Spectrophotometry of Saturn's small satellites and rings from Hubble Space Telescope images

We investigate the photometric properties of four of Saturn's inner satellites (Prometheus, Pandora, Janus, and Epimetheus) and five ring regions (the A, B, and C rings; the Cassini division; and the F ring). We have used Hubble Space Telescope images taken in July 1991 and during the Saturn's ring plane crossings by the Earth (August 1995) and the Sun (November 1995). The spectral range of this study is similar to 0.30-0.95 mu m. We confirm qualitatively but not quantitatively several interesting color properties of the main rings already derived by Estrada and Cuzzi [1996]. The derived albedos of Prometheus and Pandora are consistent with the lower limits based on Voyager images [Thomas et al., 1983], The albedos of the coorbital satellites Janus and Epimetheus are smaller than previously determined. The colors of Janus and Pandora are similar to those of the main rings, indicating the presence of ices altered by red contaminants. Epimetheus is more neutral, with spectral shape similar to that of the G ring. Our data for Prometheus are fragmentary in terms of wavelength coverage; otherwise, the satellite looks like the F ring, which is neutral or slightly blue. This unusual color could be explained if there is an inward migration of the F ring particles through Poynting-Robertson drag, which subsequently sandblast the surface of the satellite. We find a correlation of color with the distance from Saturn. This indicates that the inner satellites are spectrally closer to the main rings than to the major Saturnian satellites. The implications for the origin of the inner satellites are discussed.