Rings of Uranus
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The planet Uranus has a complex system of rings. This system is 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. More than 200 years ago, William Herschel reported that he too observed the rings around Uranus, but while there is some similarity between the rings as observed by Hershel and the real outer rings of Uranus, modern astronomers are sceptical that he could actually have noticed them, as they are very dark and faint. Two additional rings were discovered in 1986 by the Voyager 2 spacecraft, and two outer rings were found in 2003-2005 by the Hubble Space Telescope.
As of 2008 the Uranian ring system is known to consist of 13 distinct rings. In the order of increasing distance from the planet they are: ζ/1986U2R, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ rings. Their radii range from about 38,000 km for the ζ/1986U2R ring to about 98,000 km for the outermost μ ring. Additional faint dust bands and incomplete arcs may exist between the main rings. The rings of Uranus are extremely dark—the bond albedo of the rings particles does not exceed 2%. They are likely composed of water ice with addition of some dark material—probably radiation processed organics.
The majority of Uranus's rings are narrow and optically dense—they are only a few kilometres wide and have optical depth on order of unity. The ring system overall contains little dust; it consists of large bodies of 0.2–20 m in diameter. However some rings are optically thin: broad and faint ζ/1986U2R, μ, ν rings are made of small dust, while narrow and faint λ ring also contain larger bodies. The paucity of dust in the ring system is caused by the aerodynamic drag from the extended Uranian exosphere—corona, which quickly removes dust particles constantly created in collisions.
The rings of Uranus are thought to be relatively young. Their age cannot exceed 600 million years. The mechanism that confines the narrow rings is not well understood. Initially it was assumed that every ring should have a pair of moons (shepherds) corralling them into shape. However in 1986 Voyager 2 discovered such moons only near the brightest epsilon ring. The moons are now known as Cordelia and Ophelia. The origin of the Uranian ring system is probably connected with the collisional fragmentation of moons that once existed around Uranus. After colliding, the moons fragmented into numerous particles, which probably survived only in strictly confined stable zones around the planet, forming narrow and optically dense rings.
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[edit] Discovery
Uranus's ring system was the second to be discovered in the Solar System after that of the Saturn.[1] William Herschel claimed to have seen rings in 1789, however this is doubtful as in the two following centuries no rings were noted by other observers. The ring system was definitively discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. However, when their observations were analyzed, they found that the star disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet.[2] The rings were directly imaged when Voyager 2 spacecraft passed Uranian system in 1986.[3] Voyager 2 also discovered two additional faint rings bringing the total number to eleven.[3]
In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. this "outer ring system" is much farther from the planet than the inner rings.[4] Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13.[5]
[edit] General properties
The ring system of Uranus comprises thirteen distinct rings. In order of increasing distance from the planet they are: 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν, μ rings.[4] The Uranian rings can be divided into three groups: nine narrow main rings (6, 5, 4, α, β, η, γ, δ, ε),[1] two dusty rings (1986U2R/ζ, λ)[6] and two outer rings (μ, ν).[4][7] The rings of Uranus mainly consist of large particles and but little dust,[8] although the dust is known to be present in 1986U2R/ζ, η, δ, λ, ν and μ rings.[6][4] In addition to these well-known rings there may be numerous optically thin dust bands and faint rings between them.[9] These faint rings and dust bands may exist only temporarily or consist of a number of separate arcs, which are sometimes detected during occultations.[9] Some of them became visible during a series of ring plane-crossing events in 2007.[10] A few dust bands between the rings were observed in forward-scattering[a] geometry by Voyager 2.[3] All rings of Uranus show azimuthal brightness variations.[3]
The rings of Uranus are made of an extremely dark material. The geometric albedo of the ring particles does not exceed 5–6%, while the bond albedo is even lower—about 2%.[8][11] The rings particles demonstrate a steep opposition surge—an increase of the albedo when the phase angle is close to zero.[8] This means that their albedo is much lower, when they observed slightly off the opposition. The rings are slightly red in the ultraviolet and visible parts of the spectrum and grey in near-infrared.[12] They don’t demonstrate any identifiable spectral features. The chemical composition of the ring particles is not known. However they can not be made of pure water ice like the rings of Saturn because they are too dark, darker than the inner moons of Uranus.[12] This indicates that they are probably composed of a mixture of the ice and a dark material. The nature of this material is not clear, but it may be made of organic compounds considerably darkened by the charged particle irradiation from the Uranian magnetosphere. The rings particles may consist of a heavily processed material, which was initially similar to that of the inner moons.[12]
As a whole the ring system of Uranus is distinct from the ring systems of Jupiter and Saturn. The former planet has a number of faint dusty rings, and the latter has broad and complex rings made of very bright material—water ice.[1] However the narrow and dark rings of Uranus do resemble some parts of the saturnian ring system. The closest similarity exits between the F ring of Saturn and the ε ring—both rings are narrow, relatively dark and are shepherded by the moons.[1] The newly discovered outer rings of Uranus are similar to the outer G and E rings of Saturn.[13] Narrow ringlets existing in the broad saturnian rings also resemble the narrow rings of Uranus.[1] In addition, dust bands observed between the main rings of Uranus may be similar to the rings of Jupiter.[6] In contrast, the Neptunian ring system is quite similar to the Uranian rings. However the rings of Neptune are less complex, darker and contain more dust. They are also positioned further from the planet than the rings of Uranus.[6]
[edit] Narrow main rings
[edit] ε ring
The ε ring is the brightest and densest part of the Uranian ring system, and is responsible for about two-thirds of the light reflected by the rings.[3][12] While it is the most eccentric of the Uranian rings, it has negligible orbital inclination.[14] The ring's eccentricity causes its brightness to vary over the course of its orbit. The radially integrated brightness of the ε ring is highest near apoapsis and lowest near periapsis.[15] The maximum/minimum brightness ratio is about 2.5–3.0.[8] These variations are connected with the variations of the ring width, which is 19.7 km at the periapsis and 96.4 km at the apoapsis.[15] As the ring becomes wider the amount of shadowing between particles decreases and more of them come into view, leading to higher integrated brightness.[11] The width variations were measured directly from Voyager 2 images, as the ε ring was only one of the two rings resolved by Voyager’s cameras.[3] Such a behaviour indicates that the ring is not optically thin. Indeed occultation observations conducted from the ground and the spacecraft showed that its normal optical depth[c] varies between 0.5 and 2.5,[16][15] being the highest near the periapsis. The equivalent depth[d] of the ε ring is around 47 km. The equivalent depth is invariant along the orbit.[15]
The geometrical thickness of the ε ring is not precisely known, although the ring is certainly very thin—by some estimates as thin as 150 m.[9] Despite such a infinitesimal thickness it consists of several layers of particles. The ε ring is a rather crowded place with a filling factor estimated by different sources from 0.008 to 0.06 near the apoapsis.[15] The mean size of the ring particles is 0.2–20 m.[9] The mean separation of the particles is about 4.5 times of their radius.[15] The ring is almost devoid of dust, possibly due to the aerodynamical drag from Uranus's extended atmospheric corona.[17] Due to its razor thin nature the ε ring disappears when viewed edge on. This happened in 2007 when a ring plane-crossing event was observed.[10]
The Voyager 2 spacecraft observed a strange signal from the ε ring during the radio occultation experiment.[16] The signal looked like a strong enhancement of the forward-scattering at the wavelength 3.6 cm near ring’s apoapsis. Such strong scattering requires the existence of a coherent structure. That the ε ring does have such a fine structure has been confirmed by many occultation observations.[9] The ε ring seems to consist of a number of narrow and optically dense ringlets, some of which may have incomplete arcs.[9]
The ε ring is known to have interior and exterior shepherd moons—Cordelia and Ophelia, respectively.[18] The inner edge of the ring is in 24:25 resonance with Cordelia. The outer edge of the ring is in 14:13 resonance with Ophelia.[18] To confine the ring effectively, the masses of the moons should be at least three times the mass of the ring.[1] The mass estimates of the ε ring vary in the range 1015–1016 kg.[18][1]
[edit] δ ring
The δ ring is slightly eccentric and inclined.[14] It shows significant unexplained azimuthal variations in normal optical depth and width.[9] One possible explanation is that the ring has an azimuthal wave-like structure, excited by a small moonlet just inside it.[19] The sharp outer edge of the δ ring is in 23:22 resonance with Ophelia.[20] The δ ring consists of two components: a narrow optically dense component and a broad inward shoulder with low optical depth.[9] The width of the narrow component is 4.1–6.1 km and the equivalent depth is about 2.2 km, which corresponds to a normal optical depth of about 0.3–0.6.[15] The ring's broad component is about 10–12 km wide and its equivalent depth is close to 0.3 km indicating a low normal optical depth of 3×10−2.[15][21] It is known only from the occultation data because Voyager 2's imaging experiment failed to resolve the δ ring.[3][21] When observed in forward-scattering geometry by Voyager 2, the δ ring appeared relatively bright, which is compatible with the presence of dust in its broad component.[3] The broad component is geometrically thicker than the narrow component. This is supported by the observations of a ring plane-crossing event in 2007, when the δ ring increased in brightness consistent with the behaviour of a geometrically thick and simultaneously optically thin ring.[10] The outer edge of δ ring is located near 23:22 resonance with Cordelia.[20]
[edit] γ ring
The γ ring is narrow, optically dense and slightly eccentric. Its orbital inclination is almost zero.[14] The width of the ring varies in the range 3.6–4.7 km, although equivalent optical depth is constant at 3.3 km.[15] The normal optical depth of the γ ring is 0.7–0.9. During a ring plane-crossing event in 2007 the γ ring disappeared, which means it is geometrically thin like the ε ring[9] and devoid of dust.[10] The width and normal optical depth of the γ ring show significant azimuthal variations.[9] The mechanism of confinement of such a narrow ring is not known, but it has been noticed that the outer edge of the γ ring is in a 6:5 resonance with Ophelia.[20]
[edit] η ring
The η ring has zero orbital eccentricity and inclination.[14] Like δ ring it consists of two components: a narrow optically dense component and a broad outward shoulder with low optical depth.[3] The width of the narrow component is 1.9–2.7 km and equivalent depth is about 0.42 km, which correspond to the normal optical depth of about 0.16–0.25.[15] The broad component is about 40 km wide and its equivalent depth is close to 0.85 km indicating a low normal optical depth of 2×10−2.[15] It was resolved by Voyager 2 imaging experiment.[3] In forward-scattered light the η ring looked bright, which indicated the presence of lots of dust in this ring, probably, in the broad component.[3] The broad component is much thicker (geometrically) than the narrow one. This conclusion is supported by the observations of a ring plane-crossing event in 2007, when the η ring demonstrated increased brightness becoming the second brightest feature in the ring system.[10] This is consistent with the behaviour of a geometrically thick but simultaneously optically thin ring.[10] Like majority of other rings the η ring shows significant azimuthal variations in the normal optical depth and width. For instance, the narrow component vanishes in some places.[9]
[edit] α and β rings
After the ε ring, the α and β rings are the brightest of Uranus's rings.[8] Like the ε ring, they exhibit regular variations of the brightness and width.[8] They are brightest and widest 30° from the apoapsis and dimmest and narrowest 30° from the periapsis.[3][22] The α and β rings have sizable orbital eccentricity and non-negligible inclination.[14] The widths of these rings are 4.8–10 km and 6.1–11.4 km, respectively.[15] The equivalent optical depths are 3.29 km and 2.14 km resulting in normal optical depths 0.3–0.7 and 0.2–0.35, respectively.[15] During a ring plane-crossing event in 2007 the rings disappeared, which means they are geometrically thin like the ε ring and devoid of dust.[10] However the same event revealed a thick and optically thin dust band just outside the β ring, which was also noticed earlier by Voyager 2.[3]
[edit] 6, 5 and 4 rings
The 6, 5 and 4 rings are the innermost and dimmest of Uranus's narrow rings.[8] They are the most inclined rings, and their orbital eccentricities are the largest excluding the ε ring.[14] In fact, their inclinations (0.06°, 0.05° and 0.03°) were large enough for Voyager 2 spacecraft to observe their elevations above the Uranian equatorial plane, which were 24–46 km.[3] The 6, 5 and 4 rings are also the narrowest rings of Uranus measuring 1.6–2.2 km, 1.9–4.9 km and 2.4–4.4 km, respectively.[3][15] Their equivalent depths are 0.41 km, 0.91 and 0.71 km resulting in normal optical depth 0.18–0.25, 0.18–0.48 and 0.16–0.3.[15] The rings seem to be geometrically thin and free of dust and were not visible during a ring plane-crossing event in 2007.[10]
[edit] Dusty rings
[edit] λ ring
The λ ring was one of two rings discovered by Voyager 2 spacecraft in 1986.[14] It is a narrow, faint ring located just inside the ε ring, between it and the shepherd moon Cordelia.[3] This moon actually clears a dark lane just inside the λ ring. When viewed in back-scattered light[b], the λ ring is extremely narrow—about 1–2 km—and has the equivalent optical depth 0.1–0.2 km at the wavelength 2.2 μm.[17] The normal optical depth is 0.1–0.2.[3][21] The optical depth of the λ ring shows strong wavelength dependence, which is atypical for the Uranian ring system. It is as high as 0.36 km in the ultraviolet part of the spectrum, which explains why it was initially detected only in the UV stellar occultations by Voyager 2.[21] The detection during a stellar occultation at the wavelength 2.2 μm was only claimed in 1996.[17]
The appearance of the λ ring changed dramatically when it was observed in forward-scattered light in 1986.[3] In this geometry the ring became the brightest feature of the Uranian ring system, outshining the ε ring.[6] This observation, together with the wavelength dependence of the optical depth, indicates that the λ ring contains significant amount of the micrometre-sized dust.[6] The normal optical depth of this dust is 10−4–10−3.[8] Observations in 2007 by the Keck telescope during the ring plane-crossing event confirmed this conclusion, because the λ ring became one of the brightest features in the Uranian ring system.[10]
Detailed analysis of the Voyager 2 images revealed azimuthal variations in the brightness of the λ ring.[8] The variations appear to be periodical, resembling a standing wave. The origin of this fine structure in the λ ring remains unknown.[6]
[edit] 1986U2R/ζ ring
In 1986 Voyager 2 noticed a broad and faint sheet of material inward of the 6 ring.[3] This ring was given the temporary designation 1986U2R. It had the normal optical depth of 10−3 or less and was extremely faint. In fact, it was visible only in a single Voyager 2 image.[3] The ring was located between 37,000 and 39,500 km from the centre of Uranus or only about 12,000 km from the clouds.[17] It had not been observed until in 2003–2004, when the Keck telescope found a broad and faint sheet of material just inside the 6 ring. This ring was dubbed ζ ring.[17] However the position of the recovered ζ ring differs significantly from that observed in 1986. Now it is situated between 37,850 and 41,350 km from the centre of the planet. There is an inward gradually fading extension reaching to at least 32,600 km.[17]
The ζ ring was observed again during the ring plane-crossing event in 2007 when it became the brightest feature of the ring system, outshining all other rings combined.[10] The equivalent optical depth of this ring is near 1 km (0.6 km for the inward extension), while the normal optical depth is again less then 10−3.[17] Rather different appearances of the 1986U2R and ζ rings may be caused by different viewing geometries: back-scattering geometry in 2003–2007 and side-scattering geometry in 1986.[17][10] However, changes during the last 20 years in the distribution of the dust, which is thought to predominate in the ring, cannot be ruled out.[10]
[edit] Other dust bands
In addition to the 1986U2R/ζ and λ rings, there are other extremely faint dust bands in the Uranian ring system.[3] They are invisible during occultations because they have negligible optical depth, though they are bright in forward-scattered light.[6] Voyager 2's images of forward-scattered light revealed the existence of bright dust bands between the λ and δ rings, between the η and β rings, and between the α and 4 rings.[3] Many of these bands were detected again in 2003–2004 by the Keck telescope and during the 2007 ring-plane crossing event in backscattered light, but their precise locations and relative brightnesses were different from the Voyager observations.[17][10] The normal optical depth of the dust bands is about 10−5 or less. The size distribution of the dust particles is thought to obey a power law with the index p=2.5 ± 0.5.[8]
[edit] Outer ring system
In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings, now called the outer ring system, which brought the number of known Uranian rings to 13.[4] These rings were subsequently named the μ and ν rings.[7] The μ ring is the outermost of the pair, and is twice the distance from the planet as the bright η ring.[4] The outer rings differ from the inner narrow rings in a number of respects. They are broad, 17,000 and 3,800 km wide, respectively, and very faint. Their peak normal optical depths are 8.5 × 10−6 and 5.4 × 10−6, respectively. The resulting equivalent optical depths are 0.14 km and 0.012 km. The rings have triangular radial brightness profiles.[4]
The peak brightness of the μ ring lies almost exactly on the orbit of the small Uranian moon Mab, which is probably the source of the ring’s particles.[4][5] The ν ring is positioned between Portia and Rosalind and does not contain any moons inside it.[4] A reanalysis of the Voyager 2 images of forward-scattered light clearly revealed the μ and ν rings. In this geometry the rings are much brighter, which indicates that they contain a lot of micrometre-sized dust.[4] The outer rings of Uranus may be similar to the G and E rings of Saturn. The G ring also lacks any observable source bodies, while the E ring is extremely broad and receives the dust from Enceladus.[4][5]
The μ ring may consist entirely of dust, without any large particles at all. This hypothesis is supported by observations performed by the Keck telescope, which failed to detect the μ ring in the near infrared at 2.2 μm, but detected the ν ring.[13] This failure means that μ ring is blue in colour, which in turn indicates that very small (submicrometre) dust predominates within it.[13] The dust may be made of water ice.[23] In contrast, the ν ring is slightly red in colour.[13][24]
[edit] Dynamics and origin
An outstanding problem concerning the physics governing the narrow Uranian rings is their confinement. Without some mechanism to hold their particles together, the rings would quickly spread out radially.[1] The lifetime of the Uranian rings without such a mechanism cannot be more than 1 million years.[1] The most widely cited model for such confinement, proposed initially by Goldreich and Tremaine,[25] is that a pair of nearby moons, outer and inner shepherds, interact gravitationally with a ring and act like sinks and donors, respectively, for excessive and insufficient angular momentum (or equivalently, energy). The shepherds thus keep ring particles in place, but gradually move away from the ring themselves.[1] To be effective, the masses of the shepherds should exceed the mass of the ring by at least a factor of two to three. This mechanism is known to be at work in the case of the ε ring, where Cordelia and Ophelia serve as shepherds.[20] Cordelia is also the outer shepherd of the γ ring.[1] However no moon larger than 10 km is known in the vicinity of other rings.[3] The current distance of Cordelia and Ophelia from the ε ring can be used to estimate the ring’s age. The calculations show that the ε ring cannot be older than 6 × 108 years.[1][18]
Since the rings of Uranus appear to be young, they must be continuously renewed by the collisional fragmentation of larger bodies.[1] The estimates show that the lifetime against collisional disruption of a moon with the size like that of Puck is a few billion years. The lifetime of a smaller satellite is much shorter.[1] Therefore all current inner moons and rings can be products of disruption of several Puck-sized satellites during the last four and half billion years.[18] Every such disruption would have started a collisional cascade that quickly ground almost all large bodies into much smaller particles, including dust.[1] Eventually the majority of mass was lost, and particles survived only in positions that were stabilized by mutual resonances and shepherding. The end product of such a disruptive evolution would be a system of narrow rings. However, a few moonlets must still be embedded within the rings at present. The maximum size of such moonlets is probably around 10 km.[18]
The origin of the dust bands is less problematic. The dust has a very short life time, 100–1000 years, and should be continuously replenished by collisions between larger ring particles, moonlets and meteoroids from outside the Uranian system.[6][18] The belts of the parent moonlets and particles are themselves invisible due to their low optical depth, while the dust reveals itself in forward-scattered light.[18] The narrow main rings and the moonlet belts that create dust bands are expected to differ in particle size distribution. The main rings have more centimetre to metre-sized bodies. Such a distribution increases the surface area of the material in the rings, leading to high optical density in back-scattered light.[18] In contrast, the dust bands have relatively few large particles, which results in low optical depth.[18]
[edit] Exploration
The rings were thoroughly investigated during the Voyager 2 spacecraft's fly-by of Uranus in January 1986.[14] Two new faint rings—λ and 1986U2R—were discovered bringing the total number to eleven. Rings were studied by analysing results of the radio,[16] ultraviolet[21] and optical occultations.[9] Voyager 2 observed the rings in different geometries relative to the sun, producing images of back-scattered, forward-scattered and side-scattered light.[3] The analysis of them allowed derivation of the complete phase function, geometrical and bond albedo of ring particles.[8] Two rings—ε and η—were resolved in the images revealing a complicated fine structure.[3] Analysis of Voyager's images led to discovery of 10 inner moons of Uranus, including two shepherd moons of the ε ring—Cordelia and Ophelia.[3]
[edit] Herschel's observations
The first mention of a Uranian ring system comes from William Herschel's notes detailing his observations of Uranus in the 18th century, which include the following passage: "February 22, 1789: A ring was suspected".[26] Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case, at least for the ν ring.[17] Herschel's notes were published in a Royal Society journal in 1797. However, in the two centuries between 1797 and 1977 the rings are rarely mentioned, if at all. This casts serious doubt whether Herschel could have seen anything of the sort while hundreds of other astronomers saw nothing. Still, it has been claimed by some that Herschel actually gave accurate descriptions of the ν ring's size relative to Uranus, its changes as Uranus travelled around the Sun, and its colour.[27]
[edit] List
This table summarizes the properties of the planetary ring system of Uranus.
| Ring name | Radius (km)[f] | Width (km)[f] | Eq. depth (km)[d][g] | N. Opt. depth[c][i] | Thickness (m)[h] | Ecc.[e] | Incl.(°)[e] | Notes |
| ζc | 32,000–37,850 | 3,500 | 0.6 | ~10−4 | ? | ? | ? | Inward extension of the ζ ring |
| 1986U2R | 37,000–39,500 | 2,500 | ? | <10−3 | ? | ? | ? | Faint dusty ring |
| ζ | 37,850–41,350 | 3,500 | 1 | <10−3 | ? | ? | ? | |
| 6 | 41,837 | 1.6–2.2 | 0.41 | 0.18–0.25 | ? | 1.0×10−3 | 0.063 | |
| 5 | 42,234 | 1.9–4.9 | 0.91 | 0.18–0.48 | ? | 1.9×10−3 | 0.052 | |
| 4 | 42,570 | 2.4–4.4 | 0.71 | 0.16–0.3 | ? | 1.1×10−3 | 0.032 | |
| α | 44,718 | 4.8–10 | 3.39 | 0.3–0.7 | ? | 0.8×10−3 | 0.014 | |
| β | 45,661 | 6.1–11.4 | 2.14 | 0.2–0.35 | ? | 0.4×10−3 | 0.005 | |
| η | 47,175 | 1.9–2.7 | 0.42 | 0.16–0.25 | ? | 0? | 0.002 | |
| ηc | 47,176 | 40 | 0.85 | 2×10−2 | ? | 0? | 0.002 | Outward broad component of the η ring |
| γ | 47,627 | 3.6–4.7 | 3.3 | 0.7–0.9 | 150? | 0? | 0.011 | |
| δc | 48,300 | 10–12 | 0.3 | 3×10−2 | ? | 0? | 0.004 | Inward broad component of the δ ring |
| δ | 48,300 | 4.1–6.1 | 2.2 | 0.3–0.6 | ? | 0? | 0.004 | |
| λ | 50,023 | 1–2 | 0.2 | 0.1–0.2 | ? | 0? | 0? | Faint dusty ring |
| ε | 51,149 | 19.7–96.4 | 47 | 0.5–2.5 | 150? | 7.9×10−3 | 0.001 | Shepherded by Cordelia and Ophelia |
| ν | 66,100–69,900 | 3,800 | 0.012 | 5.4×10−6 | ? | ? | ? | Between Portia and Rosalind, peak brightness at 97,700 km |
| μ | 86,000–103,000 | 17,000 | 0.14 | 8.5×10−6 | ? | ? | ? | At Mab, peak brightness at 67,300 km |
[edit] Notes
- ^ The forward-scattered light is the light scattered at a small angle relative to solar light.
- ^ The back-scattered light is the light scattered at an angle close to 180° relative to solar light.
- ^ The normal optical depth τ of a ring is the ratio between the total cross-section of the ring's particles to the square area of the ring. It assumes values from 0 to infinity. A light beam passing through a ring will be attenuated by the factor e−τ.[8]
- ^ 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.[17]
- ^ Eccentricities and inclinations were taken from Stone et al, 1986.[14]
- ^ The radii of 6,5,4, α, β, η, γ, δ, λ and ε rings were taken from Esposito et al, 2002.[1] The widths of 6,5,4, α, β, η, γ, δ and ε rings are from Karkoshka et al, 2001.[15] The radii and widths of ζ and 1986U2R rings were taken from de Pater et al, 2006.[17] The width of λ ring is from Holberg et al, 1987.[21] The radii and widths of μ and ν rings are extracted from Showalter et al, 2006.[4]
- ^ The equivalent depth of 1986U2R ring is a product of its width and the normal optical depth. The equivalent depths of 6,5,4, α, β, η, γ, δ and ε rings were taken from Karkoshka et al, 2001.[15] The equivalent depths of λ and ζ, μ and ν rings are derived using μEW values from de Pater et al, 2006[17] and de Pater et al, 2006b,[13] respectively. The μEW values for these rings were multiplied by the factor 20, which corresponds to the assumed albedo of the ring's particles of 5%.
- ^ The thickness estimates are from Lane et al, 1986.[9]
- ^ The normal optical depths of all rings except 1986U2R, μ and ν were calculated as ratios of the equivalent depths to the widths. The normal optical depth of 1986U2R ring was taken from de Smith et al, 1986.[3] The normal optical depths of μ and ν rings are peak values from Showalter et al, 2006.[4]
[edit] References
- ^ a b c d e f g h i j k l m n o p Esposito, L. W. (2002). "Planetary rings" (pdf). Reports On Progress In Physics 65: 1741–1783.
- ^ J. L. Elliot, E. Dunham & D. Mink (1977). The rings of Uranus. Cornell University. Retrieved on 2007-06-09.
- ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa Smith, B.A.; Soderblom, L.A.; Beebe, A. et al. (1986). "Voyager 2 in the Uranian System: Imaging Science Results". Science 233: 97–102.
- ^ a b c d e f g h i j k l m Showalter, Mark R.; Lissauer, Jack J. (2006). "The Second Ring-Moon System of Uranus: Discovery and Dynamics". Science 311: 973–977. doi:.
- ^ a b c NASA's Hubble Discovers New Rings and Moons Around Uranus. Hubblesite (2005). Retrieved on 2007-06-09.
- ^ a b c d e f g h i Burns, J. A.; D. P. Hamilton & M. R. Showalter (2001), "Dusty Rings and Circumplanetary Dust: Observations and Simple Physics", written at Berlin, in Grun, E.; Gustafson, B. A. S.; Dermott, S. T.; Fechtig H., Interplanetary Dust, Springer, 641–725, <http://www.astro.umd.edu/~hamilton/research/preprints/BurHamSho01.pdf>
- ^ a b Showalter, Mark R.; Lissauer, J. J.; French, R. G. et al. (2008). The Outer Dust Rings of Uranus in the Hubble Space Telescope. Retrieved on 2008-05-30.
- ^ a b c d e f g h i j k l Ockert, M.E.; Cuzzin, J.N.; Porco, C.C.; and Johnson, T.V. (1987). "Uranian ring photometry: Results from Voyager 2". J.of Geophys. Res. 92: 14,969–14,978.
- ^ a b c d e f g h i j k l m Lane, Arthur L.; Hord, Charles W.; West, Robert A. et.al. (1986). "Photometry from Voyager 2: Initial results from the uranian atmosphere, satellites and rings". Science 233: 65–69.
- ^ a b c d e f g h i j k l m de Pater, Imke; Hammel, H. B.; Showalter, Mark R.; Van Dam, Marcos A. (2007). "The Dark Side of the Rings of Uranus". Science 317: 1888–1890. doi:.
- ^ a b Karkoshka, Erich (1997). "Rings and Satellites of Uranus: Colorful and Not So Dark". Icarus 125: 348–363. doi:.
- ^ a b c d Baines, Kevin H.; Yanamandra-Fisher, Padmavati A.; Lebofsky, Larry A.; et.al. (1998). "Near-Infrared Absolute Photometric Imaging of the Uranian System". Icarus 132: 266–284. doi:.
- ^ a b c d e dePater, Imke; Hammel, Heidi B.; Gibbard, Seran G.; Showalter, Mark R. (2006). "New Dust Belts of Uranus: One Ring, Two Ring, Red Ring, Blue Ring". Science 312: 92–94. doi:.
- ^ a b c d e f g h i Stone, E.C.; Miner, E.D. (1986). "Voyager 2 encounter with the uranian system". Science 233: 39–43.
- ^ a b c d e f g h i j k l m n o p q Karkoshka, Erich (2001). "Photometric Modeling of the Epsilon Ring of Uranus and Its Spacing of Particles". Icarus 151: 78–83. doi:.
- ^ a b c Tyler, J.L.; Sweetnam, D.N.; Anderson, J.D.; et.al. (1986). "Voyger 2 Radio Science Observations of the Uranian System: Atmosphere, Rings, and Satellites". Science 233: 79–84.
- ^ a b c d e f g h i j k l m de Pater, Imke; Gibbard, Seran G.; Lebofsky, Hammel, H.B. (2006). "Evolution of the dusty rings of Uranus". Icarus 180: 186–200. doi:.
- ^ a b c d e f g h i j Esposito, L.W.; Colwell, Joshua E. (1989). "Creation of The Uranus Rings and Dust bands". Nature 339: 605–607. doi:.
- ^ Horn, L.J.; Lane, A.L.; Yanamandra-Fisher, P. A.; Esposito, L. W. (1988). "Physical properties of Uranian delta ring from a possible density wave". Icarus 76: 485–492. doi:.
- ^ a b c d Porco, Carolyn, C.; Goldreich, Peter (1987). "Shepherding of the Uranian rings I: Kinematics". The Astronomical Journal 93: 724–778. doi:.
- ^ a b c d e f Holberg, J.B.; Nicholson, P. D.; French, R.G.; Elliot, J.L. (1987). "Stellar Occultation probes of the Uranian Rings at 0.1 and 2.2 km: A comparison of of Voyager UVS and Earth based results". The Astronomical Journal 94: 178–188. doi:.
- ^ Gibbard, S.G.; De Pater, I.; Hammel, H.B. (2005). "Near-infrared adaptive optics imaging of the satellites and individual rings of Uranus". Icarus 174: 253-262. doi:.
- ^ Stephen Battersby (2006). Blue ring of Uranus linked to sparkling ice. NewScientistSpace. Retrieved on 2007-06-09.
- ^ Sanders, Robert (2006-04-06). Blue ring discovered around Uranus. UC Berkeley News. Retrieved on 2006-10-03.
- ^ Goldreich, Peter; Tremaine, Scott (1979-01-11). "Towards a theory for the uranian rings". Nature 277: 97–99. Nature Publishing Group. doi:.
- ^ "Uranus rings 'were seen in 1700s'", BBC News, April 19, 2007. Retrieved on 2007-04-19.
- ^ Did William Herschel Discover The Rings Of Uranus In The 18th Century?. Physorg.com (2007). Retrieved on 2007-06-20.
[edit] External links
- Uranus' Rings by NASA's Solar System Exploration
- Uranus Rings Fact Sheet
- Hubble Discovers Giant Rings and New Moons Encircling Uranus – Hubble Space Telescope news release (2005-12-22)
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