Rings of Uranus Totally Explained

Everything about Rings Of Uranus totally explained

Uranus has a complex system of rings. This system is much less complex that of Saturn, but significantly more complex than that of either Jupiter or Neptune. The rings of Uranus were discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink. More than 200 years ago, William Herschel reported that he too observed the rings around Uranus, but while there's some similarity between the rings as observed by Hershel and the real outer rings of Uranus, modern astronomers are skeptical that he could actually have noticed them, as they're very dark and faint. Two additional rings were discovered in 1986 by the Voyager 2 spacecraft, and two outer rings were found in 2003-2005 by the Hubble Space Telescope.
   As of 2008 the Uranian ring system is known to consist of 13 distinct rings. In the order of increasing distance from the planet they are: ζ/1986U2R, 6, 5, 4, α, β, η, γ, δ, λ, ε, Nu and Mu rings. Their radii range from about 38,000 km for the ζ/1986U2R ring to about 98,000 km for the outermost Mu ring. Additional faint dust bands and incomplete arcs may exist between the main rings. The rings of Uranus are extremely dark—the bond albedo of the rings particles doesn't exceed 2%. They are likely composed of water ice with addition of some dark material—probably radiation processed organics.
   The majority of Uranus's rings are narrow and optically dense—they are only a few kilometers wide and have optical depth on order of unity. The ring system overall contains little dust; it consists of large bodies of 0.2–20 m in diameter. However some rings are optically thin: broad and faint ζ/1986U2R, Mu, Nu rings are made of small dust, while narrow and faint λ ring also contain larger bodies. The paucity of dust in the ring system is caused by the aerodynamic drag from the extended Uranian exosphere—corona, which quickly removes dust particles constantly created in collisions.
   The rings of Uranus are thought to be relatively young. Their age can not exceed 600 million years. The mechanism that confines the narrow rings isn't well understood. Initially it was assumed that every ring should have a pair of moons (shepherds) corralling them into shape. However in 1986 Voyager 2 discovered such moons only near the brightest epsilon ring. The moons are now known as Cordelia and Ophelia. The origin of the Uranian ring system is probably connected with the collisional fragmentation of moons that once existed around Uranus. After colliding, the moons fragmented into numerous particles, which probably survived only in strictly confined stable zones around the planet, forming narrow and optically dense rings.

Discovery

Uranus's ring system was the second to be discovered in the Solar System after that of the Saturn. The rings were directly imaged when Voyager 2 spacecraft passed Uranian system in 1986.

General properties

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, Nu, Mu rings. A few dust bands between the rings were observed in the forward-scattering geometry by Voyager 2.
The rings of Uranus are made of an extremely dark material. The geometric albedo of the ring particles doesn't 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. They don’t demonstrate any identifiable spectral features. The chemical composition of the ring particles isn't known. However they can not be made of pure water ice like the rings of Saturn because they're too dark, darker than the inner moons of Uranus.
The geometrical thickness of the ε ring isn't precisely known, although the ring is certainly very thin—by some estimates as thin as 150 m. Despite such a infinitesimal thickness it consists of several layers of particles. The ε ring is a rather crowded place with a filling factor estimated by different sources from 0.008 to 0.06 near the apoapsis. The signal looked like a strong enhancement of the forward-scattering at the wavelength 3.6 cm near ring’s apoapsis. Such a strong scattering requires the existence of a coherent structure within it. 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 Ophelia.

η ring

The η ring has zero orbital eccentricity and inclination. The α and β rings have sizable orbital eccentricity and non-negligible inclination. The optical depth of the λ ring shows strong wavelength dependence, which is atypical for the uranian ring system. It is as high as 0.36 km in the ultraviolet part of the spectrum, which explains why it was initially detected only in the UV stellar occultations by Voyager 2.

1986U2R/ζ ring

In 1986 Voyager 2 spacecraft noticed a broad and faint sheet of material inward of the 6 ring. It hadn't been observed until in 2003–2004, when the Keck telescope found a broad and faint sheet of material just inside the 6 ring. This ring was dubbed ζ ring. This rings were subsequently named Mu and Nu rings. The outermost of them, called the Mu ring, is twice the distance from the planet as the bright η ring. This failure means that Mu ring is blue in color, which in turn indicates that the very small (submicrometre) dust dominates in it. In contrast the Nu ring is slightly red in color.

Dynamics and origin

An outstanding problem of the physics of the narrow uranian rings is their confinement. Without some mechanism that holds their particles together the rings would quickly spread in the radial direction.
Since the rings of Uranus appear to be young, they must be continuously recreated by the collisional fragmentation of larger bodies. Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case, at least for the Nu ring.

List

This table summarizes the properties of the planetary ring system of Uranus.

Ring name	Radius (km)	Width (km)	Eq. depth (km)	N. Opt. depth	Thickness (m)	Ecc.	Incl.(°)	Notes
ζ_c	32,000–37,850	3,500	0.6	~10⁻⁴	?	?	?	Inward extension of the ζ ring
1986U2R	37,000–39,500	2,500	?	<10⁻³	?	?	?	Dusty ring
ζ	37,850–41,350	3,500	1	<10⁻³	?	?	?
6	41,837	1.6–2.2	0.41	0.18–0.25	?	1.0	0.063
5	42,234	1.9–4.9	0.91	0.18–0.48	?	1.9	0.052
4	42,570	2.4–4.4	0.71	0.16–0.3	?	1.1	0.032
α	44,718	4.8–10	3.39	0.3–0.7	?	0.8	0.014
β	45,661	6.1–11.4	2.14	0.2–0.35	?	0.4	0.005
η	47,175	1.9–2.7	0.42	0.16–0.25	?	0?	0.002
η_c	47,176	40	0.85	2	?	0?	0.002	Outward broad component of the η ring
γ	47,627	3.6–4.7	3.3	0.7–0.9	150?	0?	0.011
δ_c	48,300	10–12	0.3	3	?	0?	0.004	Inward broad component of the δ ring
δ	48,300	4.1–6.1	2.2	0.3–0.6	?	0?	0.004
λ	50,023	1–2	0.2	0.1–0.2	?	0?	0?	Dusty ring
ε	51,149	19.7–96.4	47	0.5–2.5	150?	7.9	0.001	Shepherded by Cordelia and Ophelia
Nu	66,100–69,900	3,800	0.012	5.4	?	?	?	Between Portia and Rosalind, peak brightness at 97,700 km
Mu	86,000–103,000	17,000	0.14	8.5	?	?	?	At Mab, peak brightness at 67,300 km

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