Scattering properties of Saturn’s rings in the far ultraviolet from Cassini UVIS spectra

Abstract

We use Cassini UVIS data to determine the scattering properties of Saturn’s ring particles in the FUV. We have replaced the scattering function from the classical Chandrasekhar single scattering radiative transfer equation for reflectance with a ring wake model for the A and B rings derived from stellar occultations. The free parameters in this model are the ring particle Bond albedo, AB, and the ring particle asymmetry parameter, g, which equals the cosine of the most probable scattering angle of a photon from a ring particle. The spectrum of Saturn’s rings from 140 to 190nm shows an absorption feature due to water ice shortward of 165nm. We compare our model values for I/F to lit-side data at 155nm and at 180nm for regions in both the A and B rings. We used the unmodified Chandrasekhar model for the C ring and Cassini Division, and in all cases we determined AB and g in the FUV for the first time. Values of AB vary between 0.04 and 0.091 at 180nm and between 0.012 and 0.019 at 155nm. The variations across the ring of AB at 180nm is consistent with a greater abundance of non-ice contaminant in the C ring and Cassini Division and a minimum in contaminant abundance in the outer B ring. There is little variation in AB at 155nm across the rings, which suggests that the reflectance of the water ice and non-water ice material shortward of the 165nm absorption edge are about the same. Values of g vary between −0.68 and −0.78 at 180nm and between −0.63 and −0.77 at 155nm showing that the ring particles are highly backscattering in the FUV. We find that the wavelength of the absorption feature varies with ring region and viewing geometry indicating a different photon mean path length, L, through the outer layer of the ring particle (Bradley, E.T., Colwell, J.E., Esposito, L.W., Cuzzi, J.N., Tollerud, H., Chambers, L. [2010]. Icarus 206 (2), 458–466). We compared I/F from 152 to 185nm to a radiative transfer spectral model developed by Shkuratov et al. (Shkuratov, Y., Starukhina, L., Hoffmann, H., Arnold, G. [1999]. Icarus 137, 235–246) and modified by Poulet et al. (Poulet, F., Cuzzi, J.N., Cruikshank, D.P., Roush, T., Dalle Ore, C.M. [2002]. Icarus 160, 313–324). We find that L is positively correlated with phase angle, which we attribute to multiple scattering within the particle on length scales comparable to L. We extrapolate L to zero phase angle and find values of L at zero phase ranging from ∼2 to 3μm. This provides a direct measure of the distance from the surface of a ring particle to the first scattering center. L at zero phase is roughly constant across the rings suggesting the outermost 1.25μm of the ring particles have the same structural properties in all ring regions. We azimuthally binned and interpolated observations of the unlit side of the A ring taken during Saturn orbit insertion to a 100km resolution radial profile. We see halos (enhanced brightness) surrounding the Janus 4:3 and Janus 5:4 density waves. We also computed I/F across the A ring using the SOI observational geometry along with AB and the power-law index, n, derived from the retrieval approach from lit side observations. I/F determined by this technique agrees with results from the lit side analysis for the A2 ring but diverge for the inner and outer A ring, which we attribute to multiple scattering effects.