Rings of Uranus
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[[Image:Uranian rings scheme.png|300px|right|thumb|The scheme of Uranus's ring-moon system]]
The planet [[Uranus]] has a system of [[planetary rings|ring]]s intermediate in complexity between the more extensive set around [[Rings of Saturn|Saturn]] and the simpler systems around [[Rings of Jupiter|Jupiter]] and [[Rings of Neptune|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]] also reported observing rings. However modern astronomers are skeptical 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: 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ rings. Their radii range from about 38,000 km for the 1986U2R/ζ ring to about 98,000 km for the μ ring. Additional faint dust bands and incomplete arcs may exist between the main rings. The rings are extremely dark—the [[bond albedo]] of the rings' particles does not exceed 2%. They are likely composed of water ice with the addition of some dark radiation-processed [[Organic compound|organics]].
The majority of Uranus's rings are opaque and only a few kilometres wide. The ring system contains little dust overall; it consists mostly of large bodies 0.2–20 m in diameter. However some rings are optically thin: the broad and faint 1986U2R/ζ, μ and ν rings are made of small dust particles, while the narrow and faint λ ring also contains larger bodies. The relative lack of dust in the ring system is due to [[aerodynamic drag]] from the extended Uranian [[exosphere]]—[[Uranus#Physical characteristics|corona]].
The rings of Uranus are thought to be relatively young, at not more than 600 million years. The mechanism that confines the narrow rings is not well understood. Initially it was assumed that every narrow ring had a pair of nearby [[Planetary ring|shepherd moons]] corralling them into shape. However, in 1986 ''Voyager 2'' discovered only one such shepherd pair ([[Cordelia (moon)|Cordelia]] and [[Ophelia (moon)|Ophelia]]) around the brightest ε ring. The Uranian ring system probably originated from the collisional fragmentation of a number of moons that once existed around the planet. After colliding, the moons probably broke up into numerous particles, which survived as narrow and optically dense rings only in strictly confined zones of maximum stability.
==Discovery==
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".<ref>{{Cite news|title=Uranus rings 'were seen in 1700s'|publisher=BBC News| url=http://news.bbc.co.uk/1/hi/sci/tech/6569849.stm|date=[[April 19]][[2007]]|accessdate=2007-04-19}}</ref> Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The [[W. M. Keck Observatory|Keck Telescope]] in Hawaii has since confirmed this to be the case, at least for the ν ring.<ref name=dePater2006/> 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 on 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.<ref>{{cite web|title=Did William Herschel Discover The Rings Of Uranus In The 18th Century?|work=Physorg.com|url=http://www.physorg.com/news95949762.html|year=2007| accessdate=2007-06-20}}</ref>
The definitive (and accidental) discovery of the Uranian Rings was made by astronomers [[James L. Elliot]], Edward W. Dunham, and [[Douglas J. Mink]] on [[March 10]], [[1977]] using the Kuiper Airborne Observatory. They planned to use the [[occultation]] of the star SAO 158687 by Uranus to study the planet's [[Celestial body atmosphere|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.<ref name=Elliot1977/><ref name=Elliot1977b>{{cite journal|title=The rings of Uranus|last=Elliot|first=J.L.|coauthors=Dunham, E. and Mink, D.|journal= Nature|volume=267|year=1977|pages=328–330| url=http://www.nature.com/nature/journal/v267/n5609/abs/267328a0.html}}</ref> The five occultation events that they observed were numbered by the Greek letters α, β, γ, δ and ε in their papers.<ref name=Elliot1977>{{cite web|last=Elliot|first=J.L.|coauthors=Dunham, E; Mink, D.|title=The Occultation of SAO – 15 86687 by the Uranian Satellite Belt|publisher=International Astronomical Union, Circular No. 3051|year=1977|volume=83| url=http://www.cfa.harvard.edu/iauc/03000/03051.html}}</ref> These designations have been used as the ring's names since then. Later they found four additional rings: one between β and γ rings and three inside α ring.<ref name=Nicholson1978>{{cite journal|last=Nicholson|first=P. D.|coauthors=Persson, S.E.; Matthews, K. et al.|title=The Rings of Uranus: Results from 10 April 1978 Occultations|journal=The Astronomical Journal|year=1978|volume=83|pages=1240–1248|url=http://adsabs.harvard.edu/abs/1978AJ.....83.1240N |doi=
10.1086/112318}}</ref> The former was named η ring. The latter were dubbed 4, 5 and 6 rings—according to the numbering of the occultation events in one paper.<ref name=Millis1978>{{cite journal|last=Millis|first=R.L.|coauthors=Wasserman, L.H.|title=The Occultation of BD –15 3969 by the Rings of Uranus|journal=The Astronomical Journal|year=1978|volume=83|pages=993–998|url=http://adsabs.harvard.edu/abs/1978AJ.....83..993M|doi=}}</ref> Uranus's ring system was the second to be discovered in the Solar System, after that of [[Rings of Saturn|Saturn]].<ref name=Esposito2002/>
The rings were directly imaged when the ''[[Voyager 2]]'' spacecraft flew through the Uranian system in 1986.<ref name=Smith1986/> ''[[Voyager 2]]'' also discovered two additional faint rings, bringing the total to eleven.<ref name=Smith1986/> The [[Hubble Space Telescope]] detected a pair of previously unknown rings in 2003–2005, bringing the total number to 13. This "outer ring system" is much farther from the planet than the inner rings.<ref name=Showalter2006/> Hubble also spotted two small satellites, one of which, [[Mab (moon)|Mab]], shares its orbit with the outermost newly discovered ring. <ref name=NASA2005>{{cite web|title=NASA's Hubble Discovers New Rings and Moons Around Uranus|work=Hubblesite| url=http://hubblesite.org/newscenter/archive/releases/2005/33/| year=2005|accessdate=2007-06-09}}</ref>
==General properties==
[[Image:Uranian rings PIA01977 modest.jpg|thumb|right|250px|Uranus's inner rings. The bright outer ring is the epsilon ring; eight other rings are visible.]]
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.<ref name=Showalter2006/> They can be divided into three groups: nine narrow main rings (6, 5, 4, α, β, η, γ, δ, ε),<ref name=Esposito2002/> two dusty rings (1986U2R/ζ, λ)<ref name=Burns2001/> and two outer rings (μ, ν).<ref name=Showalter2006/><ref name=Showalter2008b/> The rings of Uranus consist mainly of macroscopic particles and little [[dust]],<ref name=Ockert1987/> although dust is known to be present in 1986U2R/ζ, η, δ, λ, ν and μ rings.<ref name=Burns2001/><ref name=Showalter2006/> In addition to these well-known rings, there may be numerous optically thin dust bands and faint rings between them.<ref name=Lane1986/> These faint rings and dust bands may exist only temporarily or consist of a number of separate arcs, which are sometimes detected during [[occultation]]s.<ref name=Lane1986/> Some of them became visible during a series of ring plane-crossing events in 2007.<ref name=dePater2007>{{cite journal|last=de Pater|first=Imke|coauthors=Hammel, H. B.; Showalter, Mark R.; Van Dam, Marcos A.|title=The Dark Side of the Rings of Uranus |journal=Science|year=2007|volume=317|pages=1888–1890|url=http://adsabs.harvard.edu/abs/2007Sci...317.1888D| doi=10.1126/science.1148103|pmid=17717152}}</ref> A number of dust bands between the rings were observed in forward-scattering{{Ref_label|A|a|none}} geometry by ''[[Voyager 2]]''.<ref name=Smith1986/> All rings of Uranus show [[azimuth]]al brightness variations.<ref name=Smith1986>{{cite journal|last= Smith|first=B.A.|coauthors=Soderblom, L.A.; Beebe, A. et al. |title=Voyager 2 in the Uranian System: Imaging Science Results|journal=Science|volume=233|pages=97–102|year=1986| url=http://adsabs.harvard.edu/abs/1986Sci...233...43S |doi=10.1126/science.233.4759.43 |pmid=17812889}}</ref>
The rings 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%.<ref name=Ockert1987>{{cite journal|last=Ockert|first=M.E.|coauthors=Cuzzin, J.N.; Porco, C.C.; and Johnson, T.V.|title=Uranian ring photometry: Results from Voyager 2|journal=J.of Geophys. Res.|year=1987|volume=92|pages=14,969–14,978| url=http://adsabs.harvard.edu/abs/1987JGR....9214969O|doi=}}</ref><ref name=Karkoshka1997>{{cite journal|last=Karkoshka| first=Erich|title=Rings and Satellites of Uranus: Colorful and Not So Dark|journal=Icarus|year=1997|volume=125| pages=348–363 |url=http://adsabs.harvard.edu/abs/1997Icar..125..348K|doi=10.1006/icar.1996.5631}}</ref> The rings particles demonstrate a steep opposition surge—an increase of the albedo when the [[phase angle]] is close to zero.<ref name=Ockert1987/> This means that their albedo is much lower when they are observed slightly off the opposition.{{Ref_label|J|j|none}} The rings are slightly red in the [[ultraviolet]] and visible parts of the [[spectrum]] and grey in [[near-infrared]].<ref name=Baines1998>{{cite journal|last=Baines|first=Kevin H.|coauthors=Yanamandra-Fisher, Padmavati A.; Lebofsky, Larry A.; et.al. |title=Near-Infrared Absolute Photometric Imaging of the Uranian System |journal=Icarus|year=1998|volume=132 |pages=266–284|url=http://adsabs.harvard.edu/abs/1998Icar..132..266B|doi=10.1006/icar.1998.5894}}</ref> They exhibit no identifiable [[spectral analysis|spectral feature]]s. The [[chemical composition]] of the ring particles is not known. However, they cannot be made of pure water ice like the [[rings of Saturn]] because they are too dark, darker than the [[moons of Uranus|inner moons of Uranus]].<ref name=Baines1998/> 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 [[organic compound]]s 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.<ref name=Baines1998/>
As a whole, the ring system of Uranus is unlike the faint dusty [[rings of Jupiter]] or the broad and complex [[rings of Saturn]], some of which are composed of very bright material—water ice.<ref name=Esposito2002/> However, there are similarities with some parts of the latter ring system; the Saturnian [[F ring]] and the ε ring are both narrow, relatively dark and are shepherded by a pair of moons.<ref name=Esposito2002/> The newly discovered outer rings of Uranus are similar to the outer G and E rings of [[Saturn (planet)|Saturn]].<ref name=dePater2006b/> Narrow ringlets existing in the broad Saturnian rings also resemble the narrow rings of Uranus.<ref name=Esposito2002/> In addition, dust bands observed between the main rings of Uranus may be similar to the rings of Jupiter.<ref name=Burns2001/> In contrast, the [[Rings of Neptune|Neptunian ring]] system is quite similar to that of Uranus, although is less complex, darker and contains more dust; the Neptunian rings are also positioned further from the planet.<ref name=Burns2001/>
==Narrow main rings==
===ε ring===
[[Image:Epsilon ring of Uranus.jpg|250px|right|thumb|A close-up view of the ε ring of Uranus]]
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.<ref name=Smith1986/><ref name=Baines1998/> While it is the most [[orbital eccentricity|eccentric]] of the Uranian rings, it has negligible [[orbital inclination]].<ref name=Stone1986>{{cite journal|last=Stone|first=E.C.|coauthors=Miner, E.D.|title=Voyager 2 encounter with the uranian system|journal=Science|year=1986|volume=233|pages=39–43|url=http://adsabs.harvard.edu/abs/1986Sci...233...39S| doi=}}</ref> 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]].<ref name=Karkoshka2001b/> The maximum/minimum brightness ratio is about 2.5–3.0.<ref name=Ockert1987/> 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.<ref name=Karkoshka2001b/> As the ring becomes wider, the amount of shadowing between particles decreases and more of them come into view, leading to higher integrated brightness.<ref name=Karkoshka1997/> The width variations were measured directly from ''Voyager 2'' images, as the ε ring was one of only two rings resolved by Voyager’s cameras.<ref name=Smith1986/> 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]]{{Ref_label|C|c|none}} varies between 0.5 and 2.5,<ref name=1986Tyler/><ref name=Karkoshka2001b/> being the highest near the periapsis. The equivalent depth{{Ref_label|D|d|none}} of the ε ring is around 47 km. The equivalent depth is invariant along the orbit.<ref name=Karkoshka2001b>{{cite journal|last=Karkoshka|first=Erich|title=Photometric Modeling of the Epsilon Ring of Uranus and Its Spacing of Particles|journal=Icarus|year=2001|volume=151|pages=78–83| url=http://adsabs.harvard.edu/abs/2001Icar..151...78K|doi=10.1006/icar.2001.6598}}</ref>
[[Image:Rings of Uranus.jpg|250px|left|thumb|A close-up view of the (from top to bottom) δ, γ, η, β and α rings of Uranus. The resolved η ring demonstrates the optically thin broad component.]]
The geometric thickness of the ε ring is not precisely known, although the ring is certainly very thin—by some estimates as thin as 150 m.<ref name=Lane1986>{{cite journal|last=Lane|first=Arthur L.|coauthors=Hord, Charles W.; West, Robert A. et.al.|title=Photometry from Voyager 2: Initial results from the uranian atmosphere, satellites and rings|journal=Science|year=1986|volume=233|pages=65–69|url=http://adsabs.harvard.edu/abs/1986Sci...233...65L|doi=}}</ref> Despite such 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.<ref name=Karkoshka2001b/> The mean size of the ring particles is 0.2–20.0 m,<ref name=Lane1986/> and the mean separation is around 4.5 times their radius.<ref name=Karkoshka2001b/> The ring is almost devoid of [[Cosmic dust|dust]], possibly due to the aerodynamic drag from Uranus's extended atmospheric corona.<ref name=dePater2006/> Due to its razor-thin nature the ε ring disappears when viewed edge-on. This happened in 2007 when a ring plane-crossing was observed.<ref name=dePater2007/>
The ''Voyager 2'' spacecraft observed a strange signal from the ε ring during the [[radio occultation]] experiment.<ref name=1986Tyler>{{cite journal|last=Tyler|first=J.L.|coauthors=Sweetnam, D.N.; Anderson, J.D.; et.al. |title=Voyger 2 Radio Science Observations of the Uranian System: Atmosphere, Rings, and Satellites|journal=Science|volume=233|pages=79–84|year=1986|url=http://adsabs.harvard.edu/abs/1986Sci...233...79T |doi=10.1126/science.233.4759.79 |pmid=17812893}}</ref> The signal looked like a strong enhancement of the [[forward scattering|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.<ref name=Lane1986/> The ε ring seems to consist of a number of narrow and optically dense ringlets, some of which may have incomplete arcs.<ref name=Lane1986/>
The ε ring is known to have interior and exterior [[shepherd moon]]s—[[Cordelia (moon)|Cordelia]] and [[Ophelia (moon)|Ophelia]], respectively.<ref name=Esposito1989/> The inner edge of the ring is in 24:25 resonance with Cordelia, and the outer edge is in 14:13 [[orbital resonance|resonance]] with Ophelia.<ref name=Esposito1989/> The masses of the moons need to be at least three times the mass of the ring to confine it effectively.<ref name=Esposito2002/> The mass of the ε ring is estimated to be about 10<sup>16</sup> kg.<ref name=Esposito1989/><ref name=Esposito2002/>
===δ ring===
[[Image:Forward Back Uranus Rings.png|250px|right|thumb|A merge of two images (forward-scattering and back-scattering) of Uranian rings obtained by ''[[Voyager 2]]'' in 1986]]
The δ ring is circular and slightly inclined.<ref name=Stone1986/> It shows significant unexplained azimuthal variations in normal optical depth and width.<ref name=Lane1986/> One possible explanation is that the ring has an azimuthal wave-like structure, excited by a small moonlet just inside it.<ref name=Horn1988>{{cite journal|last=Horn|first=L.J.|coauthors=Lane, A.L.; Yanamandra-Fisher, P. A.; Esposito, L. W.|title=Physical properties of Uranian delta ring from a possible density wave|journal=Icarus |year=1988|volume=76|pages=485–492|url=http://adsabs.harvard.edu/abs/1988Icar...76..485H |doi=10.1016/0019-1035(88)90016-4}}</ref> The sharp outer edge of the δ ring is in 23:22 resonance with Cordelia.<ref name=Porco1987/> The δ ring consists of two components: a narrow optically dense component and a broad inward shoulder with low optical depth.<ref name=Lane1986/> 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.<ref name=Karkoshka2001b/> 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<sup>−2</sup>.<ref name=Karkoshka2001b/><ref name=Holberg1987/> This is known only from occultation data because ''Voyager 2s''' imaging experiment failed to resolve the δ ring.<ref name=Smith1986/><ref name=Holberg1987/> 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.<ref name=Smith1986/> 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.<ref name=dePater2007/>
===γ ring===
The γ ring is narrow, optically dense and slightly eccentric. Its orbital inclination is almost zero.<ref name=Stone1986/> The width of the ring varies in the range 3.6–4.7 km, although equivalent optical depth is constant at 3.3 km.<ref name=Karkoshka2001b/> 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<ref name=Lane1986/> and devoid of dust.<ref name=dePater2007/> The width and normal optical depth of the γ ring show significant [[azimuthal]] variations.<ref name=Lane1986/> The mechanism of confinement of such a narrow ring is not known, but it has been noticed that the sharp inner edge of the γ ring is in a 6:5 resonance with Ophelia.<ref name=Porco1987>{{cite journal|last=Porco|first=Carolyn, C.|coauthors=Goldreich, Peter|title= Shepherding of the Uranian rings I: Kinematics|journal=The Astronomical Journal|year=1987|volume=93|pages=724–778| url=http://adsabs.harvard.edu/abs/1987AJ.....93..724P|doi=10.1086/114354}}</ref><ref name=French1988/>
===η ring===
The η ring has zero orbital eccentricity and inclination.<ref name=Stone1986/> Like the δ ring, it consists of two components: a narrow optically dense component and a broad outward shoulder with low optical depth.<ref name=Smith1986/> The width of the narrow component is 1.9–2.7 km and the equivalent depth is about 0.42 km, which corresponds to the normal optical depth of about 0.16–0.25.<ref name=Karkoshka2001b/> 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<sup>−2</sup>.<ref name=Karkoshka2001b/> It was resolved in ''Voyager 2'' images.<ref name=Smith1986/> In forward-scattered light, the η ring looked bright which indicated the presence of considerable dust in this ring, probably in the broad component.<ref name=Smith1986/> 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.<ref name=dePater2007/> This is consistent with the behaviour of a geometrically thick but simultaneously optically thin ring.<ref name=dePater2007/> Like the majority of other rings, the η ring shows significant azimuthal variations in the normal optical depth and width. The narrow component even vanishes in some places.<ref name=Lane1986/>
===α and β rings===
After the ε ring, the α and β rings are the brightest of Uranus's rings.<ref name=Ockert1987/> Like the ε ring, they exhibit regular variations in brightness and width.<ref name=Ockert1987/> They are brightest and widest 30° from the [[apoapsis]] and dimmest and narrowest 30° from the [[periapsis]].<ref name=Smith1986/><ref name=Gibbard2005>{{cite journal|last=Gibbard|first=S.G.|coauthors=De Pater, I.; Hammel, H.B.|title=Near-infrared adaptive optics imaging of the satellites and individual rings of Uranus|journal=Icarus|year=2005|volume=174|pages=253–262| url=http://adsabs.harvard.edu/abs/2005Icar..174..253G |doi=10.1016/j.icarus.2004.09.008}}</ref> The α and β rings have sizable orbital eccentricity and non-negligible inclination.<ref name=Stone1986/> The widths of these rings are 4.8–10 km and 6.1–11.4 km, respectively.<ref name=Karkoshka2001b/> 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.<ref name=Karkoshka2001b/> 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.<ref name=dePater2007/> However the same event revealed a thick and optically thin dust band just outside the β ring, which was also observed earlier by ''Voyager 2''.<ref name=Smith1986/> The masses of the α and β rings are estimated to about 5{{Esp|15}} kg (each)—half the mass of the ε ring.<ref name=Chiang2003>{{cite journal|last=Chiang|first=Eugene I.|coauthors=Culter, Cristopher J.|title=Three-Dimensional Dynamics of Narrow Planetary Rings|journal=The Astrophysical Journal |year=2003|volume=599|pages=675–685 |url=http://adsabs.harvard.edu/abs/2003ApJ...599..675C|doi=10.1086/379151}}</ref>
=== 6, 5 and 4 rings===
The 6, 5 and 4 rings are the innermost and dimmest of Uranus's narrow rings.<ref name=Ockert1987/> They are the most inclined rings, and their orbital eccentricities are the largest excluding the ε ring.<ref name=Stone1986/> In fact, their inclinations (0.06°, 0.05° and 0.03°) were large enough for ''Voyager 2'' to observe their elevations above the Uranian equatorial plane, which were 24–46 km.<ref name=Smith1986/> 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 wide, respectively.<ref name=Smith1986/><ref name=Karkoshka2001b/> 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.<ref name=Karkoshka2001b/> They were not visible during a ring plane-crossing event in 2007 due to their narrowness and lack of dust.<ref name=dePater2007/>
==Dusty rings==
===λ ring===
[[Image:FDS 26852.19 Rings of Uranus.gif|thumb|right|250px|A long-exposure, high [[Phase angle (astronomy)|phase angle]] (back-illuminated) ''[[Voyager 2]]'' image of Uranus's inner rings. In [[Forward scatter|forward-scattered]] light, dust bands not visible in other images can be seen, as well as the recognized rings.]]
The λ ring was one of two rings discovered by ''Voyager 2'' in 1986.<ref name=Stone1986/> It is a narrow, faint ring located just inside the ε ring, between it and the shepherd moon [[Cordelia (moon)|Cordelia]].<ref name=Smith1986/> This moon actually clears a dark lane just inside the λ ring. When viewed in [[backscatter|back-scattered]] light{{Ref_label|B|b|none}}, 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.<ref name=dePater2006/> The normal optical depth is 0.1–0.2.<ref name=Smith1986/><ref name=Holberg1987>{{cite journal|last=Holberg|first=J.B.|coauthors=Nicholson, P. D.; French, R.G.; Elliot, J.L.|title=Stellar Occultation probes of the Uranian Rings at 0.1 and 2.2 km: A comparison of of Voyager UVS and Earth based results|journal=The Astronomical Journal|year=1987|volume=94|pages=178–188| url=http://adsabs.harvard.edu/abs/1987AJ.....94..178H|doi=10.1086/114462}}</ref> 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 UV stellar [[occultation]]s by ''Voyager 2''.<ref name=Holberg1987/> The detection during a stellar occultation at the wavelength 2.2 μm was only announced in 1996.<ref name=dePater2006/>
The appearance of the λ ring changed dramatically when it was observed in forward-scattered light in 1986.<ref name=Smith1986/> In this geometry the ring became the brightest feature of the Uranian ring system, outshining the ε ring.<ref name=Burns2001/> This observation, together with the wavelength dependence of the optical depth, indicates that the λ ring contains significant amount of [[micrometre]]-sized dust.<ref name=Burns2001/> The normal optical depth of this dust is 10<sup>−4</sup>–10<sup>−3</sup>.<ref name=Ockert1987/> Observations in 2007 by the [[Keck telescopes|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.<ref name=dePater2007/>
Detailed analysis of the ''Voyager 2'' images revealed azimuthal variations in the brightness of the λ ring.<ref name=Ockert1987/> The variations appear to be periodic, resembling a [[standing wave]]. The origin of this fine structure in the λ ring remains a mystery.<ref name=Burns2001>{{cite encyclopedia|last=Burns|first=J.A.|coauthors=Hamilton, D.P.; Showalter, M.R.|title=Dusty Rings and Circumplanetary Dust: Observations and Simple Physics |encyclopedia=Interplanetary Dust|year=2001 |publisher=Springer |place=Berlin |editor=Grun, E.; Gustafson, B. A. S.; Dermott, S. T.; Fechtig H. |pages=641–725|url=http://www.astro.umd.edu/~hamilton/research/preprints/BurHamSho01.pdf|format=pdf}}</ref>
===1986U2R/ζ ring===
[[Image:Uranus' rings dim.jpg|250px|right|thumb|The discovery image of 1986U2R ring]]
In 1986 ''[[Voyager 2]]'' detected a broad and faint sheet of material inward of the 6 ring.<ref name=Smith1986/> This ring was given the temporary designation 1986U2R. It had a normal optical depth of 10<sup>−3</sup> or less and was extremely faint. In fact, it was visible only in a single ''Voyager 2'' image.<ref name=Smith1986/> The ring was located between 37,000 and 39,500 km from the centre of Uranus, or only about 12,000 km above the clouds.<ref name=dePater2006>{{cite journal|last=de Pater|first=Imke|coauthors=Gibbard, Seran G.; Lebofsky, Hammel, H.B. |title=Evolution of the dusty rings of Uranus|journal=Icarus| year=2006|volume=180|pages=186–200|url=http://adsabs.harvard.edu/abs/2006Icar..180..186D| doi=10.1016/j.icarus.2005.08.011}}</ref> It was not observed again until 2003–2004, when the [[Keck telescopes|Keck telescope]] found a broad and faint sheet of material just inside the 6 ring. This ring was dubbed the ζ ring.<ref name=dePater2006/> 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.<ref name=dePater2006/>
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.<ref name=dePater2007/> 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<sup>−3</sup>.<ref name=dePater2006/> 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.<ref name=dePater2006/><ref name=dePater2007/> However, changes during the past 20 years in the distribution of dust, which is thought to predominate in the ring, cannot be ruled out.<ref name=dePater2007/>
===Other dust bands===
In addition to the 1986U2R/ζ and λ rings, there are other extremely faint dust bands in the Uranian ring system.<ref name=Smith1986/> They are invisible during occultations because they have negligible optical depth, though they are bright in forward-scattered light.<ref name=Burns2001/> ''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.<ref name=Smith1986/> Many of these bands were detected again in 2003–2004 by the [[W. M. Keck Observatory|Keck Telescope]] and during the 2007 ring-plane crossing event in backscattered light, but their precise locations and relative brightnesses were different than during the ''Voyager'' observations.<ref name=dePater2006/><ref name=dePater2007/> The normal optical depth of the dust bands is about 10<sup>−5</sup> or less. The dust particle size distribution is thought to obey a [[power law]] with the index p = 2.5 ± 0.5.<ref name=Ockert1987/>
==Outer ring system==
[[Image:Outer Uranian rings.jpg|thumb|250px|μ and ν rings of Uranus (R/2003 R1 and R2) as observed by the [[Hubble Space Telescope]] in 2005]]
In 2003–2005, 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.<ref name=Showalter2006>{{cite journal|last= Showalter|first=Mark R.|coauthors=Lissauer, Jack J.|title=The Second Ring-Moon System of Uranus: Discovery and Dynamics |journal=Science|year=2006|volume=311|pages=973–977|url=http://adsabs.harvard.edu/abs/2006Sci...311..973S|doi=10.1126/science.1122882|pmid=16373533}}</ref> These rings were subsequently named the μ and ν rings.<ref name=Showalter2008b>{{cite web|url=http://adsabs.harvard.edu/abs/2008DDA....39.1602S|title=The Outer Dust Rings of Uranus in the Hubble Space Telescope|author=Showalter, Mark R.; Lissauer, J. J.; French, R. G. et al.|year=2008|accessdate=2008-05-30|publisher=American Astronomical Society}}</ref> The μ ring is the outermost of the pair, and is twice the distance from the planet as the bright η ring.<ref name=Showalter2006/> 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<sup>−6</sup> and 5.4 × 10<sup>−6</sup>, respectively. The resulting equivalent optical depths are 0.14 km and 0.012 km. The rings have triangular radial brightness profiles.<ref name=Showalter2006/>
The peak brightness of the μ ring lies almost exactly on the orbit of the small Uranian moon [[Mab (moon)|Mab]], which is probably the source of the ring’s particles.<ref name=Showalter2006/><ref name=NASA2005/> The ν ring is positioned between [[Portia (moon)|Portia]] and [[Rosalind (moon)|Rosalind]] and does not contain any moons inside it.<ref name=Showalter2006/> A reanalysis of the ''Voyager 2'' images of forward-scattered light clearly reveals the μ and ν rings. In this geometry the rings are much brighter, which indicates that they contain a lot of micrometre-sized dust.<ref name=Showalter2006/> The outer rings of Uranus may be similar to the [[Rings of Saturn #G Ring|G and E rings of Saturn]]. The G ring also lacks any observable source bodies, while the E ring is extremely broad and receives dust from [[Enceladus (moon)|Enceladus]].<ref name=Showalter2006/><ref name=NASA2005/>
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.<ref name=dePater2006b>{{cite journal|last=dePater|first=Imke|coauthors=Hammel, Heidi B.; Gibbard, Seran G.; Showalter, Mark R. |title=New Dust Belts of Uranus: One Ring, Two Ring, Red Ring, Blue Ring |journal=Science|year=2006|volume=312|pages=92–94|url=http://adsabs.harvard.edu/abs/2006Sci...312...92D| doi=10.1126/science.1125110 |pmid=16601188}}</ref> This failure means that the μ ring is blue in colour, which in turn indicates that very small (submicrometre) dust predominates within it.<ref name=dePater2006b/> The dust may be made of water ice.<ref>{{cite web|title=Blue ring of Uranus linked to sparkling ice|author=Stephen Battersby|work=NewScientistSpace| url=http://space.newscientist.com/article/dn8960|year=2006|accessdate=2007-06-09}}</ref> In contrast, the ν ring is slightly red in colour.<ref name=dePater2006b/><ref>{{Cite web|title=Blue ring discovered around Uranus|publisher=UC Berkeley News|last=Sanders|first=Robert|url=http://www.berkeley.edu/news/media/releases/2006/04/06_bluering.shtml| date=[[2006-04-06]]|accessdate=2006-10-03}}</ref>
==Dynamics and origin==
[[Image:Uranus rings.png|250px|right|thumb|An enhanced-colour schematic of the inner rings derived from ''Voyager 2'' images]]
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.<ref name=Esposito2002/> The lifetime of the Uranian rings without such a mechanism cannot be more than 1 million years.<ref name=Esposito2002/> The most widely cited model for such confinement, proposed initially by [[Peter Goldreich|Goldreich]] and [[Scott Tremaine|Tremaine]],<ref>{{cite journal
| last = Goldreich
| first = Peter
| authorlink = Peter Goldreich
| coauthors = [[Scott Tremaine|Tremaine, Scott]]
| title = Towards a theory for the uranian rings
| journal = Nature
| volume = 277
| issue =
| pages = 97–99
| publisher = [[Nature Publishing Group]]
| location =
| year = 1979
| url = http://www.nature.com/nature/journal/v277/n5692/abs/277097a0.html
| doi = 10.1038/277097a0
| id =
| accessdate = }}</ref> 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.<ref name=Esposito2002/> 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 (moon)|Cordelia]] and [[Ophelia (moon)|Ophelia]] serve as shepherds.<ref name=Porco1987/> Cordelia is also the outer shepherd of the δ ring, and Ophelia is the outer shepherd of the γ ring.<ref name=Porco1987/> However no moon larger than 10 km is known in the vicinity of other rings.<ref name=Smith1986/> 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 × 10<sup>8</sup> years.<ref name=Esposito2002>{{cite journal|last=Esposito|first=L. W.|authorlink=Larry W. Esposito|title=Planetary rings |journal=Reports On Progress In Physics|year=2002|volume=65|pages=1741–1783 |url=http://www.iop.org/EJ/article/0034-4885/65/12/201/r21201.pdf|format=pdf|doi=10.1088/0034-4885/65/12/201}}</ref><ref name=Esposito1989>{{cite journal|last=Esposito|first=L.W.|coauthors=Colwell, Joshua E. |title=Creation of The Uranus Rings and Dust bands|journal=Nature|year=1989|volume=339|pages=605–607|url=http://adsabs.harvard.edu/abs/1989Natur.339..605E| doi=10.1038/339605a0}}</ref>
Since the rings of Uranus appear to be young, they must be continuously renewed by the collisional fragmentation of larger bodies.<ref name=Esposito2002/> The estimates show that the lifetime against collisional disruption of a moon with the size like that of [[Puck (moon)|Puck]] is a few billion years. The lifetime of a smaller satellite is much shorter.<ref name=Esposito2002/> 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.<ref name=Esposito1989/> Every such disruption would have started a collisional cascade that quickly ground almost all large bodies into much smaller particles, including dust.<ref name=Esposito2002/> 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 [[moonlet]]s must still be embedded within the rings at present. The maximum size of such moonlets is probably around 10 km.<ref name=Esposito1989/>
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 [[meteoroid]]s from outside the Uranian system.<ref name=Burns2001/><ref name=Esposito1989/> 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.<ref name=Esposito1989/> 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.<ref name=Esposito1989/> In contrast, the dust bands have relatively few large particles, which results in low optical depth.<ref name=Esposito1989/>
==Exploration==
The rings were thoroughly investigated during the [[Voyager 2]] spacecraft's flyby of Uranus in January 1986.<ref name=Stone1986/> Two new faint rings—λ and 1986U2R—were discovered bringing the total number to eleven. Rings were studied by analysing results of radio,<ref name=1986Tyler/> ultraviolet<ref name=Holberg1987/> and optical occultations.<ref name=Lane1986/> ''Voyager 2'' observed the rings in different geometries relative to the sun, producing images of back-scattered, forward-scattered and side-scattered light.<ref name=Smith1986/> Analysis of these images allowed derivation of the complete phase function, geometrical and bond albedo of ring particles.<ref name=Ockert1987/> Two rings—ε and η—were resolved in the images revealing a complicated fine structure.<ref name=Smith1986/> Analysis of Voyager's images also led to discovery of 10 inner [[moons of Uranus]], including the two shepherd moons of the ε ring—Cordelia and Ophelia.<ref name=Smith1986/>
==List==
This table summarizes the properties of the '''[[planetary ring]] system of [[Uranus (planet)|Uranus]]'''.
{| class="wikitable"
| '''Ring name''' || '''Radius (km)'''{{Ref_label|F|f|none}} || '''Width (km)'''{{Ref_label|F|f|none}} ||'''Eq. depth (km)'''{{Ref_label|D|d|none}}{{Ref_label|G|g|none}} ||'''N. Opt. depth'''{{Ref_label|C|c|none}}{{Ref_label|I|i|none}} ||'''Thickness (m)'''{{Ref_label|H|h|none}} ||Ecc.{{Ref_label|E|e|none}} ||'''Incl.(°)'''{{Ref_label|E|e|none}}||'''Notes'''
|-
|ζ<sub>c</sub>||32,000–37,850||3,500||0.6||~ 10<sup>−4</sup>||?||?||?||Inward extension of the ζ ring
|-
|1986U2R ||37,000–39,500||2,500||?||< 10<sup>−3</sup> ||?||?||?||Faint dusty ring
|-
|ζ||37,850–41,350||3,500||1||< 10<sup>−3</sup>||?||?||?||
|-
|6||41,837||1.6–2.2||0.41||0.18–0.25||?||1.0 × 10<sup>−3</sup>||0.062||
|-
|5||42,234||1.9–4.9||0.91||0.18–0.48||?||1.9 × 10<sup>−3</sup>||0.054||
|-
|4||42,570||2.4–4.4||0.71||0.16–0.30||?||1.1 × 10<sup>−3</sup>||0.032||
|-
|α||44,718||4.8–10.0||3.39||0.3–0.7||?||0.8 × 10<sup>−3</sup>||0.015||
|-
|β||45,661||6.1–11.4||2.14||0.20–0.35||?||0.4 × 10<sup>−3</sup>||0.005||
|-
|η||47,175||1.9–2.7||0.42||0.16–0.25||?||0||0.001||
|-
|η<sub>c</sub>||47,176||40||0.85||2 × 10<sup>−2</sup>||?||0||0.001||Outward broad component of the η ring
|-
|γ||47,627||3.6–4.7||3.3||0.7–0.9||150?||0.1 × 10<sup>−3</sup>||0.002||
|-
|δ<sub>c</sub>||48,300||10–12||0.3||3 × 10<sup>−2</sup>||?||0||0.001||Inward broad component of the δ ring
|-
|δ||48,300||4.1–6.1||2.2||0.3–0.6||?||0||0.001||
|-
|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<sup>−3</sup>||0||Shepherded by [[Cordelia (moon)|Cordelia]] and [[Ophelia (moon)|Ophelia]]
|-
|ν||66,100–69,900||3,800||0.012||5.4 × 10<sup>−6</sup>||?||?||?||Between [[Portia (moon)|Portia]] and [[Rosalind (moon)|Rosalind]], peak brightness at 97,700 km
|-
|μ||86,000–103,000||17,000||0.14||8.5 × 10<sup>−6</sup>||?||?||?||At [[Mab (moon)|Mab]], peak brightness at 67,300 km
|}
==Notes==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}} Forward-scattered light is light scattered at a small angle relative to solar light ([[Phase angle (astronomy)|phase angle]] close to 180°).
<li>{{Note_label|B|b|none}} Back-scattered light is light scattered at an angle close to 180° relative to solar light ([[Phase angle (astronomy)|phase angle]] close to 0°).
<li>{{Note_label|C|c|none}} The normal optical depth τ of a ring is the ratio of the total geometrical [[Cross section (geometry)|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<sup>−τ</sup>.<ref name=Ockert1987/>
<li>{{Note_label|D|d|none}} 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.<ref name=dePater2006/>
<li>{{Note_label|E|e|none}} Eccentricities and inclinations were taken from Stone et al, 1986 and French et al, 1989.<ref name=Stone1986/><ref name=French1988>{{cite journal|last=French|first=Richard D.|coauthors=Elliot, J.L.; French, Linda M. et al.|title=Uranian Ring Orbits from Earth-based and Voyager Occultation Observations|journal=Icarus |year=1988|volume=73|pages=349–478 |url=http://adsabs.harvard.edu/abs/1988Icar...73..349F |doi=10.1016/0019-1035(88)90104-2}}</ref>
<li>{{Note_label|F|f|none}} The radii of 6,5,4, α, β, η, γ, δ, λ and ε rings were taken from Esposito et al, 2002.<ref name=Esposito2002/> The widths of 6,5,4, α, β, η, γ, δ and ε rings are from Karkoshka et al, 2001.<ref name=Karkoshka2001b/> The radii and widths of ζ and 1986U2R rings were taken from de Pater et al, 2006.<ref name=dePater2006/> The width of λ ring is from Holberg et al, 1987.<ref name=Holberg1987/> The radii and widths of μ and ν rings were extracted from Showalter et al, 2006.<ref name=Showalter2006/>
<li>{{Note_label|G|g|none}} 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.<ref name=Karkoshka2001b/> The equivalent depths of λ and ζ, μ and ν rings are derived using μEW values from de Pater et al, 2006<ref name=dePater2006/> and de Pater et al, 2006b,<ref name=dePater2006b/> 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%.
<li>{{Note_label|H|h|none}} The thickness estimates are from Lane et al, 1986.<ref name=Lane1986/>
<li>{{Note_label|I|i|none}} 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.<ref name=Smith1986/> The normal optical depths of μ and ν rings are peak values from Showalter et al, 2006.<ref name=Showalter2006/>
<li>{{Note_label|J|j|none}} ''Off opposition'' means that the angle between the object-sun direction and object-Earth direction is not zero.
</li>
</ol>
</div>
==References==
{{reflist|2}}
==External links==
*[http://solarsystem.nasa.gov/planets/profile.cfm?Object=Uranus&Display=Rings Uranus' Rings] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
*[http://nssdc.gsfc.nasa.gov/planetary/factsheet/uranringfact.html Uranus Rings Fact Sheet]
*[http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/33/ Hubble Discovers Giant Rings and New Moons Encircling Uranus] – Hubble Space Telescope news release ([[2005-12-22]])
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