication of rings, showing an extra "blink" just before or after the planet occulted the star. This was evidence that the rings are incomplete (or transient).[1][2]
The flyby of Neptune by Voyager 2 in 1989 provided nearly all of our current knowledge about the rings, and also settled the incompleteness issue by imaging the ring arcs. Dustiness was assessed by comparing the brightness of the rings in forward and backscattered light (microscopic dust brightens when backlit by the Sun, while macroscopic particles become dark because only their "night" side is visible). For the outer planets only spacecraft can get the backlit view required for such an analysis.
Recently, the brightest parts of the rings (the arcs in the Adams ring) have been imaged with 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.
[edit] Inventory
| Name | orbital radius (km) |
Width (km) | Optical depth | Dust fraction | Named after |
|---|---|---|---|---|---|
| Galle | 41,900 | 2000 | 0.00008 | 0.4 - 0.75 | Johann Galle |
| uncertain[3] | < ≈ 50,000 | wide | |||
| LeVerrier | 53,200 | 110 | 0.002 | 0.4 - 0.8 | Urbain Le Verrier |
| Lassell | 53,200 — 57,200 | 4000 | 0.00015 | 0.13 - 0.45 | William Lassell |
| Arago | 57,200 | <100 | François Arago | ||
| unnamed | 61,950 | narrow | |||
| Adams | 62,933 | 50 | 0.0045 | 0.17 - 0.55 | John Couch Adams |
| Name | Width (km)[4] 1989 |
Relative Longitude | Length | Strength | Notes | |||
|---|---|---|---|---|---|---|---|---|
| 1989[5] | 2003[6] | 1989 | 2003 | 1989 | 2003 | |||
| Fraternité | 15 | ~0° | ~0° | 10° | ~8° | strong | strong | "Trailing" arc |
| Égalité | 15 | ~11° | ~13° | ~5° | ~8° | strong | strong | "Middle" arc |
| Liberté | 15 | ~26° | ~25° | 4° | ~4° | strong | weak | "Leading" arc |
| Courage | 15 | ~33° | ~41° | ~2° | ~4° | weak | weak | |
| All arcs have optical depths of the order of 0.12 and Dust fractions 0.4 − 0.8 | ||||||||
The rings are named after astronomers who made significant contributions to the study of Neptune.
Four of Neptune's moons: Naiad, Thalassa, Despina, and Galatea orbit within the ring region.
[edit] Inner rings
The ring structures proceeding outward away from Neptune are:[7]
The innermost Galle Ring (1989 N3R) is faint and poorly understood. It lies well within the orbit of the innermost of Neptune's moons, Naiad. The Galle Ring is very dusty, similarly to the ring arcs. (Voyager images appear to also indicate a broad disc of diffuse material extending inside of the 50,000 km radius which envelops the Galle ring, but it difficult to distinguish it unambiguously from Neptune's glare, and hence its presence is uncertain.[3])
The narrow LeVerrier ring (1989 N2R) is the second most prominent of Neptune's rings, and lies just outside the orbit of the moon Despina by about 700 km. It is very dusty, similarly to the ring arcs.
The broad Lassell Ring (1989 N4R), is a faint sheet extending for 4000 km inwards of about 59,200 km radius. It is dusty but less so than some of the other rings, being comparable in this respect to the continuous portion of the Adams ring. There is a brightness enhencement on its outer edge which has been called the Arago Ring (1989 N5R), and the Lassell Ring's inner edge abuts the LeVerrier Ring. [8][4]
An unnamed, indistinct, clumpy ring of dust lies in the orbit of Galatea, 1000 km inside the Adams ring.
[edit] Adams ring and the Ring Arcs
The most prominent ring is the narrow outer Adams ring, although in comparison with Saturn's or Uranus' rings, it is still very faint. Its original designation was 1989 N1R. The Adams ring is about 1000 km outside the orbit of Galatea.
It includes several longitudinal "arcs" spanning 4-10° each along their length that are much brighter and much more opaque than the remainder of the ring. These are somewhat similar to the arc in Saturn's G ring, and the method of their longitudinal confinement has been a matter of lively scientific investigation. Their existence was initially a puzzle because basic orbital dynamics implies that they should spread out into a uniform ring over very short timescales. It now appears that resonances with the satellite Galatea are largely or wholly responsible for the confinement, although the details are still somewhat uncertain. The leading theory for how the arcs are confined longitudinally invokes a 43:42 corotation eccentricity resonance with Galatea that produces 43 equally spaced stable regions of length 8.37°. The influence of an additional associated inclination resonance is invoked to make the ~4° width of the shorter arcs plausible [9].
The three most prominent arcs are named Liberté, Égalité, and Fraternité (Liberty, equality, and brotherhood after the famous motto of the French Republic and French Revolution), a terminology suggested by their original discoverers who had found them during stellar occultations in 1984 and 1985 [1]. All the arcs are clustered close by, altogether spanning under 40° of longitude.
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 [3].
As with all of Neptune's rings, microscopic dust is an important constituent. While there is already much dust in the continuous background ring, its importance is markedly even greater in the arcs, where the dust accounts for the majority of scattered light. This is in contrast to e.g. Saturn's main rings which contain much less than 1% dust. The ring is very red, and the diffuse background ring varies in brightness with longitude, being about 50% dimmer roughly opposite the ring arcs [10].
The Adams ring has 42 radial wiggles with an amplitude of about 30 km. These are caused by the gravitational influence of the moon Galatea that orbits just 1000 km inside the ring. Their amplitude has been used to infer Galatea's mass.[5].
[edit] Arc dynamics
With the advent of the Hubble Space Telescope and adaptive optics ground-based telescopes the ring arcs have been re-observed several times, starting in July 1998.[11][12][6][13][14] This has revealed that they are surprisingly dynamic, changing significantly over only a few years: Fraternité and Égalité have been swapping material and changing their lengths appreciably. Even more dramatically, Liberté appears to be fading away, being only 30% of its original 1989 brightness in 2003, and barely visible in Hubble Space Telescope images from June 2005. In the meantime, it seems to have acquired a split two-humped profile, and wandered several degrees closer to the more stable Égalité. 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 but had advanced by an additional 8° ahead of the other arcs. That is, it appears to have jumped over to the next stable corotation resonance position. There were also some indications that the ring arcs may have been generally fading away.[6][13] 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.[14]
The dynamics of the ring arcs are not presently understood.
[edit] External links
[edit] References
- ^ a b B. Sicardy et al (1991). "Neptune's Rings, 1983-1989 Ground-Based Stellar Occultation Observations". Icarus 89: 220. doi:.
- ^ P.D. Nicholson et al (1990). "Five Stellar Occultations by Neptune: Further Observations of Ring Arcs". Icarus 87: 1. doi:.
- ^ a b c d B.A. Smith et al (1989). "Voyager 2 at Neptune: Imaging Science Results". Science 246: 1422. doi:.
- ^ a b c Nasa Neptunian rings factsheet
- ^ a b C.C. Porco (1991). "An Explanation for Neptune's Ring Arcs". Science 253: 995. doi:.
- ^ a b c I. de Pater et al. (2005). "The dynamic neptunian ring arcs: evidence for a gradual disappearance of Liberté and resonant jump of courage". Icarus 174: 263.
- ^ Planetary Society description
- ^ IAUC 4867
- ^ F. Namouni & C. Porco (2002). "The confinement of Neptune's ring arcs by the moon Galatea". Nature 417: 45. doi:.
- ^ M.R. Showalter & J.N. Cuzzi (1992). "Physical Properties of Neptune's Ring System". Bulletin of the American Astronomical Society 24: 1029.
- ^ B. Sicardy et al (1999). "Images of Neptune's ring arcs obtained by a ground-based telescope". Nature 400: 731. doi:.
- ^ C. Dumas et al (1999). "Stability of Neptune's ring arcs in question". Nature 400: 733. doi:.
- ^ a b "Neptune's rings are fading away" (2005). New Scientist 2492: 21.
- ^ a b M.R. Showalter et al (2005). "Updates on the dusty rings of Jupiter, Uranus and Neptune". Dust in Planetary Systems, Proceedings of the conference held September 26-28, 2005 in Kaua'i, Hawaii: 130.
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