Rings of Uranus

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". The definitive discovery of the Uranian rings was made by astronomers James L. Elliot, Edward W. Dunham, and Jessica Mink (known at the time as Douglas Mink) on March 10, 1977, using the Kuiper Airborne Observatory, and was serendipitous. They planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. When their observations were analysed, they found that the star disappeared briefly from view five times both before and after it was eclipsed by the planet. They deduced that a system of narrow rings was present. The five occultation events they observed were denoted by the Greek letters α, β, γ, δ and ε in their papers. These designations have been used as the rings' names since then. Later they found four additional rings: one between the β and γ rings and three inside the α ring. The former was named the η ring. The latter were dubbed rings 4, 5 and 6—according to the numbering of the occultation events in one paper. It was suggested that the 4, 5, and 6 rings be renamed θ, ι, and κ rings, respectively, but this was not taken up by the IAU or the astronomical community. Regardless, rings discovered later still used the Greek lettering scheme but resumed at λ, skipping the previous three letters. The rings were directly imaged when the Voyager 2 spacecraft flew through the Uranian system in 1986. == General properties ==

's ring-moon system. Solid lines denote rings; dashed lines denote orbits of moons. This diagram excludes the moon Uranus XXVIII, which was not yet discovered at the time of the diagram's creation. |473x473px As currently understood, 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. A number of dust bands between the rings were observed in forward-scattering geometry by Voyager 2. The rings particles demonstrate a steep opposition surge—an increase of the albedo when the phase angle is close to zero. The rings are slightly red in the ultraviolet and visible parts of the spectrum and grey in near-infrared. They exhibit no identifiable spectral features. The chemical composition of the ring particles is not known. They cannot be made of pure water ice like the rings of Saturn because they are too dark, darker than the inner moons of Uranus. As a whole, the ring system of Uranus is unlike either 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. There are similarities with some parts of the latter ring system; the Saturnian F ring and the Uranian ε ring are both narrow, relatively dark and are shepherded by a pair of moons. The newly discovered outer ν and μ rings of Uranus are similar to the outer G and E rings of Saturn. Narrow ringlets existing in the broad Saturnian rings also resemble the narrow rings of Uranus. In addition, dust bands observed between the main rings of Uranus may be similar to the rings of Jupiter. In contrast, the Neptunian ring system is quite similar to that of Uranus, although it is less complex, darker and contains more dust; the Neptunian rings are also positioned further from the planet. == Narrow main rings ==

ε (epsilon) 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. 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. 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. Despite such infinitesimal thickness, it consists of several layers of particles. The ε ring is a rather crowded place with a filling factor near the apoapsis estimated by different sources at from 0.008 to 0.06. The Voyager 2 spacecraft observed a strange signal from the ε ring during the radio occultation experiment. 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. The sharp outer edge of the δ ring is in 23:22 resonance with Cordelia. α (alpha) and β (beta) rings After the ε ring, the α and β rings are the brightest of Uranus's rings. The α and β rings have sizable orbital eccentricity and non-negligible inclination. Rings 6, 5 and 4 Rings 6, 5 and 4 are the innermost and dimmest of Uranus's narrow rings. They are the most inclined rings, and their orbital eccentricities are the largest excluding the ε ring. 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. Rings 6, 5 and 4 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. 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. They were not visible during a ring plane-crossing event in 2007 due to their narrowness and lack of dust. ==Dusty rings==

λ (lambda) ring (172.5°) 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. The optical depth of the λ ring shows strong wavelength dependence, which is atypical for the Uranian ring system. The equivalent depth is as high as 0.36 km in the ultraviolet part of the spectrum, which explains why λ ring was initially detected only in UV stellar occultations by Voyager 2. 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. It was not observed again until 2003–2004, when the Keck telescope found a broad and faint sheet of material just inside ring 6. This ring was dubbed the ζ ring. 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. ==μ (mu) and ν (nu) rings==

images from 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. The μ ring is the outermost of the pair, and is twice the distance from the planet as the bright η ring. This failure means that the μ ring is blue in colour, which in turn indicates that very small (submicrometer) dust predominates within it. In contrast, the ν ring is slightly red in colour. ==Dynamics and origin==

'' 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. 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. Since the rings of Uranus appear to be young, they must be continuously renewed by the collisional fragmentation of larger bodies. 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. 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. Every such disruption would have started a collisional cascade that quickly ground almost all large bodies into much smaller particles, including dust. 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. A few moonlets must still be embedded within the rings at present. The maximum size of such moonlets is probably around 10 km. The origin of the dust bands is less problematic. The dust has a very short lifetime, 100–1,000 years, and should be continuously replenished by collisions between larger ring particles, moonlets and meteoroids from outside the Uranian system. 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. 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 meter-sized bodies. Such a distribution increases the surface area of the material in the rings, leading to high optical density in back-scattered light. In contrast, the dust bands have relatively few large particles, which results in low optical depth. ==Exploration==

The rings were thoroughly investigated by the Voyager 2 spacecraft in January 1986. Two new faint rings—λ and 1986U2R—were discovered bringing the total number then known to eleven. Rings were studied by analysing results of radio, ultraviolet and optical occultations. Voyager 2 observed the rings in different geometries relative to the Sun, producing images with back-scattered, forward-scattered and side-scattered light. Analysis of these images allowed derivation of the complete phase function, geometrical and Bond albedo of ring particles. Two rings—ε and η—were resolved in the images revealing a complicated fine structure. Analysis of Voyager's images also led to discovery of eleven inner moons of Uranus, including the two shepherd moons of the ε ring—Cordelia and Ophelia. ==List of properties==

This table summarizes the properties of the planetary ring system of Uranus. ==Notes==