RINGS AND SATELLITES OF URANUS - COLORFUL AND NOT SO DARK

Photometric properties of nine uranian satellites and four rings, base d on six Hubble Space Telescope images taken in 1995, are presented. D erived albedos are consistent with previous data taken at the same pha se angle of 1 degrees, but inconsistent with most Voyager-based estima tes extrapolated from observations at phase angles above 15 degrees. T he shape of phase functions in the range 1-90 degrees is similar to th at of asteroids. Darker surfaces have steeper phase functions than bri ghter ones, except for the four brightest satellites, which have the s ame 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. T he satellites smaller than Puck may be 10% larger than Voyager-based e stimates. Ring particles have a geometric albedo of 0.061 +/- 0.006, m uch larger than the Voyager-based value of 0.032 +/- 0.003. The longit udinal variation of brightness of the epsilon ring indicates that the mean separation of particles in the ring is four to five times their d iameter. While the uranian rings and satellites seemed to be all gray heretofore, the wide wavelength range of this study, 340-910 nm, detec ted 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 as teroids. (C) 1997 Academic Press.