Rings of Uranus

In 1977, the rings of Uranus were discovered by the astronomical team consisting of Edward W. Dunham, James L. Elliot, and Douglas J. Mink. These rings begin at a distance of nearly 38,000 km from the center of Uranus, and extend up to 98,000 km.

They are thin and only a few kilometer's thick. These rings are not bright, and are composed of water ice with some dark radiation-processed organics.

The rings of Uranus are nearly 600 million years old, and developed from the collisional fragmentation of a number of moons that existed around the planet. After the initial collision, the moons broke up into many particles, that formed narrow and optically dense rings in the zones of maximum stability.

The ε ring is the densest and the brightest ring of the Uranian ring system. It is responsible for about two-thirds of the light reflected by the rings, and has a brightness ratio of 2.5-3.0 approximately. Out of all the rings, it is the most eccentric, and hence, has negligible orbital inclination.

This ring is very thin, and has an estimated thickness of 150 m. It is known for its interior and exterior shepherd moons: Cordelia and Ophelia. The estimated weight of the ring is 10¹⁶ kg.

The δ ring is circular and has a slight orbital inclination. It shows substantial azimuthal variations in normal optical depth and width. The possible reason for that being an azimuthal wave-like structure that the ring possesses, excited by a small moonlet inside it. The ring appears relatively bright, and comprises two components: a narrow optically dense component and a broad inward shoulder with low optical depth.

The γ ring has no orbital inclination. It is narrow, optically dense, and slightly eccentric. The ring's width varies from 3.6-4.7 km, with a constant optical depth of 3.3 km. Geometrically, it is thin like the δ ring, and devoid of dust, showing significant azimuthal variations.

The η ring has negligible orbital eccentricity and inclination. Similar to the δ ring, it comprises two components: a narrow optically dense component and a broad outward shoulder with low optical depth. The ring appears bright, which indicates the presence of a sufficient amount of dust in it. Like other rings, this one also shows considerable azimuthal variations in the normal optical depth and width.

The α and β rings are considered to be the second brightest rings of Uranus. Like the ε ring, these rings show periodic variations in their brightness and width. They also possess a sizable orbital eccentricity and non-negligible inclination.

The widths of the α and β rings are 4.8-10 km and 6.1-11.4 km, respectively. They are geometrically thin and devoid of dust. The masses of these rings are approximately 5×10¹⁵ kg each, nearly half the mass of the ε ring.

Rings 6, 5, and 4 are the innermost and dimmest of all Uranus rings. They are inclined and have large orbital eccentricities. These rings are the narrowest, measuring about 1.6-2.2 km, 1.9-4.9 km, and 2.4-4.4 km in width respectively.

The λ ring is a narrow, dull ring located between the ε ring and the shepherd moon Cordelia. The ring was discovered by the Voyager 2 spacecraft in 1986. This ring is extremely narrow, measuring about 1-2 km, having the equivalent optical depth of 0.1-0.2 km at the wavelength 2.2 μm. Its optical depth shows strong wavelength dependence, which is uncommon for the Uranian ring system.

In 1986, Voyager 2 detected a wide and faint sheet of material inbound of the ring 6. It was given a temporary designation as 1986U2R. It had a normal optical depth of 10⁻³ or less and was located between 37,000 and 39,500 km from the center of Uranus. In late 2003, through the Keck telescope a broad and dull sheet of material was observed inside the ring 6.

The ring was later renamed as the ζ ring. However, the position of the ζ ring differs significantly from the one observed in 1986, and is located between 37,850 and 41,350 km from the center of the planet.

In 2003, the Hubble Space telescope detected a pair of unknown rings μ and ν, later called the outer ring system. These rings are broad, dull, and 17,000 and 3,800 km wide respectively. They have triangular radial brightness profiles, and peak optical depths of 8.5×10⁻⁶ and 5.4×10⁻⁶ respectively.

The μ ring is the outermost of the pair, having twice the distance from the planet as compared to the η ring. Geometrically, the rings are much brighter, which indicates the presence of many micrometer-sized dust particles.

The outer rings of Uranus appear similar to the G and E rings of planet Saturn. The μ ring is blue in color, due to the slight amount of dust that predominates in it, which is probably made of water ice. On the other hand, the ν ring is slightly red in color.

Uranus is one of the 4 planets in our solar system which is orbited by rings. Along with the 1986U2R/ζ and λ rings, there are other extremely dull dust rings in the Uranian ring system. These rings appear bright in forward scattered light, but are invisible during occultations due to negligible optical depth.