Rings and Satellites of Uranus: Colorful and Not So Dark

Abstract

Photometric properties of nine uranian satellites and four rings, based on six Hubble Space Telescope images taken in 1995, are presented. Derived albedos are consistent with previous data taken at the same phase angle of 1°, but inconsistent with most Voyager-based estimates extrapolated from observations at phase angles above 15°. The shape of phase functions in the range 1–90° is similar to that of asteroids. Darker surfaces have steeper phase functions than brighter ones, except for the four brightest satellites, which have the same phase function. Puck's geometric albedo in the visible is 0.11 ± 0.015, much larger than the Voyager-based value of 0.074 ± 0.008. The satellites smaller than Puck may be 10% larger than Voyager-based estimates. Ring particles have a geometric albedo of 0.061 ± 0.006, much larger than the Voyager-based value of 0.032 ± 0.003. The longitudinal variation of brightness of the ϵ ring indicates that the mean separation of particles in the ring is four to five times their diameter. While the uranian rings and satellites seemed to be all gray heretofore, the wide wavelength range of this study, 340–910 nm, detected their subtle, distinct colors. Rings and the minor satellites are brown, Miranda is blue, Umbriel is red, and Ariel, Titania, and Oberon are yellow. Rings and minor satellites belong spectrally to M-type asteroids.