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P. D. Nicholson (Cornell), R. G. French (Wellesley College)
An atlas of optical depth profiles of Saturn's rings at a uniform radial resolution of 20~km has been assembled from observations by the Voyager spacecraft in 1980/81 (UVS, PPS, imaging and RSS datasets) and Earth-based observations of the 28~Sgr stellar occultation in July 1989 (at wavelengths of 0.9, 2.1, 3.3 and 3.9~\mum). The radius scale is that derived by French et al. [{\sl Icarus} {\bf 103}, 163, 1993]. Ratios of the optical depths measured in the near-IR to the PPS optical depths at \lambda0.27~\mum fall generally between 0.9 and 1.5, with systematic variations between the major ring regions which we attribute to variations in the angular width of the rings' forward scattering cross section (Cuzzi [{\sl Icarus} {\bf 63}, 312, 1985]; Nicholson et al. [{\sl B.A.A.S.} {\bf 23}, 1178, 1991]). The data are consistent with previous inferences that the fraction of micron-sized dust in the main rings is very small. Moreover, narrow ring features in the Earth-based data show {\it enhanced} contrast relative to the PPS data in the low-optical depth C Ring and Cassini Division but {\it reduced} contrast in the more opaque B and A Rings, while in several narrow gaps and adjacent to sharp edges in the rings the observed flux significantly {\it exceeds} the unocculted stellar flux. Both effects can again be explained by the presence of a significant component of diffuse, forward-scattered light in the 28~Sgr data.
Using the Voyager PPS optical depth profile, \tau'(r) to predict the attenuated, directly-transmitted flux, we extracted the forward-scattered component of the flux in the 28~Sgr lightcurves via the equation: I\rm sc = I\rm obs - e-2\tau'/\mu. (\mu is the usual geometric projection factor while the factor of 2 is due to the extinction efficiency for macroscopic particles). The resulting radial profiles of scattered light at \lambda0.9, 2.1 and 3.9~\mum were modelled by adjusting the parameters of a simple power law particle size distribution, n(a)da \propto a-qda. We find satisfactory fits to the 28~Sgr data with size distributions similar to those derived by Zebker et al. [{\sl Icarus} {\bf 64}, 531, 1985] from the Voyager RSS data. Best-fit power law indices are q~q3.1 in the C Ring and q~q2.75 in the A and B Rings, with upper size cutoffs at radii of 10--20~m in all regions. Our models also yield a lower size cutoff of ~0~cm in the A and B Rings, ~~cm in the C Ring and outermost A Ring, and perhaps as small as 0.1~cm in the Cassini Division.