<P> 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% . The rings particles demonstrate a steep opposition surge--an increase of the albedo when the phase angle is close to zero . This means that their albedo is much lower when they are observed slightly off the opposition . 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 . 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 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 . </P> <P> 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 . </P> <P> 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 . While it is the most eccentric of the Uranian rings, it has negligible orbital inclination . 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 maximum / minimum brightness ratio is about 2.5--3.0 . 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 . As the ring becomes wider, the amount of shadowing between particles decreases and more of them come into view, leading to higher integrated brightness . 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 . Such behavior indicates that the ring is not optically thin . Indeed, occultation observations conducted from the ground and the spacecraft showed that its normal optical depth varies between 0.5 and 2.5, being highest near the periapsis . The equivalent depth of the ε ring is around 47 km and is invariant around the orbit . </P> <P> 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 mean size of the ring particles is 0.2--20.0 m, and the mean separation is around 4.5 times their radius . The ring is almost devoid of dust, possibly due to the aerodynamic drag from Uranus's extended atmospheric corona . Due to its razor - thin nature the ε ring is invisible when viewed edge - on . This happened in 2007 when a ring plane - crossing was observed . </P>

Which describes how the rings of uranus were discovered