[9] The broad component is much thicker (geometrically) than the narrow one. [9][21] 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. [18] This indicates that they are probably composed of a mixture of the ice and a dark material. The lifetime of a smaller satellite is much shorter. 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". [19] The dust may be made of water ice. In order of increasing distance from Uranus, they are: 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ. 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. Resolution here is about 10 km (6 mi). [21][26] This is known only from occultation data because Voyager 2's imaging experiment failed to resolve the δ ring. The resulting equivalent optical depths are 0.14 km and 0.012 km. 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. The maximum size of such moonlets is probably around 10 km. [15] Some of them became visible during a series of ring plane-crossing events in 2007. [20] The ring's eccentricity causes its brightness to vary over the course of its orbit. [15] 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. [9][26] The optical depth of the λ ring shows strong wavelength dependence, which is atypical for the Uranian ring system. Uranus’s atmospheric drag appears to be so large that the present rings themselves may be short-lived. It wasn't firmly established for 70 years after its discovery. [14] Like the ε ring, they exhibit regular variations in brightness and width. [20] The width of the ring varies in the range 3.6–4.7 km, although equivalent optical depth is constant at 3.3 km. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. [20] It is a narrow, faint ring located just inside the ε ring, between it and the shepherd moon Cordelia. [23] Every such disruption would have started a collisional cascade that quickly ground almost all large bodies into much smaller particles, including dust. Later they found four additional rings: one between the β and γ rings and three inside the α ring. In 1986 'Voyager 2' discovered only one such shepherd pair (Cordelia and Ophelia) around the brightest ring (ε). Uranus rings fall into three groups based on size, location, and composition. The discovery of these outer rings doubled the known radius of the ring system. [8] The newly discovered outer ν and μ rings of Uranus are similar to the outer G and E rings of Saturn. [14] They are the most inclined rings, and their orbital eccentricities are the largest excluding the ε ring. A few moonlets must still be embedded within the rings at present. [25] No moon larger than 10 km is known in the vicinity of other rings. [16], Detailed analysis of the Voyager 2 images revealed azimuthal variations in the brightness of the λ ring. Uranus has two sets of rings. [12] 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 ring and ring 4. Moons In this kind of relationship, as the moon and the particles pass one another periodically, they interact gravitationally in a way that tends to maintain the regularity of the encounters. **Equivalent width is the product of the observed width and the fraction of light attenuated and is given for visible light. Certain orbits that lie inside or outside the orbit of a given ring are at the proper radius for a moon in such an orbit to establish a stable dynamic resonance with the ring particles. This table summarizes the properties of the planetary ring system of Uranus. [23], The origin of the dust bands is less problematic. [9] 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. [9] In forward-scattered light, the η ring looked bright, which indicated the presence of a considerable amount of dust in this ring, probably in the broad component. Moons more massive than the rings can halt this spreading in a process called shepherding. 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. 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. [12] 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. By 1978, nine distinct rings were identified. Uranus - Uranus - The ring system: The rings of Uranus were the first to be found around a planet other than Saturn. The relative lack of dust in the ring system may be due to aerodynamic drag from the extended Uranian exosphere. [1] Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". [30], 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. [23] In contrast, the dust bands have relatively few large particles, which results in low optical depth. [15] Voyager 2 observed the rings in different geometries relative to the sun, producing images with back-scattered, forward-scattered and side-scattered light. [8], The rings were directly imaged when the Voyager 2 spacecraft flew through the Uranian system in 1986. Unlike the rings of Saturn, which are very bright and composed of water ice, the rings of Uranus are relatively dark. [15] The ε ring seems to consist of a number of narrow and optically dense ringlets, some of which may have incomplete arcs. [8] The estimates show that the lifetime against collisional disruption of a moon with the size like that of Puck is a few billion years. [9] Such behavior indicates that the ring is not optically thin. [9] The Hubble Space Telescope detected an additional pair of previously unseen rings in 2003–2005, bringing the total number known to 13. Combining the brightness of the rings observed in Voyager images with the equivalent widths from occultations shows that the ring particles reflect less than 5 percent of the incident sunlight. The scattering effects on Voyager’s radio signal propagated through the rings to Earth revealed that the rings consist of mostly large particles, objects greater than 140 cm (4.6 feet) across. Uranus Image: Uranus' Rings. [2], The appearance of the λ ring changed dramatically when it was observed in forward-scattered light in 1986. [16], In addition to the 1986U2R/ζ and λ rings, there are other extremely faint dust bands in the Uranian ring system. [9] Two more faint rings were revealed, bringing the total to eleven. [12] The normal optical depth of this dust is 10−4–10−3. The equivalent depth of the 1986U2R and ζ, The normal optical depths of all rings except ζ, ζ. [9] All rings of Uranus show azimuthal brightness variations. The geometric albedo of the ring particles does not exceed 5–6%, while the Bond albedo is even lower—about 2%. [21] 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. The scheme of Uranus's ring-moon system.

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