page_title: Rings of Uranus
section_title: ε ring
hierarchical_section_title: Rings of Uranus / Narrow main rings / ε ring
context_page_description: The rings of Uranus 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 1978, 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.
context_section_description: 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.
  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.
  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 ε ring seems to consist of a number of narrow and optically dense ringlets, some of which may have incomplete arcs.
  The ε ring is known to have interior and exterior shepherd moons—Cordelia and Ophelia, respectively. The inner edge of the ring is in 24:25 resonance with Cordelia, and the outer edge is in 14:13 resonance with Ophelia. The masses of the moons need to be at least three times the mass of the ring to confine it effectively. The mass of the ε ring is estimated to be about 10¹⁶ kg.
caption: 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.
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