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F. Poulet, D.P. Cruikshank, J.N. Cuzzi, T. Roush (NASA Ames Research Center)
We used the NASA IRTF spectrograph Spex to obtain near-ir spectra (0.9-5.1 microns) of the Saturn's rings, achieving spectral resolution of about 1500. The spatial resolution (about 1 arcsec) is sufficient to distinguish the three main ring components (A, B and C rings) from one another. We combined these data with recent photometric observations acquired by HST in the range 0.3-1.0 microns (Cuzzi et al., submitted to Icarus). The spectra of all rings are dominated by strong absorption bands due to crystalline water ice. The shape and the depth of these absorptions differ for each ring, which indicates different water ice grain sizes. Our preliminary analysis show no spectral evidence for ices of CO2, CO, NH3 or CH4. At 0.95 microns, there is a broad absorption in the spectrum of the C ring due to a yet-unidentified contaminant.
With the Shkuratov model (Shkuratov et al. 1999), we constrained the relative abundances and the grain sizes of the water ice, of a dark component (amorphous carbon) to adjust the albedo and the presence of a second contaminant to reproduce the reddening in the UV-visible range. The preferred contaminant is some organic material represented here by tholins. The tholin particles are mixed with water ice particles as an intra-mixture, namely some individual water ice particles contain themselves small inclusions of tholins in their bulk or near their surface.
This work was performed while FP held a National Research Council research associateship.