thumb|An updated image of Uranus's rings system (the epsilon/ε, zeta/ζ, mu/μ and nu/ν rings are annotated) as imaged by the [[James Webb Space Telescope's near-infrared camera on September 4, 2023|300x300px]]

The rings of Uranus consist of 13 planetary rings. They are 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 Jessica Mink. William Herschel had also reported observing rings in 1789; modern astronomers are divided on whether he could have seen them, as they are very dark and faint.

By 1977, nine distinct rings were identified. Two additional rings were discovered in 1986 in images taken by the Voyager 2 spacecraft, and two outer rings were found in 2003–2005 in Hubble Space Telescope photos. In the order of increasing distance from the planet the 13 known rings are designated 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ. 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 probably composed of water ice with the addition of some dark radiation-processed 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 20 cm to 20 m in diameter. 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 may be due to aerodynamic drag from the extended Uranian exosphere.

The rings of Uranus are thought to be relatively young, and not more than 600 million years old. The Uranian ring system probably originated from the collisional fragmentation of several moons that once existed around the planet. After colliding, the moons probably broke up into many particles, which survived as narrow and optically dense rings only in strictly confined zones of maximum stability.

The mechanism that confines the narrow rings is not well understood. Initially it was assumed that every narrow ring had a pair of nearby shepherd moons corralling it into shape. In 1986 Voyager 2 discovered only one such shepherd pair (Cordelia and Ophelia) around the brightest ring (ε), though the faint ν would later be discovered shepherded between Portia and Rosalind.

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".

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

upright=1.8|thumb|The scheme of [[Uranus'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. 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.

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. 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. 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

thumb|The μ and ν rings of Uranus (R/2003 U1 and U2) in [[Hubble Space Telescope 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

upright|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. 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.|name=footnoteE|group=lower-alpha ||data-sort-type=number|Incl.(°)

  • 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 (22 December 2005)
  • Gazetteer of Planetary Nomenclature – Ring and Ring Gap Nomenclature (Uranus), USGS