indicates that they contain much micrometer-sized dust. The outer rings of Uranus may be similar to the G and E rings of Saturn as E ring is extremely broad and receives dust from Enceladus. The μ ring may consist entirely of dust, without any large particles at all. This hypothesis is supported by observations performed by the Keck telescope, which failed to detect the μ ring in the near infrared at 2.2 μm, but detected the ν ring. This failure means that the μ ring is blue in color, which in turn indicates that very small (submicrometer) dust predominates within it. The dust may be made of water ice. In contrast, the ν ring is slightly red in color. Dynamics and origin An outstanding problem concerning the physics governing the narrow Uranian rings is their confinement. Without some mechanism to hold their particles together, the rings would quickly spread out radially. The lifetime of the Uranian rings without such a mechanism cannot be more than 1 million years. The most widely cited model for such confinement, proposed initially by Goldreich and Tremaine, is that a pair of nearby moons, outer and inner shepherds, interact gravitationally with a ring and act like sinks and donors, respectively, for excessive and insufficient angular momentum (or equivalently, energy). The shepherds thus keep ring particles in place, but gradually move away from the ring themselves. To be effective, the masses of the shepherds should exceed the mass of the ring by at least a factor of two to three. This mechanism is known to be at work in the case of the ε ring, where Cordelia and Ophelia serve as shepherds. Cordelia is also the outer shepherd of the δ ring, and Ophelia is the outer shepherd of the γ ring. No moon larger than 10 km is known in the vicinity of other rings. The current distance of Cordelia and Ophelia from the ε ring can be used to estimate the ring’s age. The calculations show that the ε ring cannot be older than 600 million years. Since the rings of Uranus appear to be young, they must be continuously renewed by the collisional fragmentation of larger bodies. The estimates show that the lifetime against collisional disruption of a moon with the size like that of Puck is a few billion years. The lifetime of a smaller satellite is much shorter. Therefore, all current