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F. Poulet, J.N. Cuzzi (NASA Ames Research Center), L. Dones (Southwest Research Institute), P. Drossart (DESPA Observatoire de Paris), R.G. French (Wellesley College)
During several HST orbits, the Wide-Field camera acquired pictures of Saturn's ring system in several wideband filters as presented by Cuzzi et al. (this meeting). We selected a part of these images for measuring the spectrophotometric behavior of four main rings (A, B, C rings and Cassini Division) over several phase angles below 6 degrees and in two wavelengths (U and V filters). The data are particularly useful in helping to disentangle whether individual or collective properties of the ring particles create the observed color variation with illumination and viewing geometry. The geometrical dependence of color variations is confined almost entirely to phase angle and almost lacking with the ring opening angle. This implies that the collective properties of the ring particles are less important than the individual properties of the ring particles. An unusually large ring reddening with increasing phase angles is observed implying that the phase function is dependent on wavelength. We test the hypothesis that this effect is due to intraparticle shadow-hiding, multiple scattering, or some combination of two phenomena. We use Drossart's model (Drossart, PSS 41, 381-393, 1995) which we extended to include multiple scattering, Hapke's model (Hapke, Icarus 67, 264-280, 1986) and Shkuratov's model (Shkuratov et al., Icarus 141, 132-155, 1999) to describe the properties of the surfaces of the ring particles. Our results will be presented and discussed in terms of the surface texture.
This work was performed while the author held a National Research Council - NASA ARC Research Associateship.