Introduction

Voyager 2 provided the first resolved images of the rings of Uranus and of many small satellites which might gravitationally influence them. It also obtained data from a variety of sensors and over a variety of viewing and illumination conditions which help to determine the reflective properties and composition of the ring particles as well as their particle size distribution. However, continuing Earth-based measurements of stellar ring occultations (i.e., blockage of starlight by the rings as viewed from Earth) have provided the longest time base and most accurate data on ring radii, shapes, inclinations, widths, optical depths (transparency), precession rates (how fast the orbit changes orientation), and dynamic stability, all data that were not obtained by Voyager 2 in the short time period during which it was in the vicinity of Uranus. This chapter will follow up the discussions of Chapter 3 on the discovery of the Uranus rings with a discussion of what we really know about the rings 30 years after their initial discovery.

Except for three broad and indistinct rings (the innermost and the two outermost), none of which have yet been given official names by the Nomenclature Committee of the International Astronomical Union, the rings of Uranus are all narrow, relatively sharp-edged bands of intrinsically dark particles. Early Earth-based attempts to image the rings between the 1977 discovery and the 1986 Voyager 2 encounter were hampered by their narrowness, their low reflectivity, their great distance from Earth, and their close proximity to Uranus.

Even following the Voyager 2 encounter and more than 20 years of additional theoretical and observational data since 1986, ring scientists are hard-pressed to explain the mechanisms which confine the rings. Cordelia is about 1,400 km interior to the Epsilon ring and Ophelia is about 2,600 km exterior to the Epsilon ring; it is tempting to assume that these two small satellites gravitationally impede radial spreading in the Epsilon ring. The case for shepherding of the Epsilon ring by Cordelia

(inner edge) and Ophelia (outer edge) is in fact stronger than for any other Uranus ring. Three other rings may possibly be gravitationally constrained by a known satellite at either a ring inner edge or an outer edge, but not both. For the vast majority of the Uranus rings, no known satellites can provide the gravitational forces needed to confine the ring particles. A thorough search by Voyager 2 and subsequent searches by Earth-based observers have not been successful in locating satellites closer to the planet than Cordelia. Either the satellites which might shape the rings of Uranus are too small to have been discovered as yet or else some mechanism other than gravitational shepherding is acting within this unique ring system.

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