Spectrophotometric Modeling of Rhea's Surface from Vims Data

Abstract

The huge amount of hyperspectral data from the VIMS (Visual Infrared Mapping Spectrometer) instrument onboard Cassini spacecraft allows the study of the surface properties of icy bodies in Saturnian system in the 0.35 - 5.1 um spectral range. In this work we have analyzed 112 full disk images of Rhea with solar phase angle range 0°-110°. The goal of this work is to perform a quantitative evaluation of physical parameters affecting the spectrophotometric behavior of the satellite's surface. We have applied the Hapke model (Hapke, 1993) in order to describe both the full-disk phase function at each wavelength and the spectrum at given phase angles. With this approach we are able to constrain ice grain size and the amount of organic contaminants as well as the opposition effect surge and the surface roughness of Rhea. The best fit model is represented by an intraparticle mixture of water ice and Triton tholin (99.6%-0.4%) and particle radius of 38 um. What emerges from the analysis is that wavelength dependent parameters, i.e. opposition surge width and amplitude (h, Bo) and single particle phase function parameters (b,v), are correlated with the estimated single scattering albedo of particles (w), as expected for media with grain size larger than the wavelength. Regarding the opposition effect, we find that both Shadow Hiding and Coherent Backscattering contribute. The surface roughness parameter we obtain is theta=33°. This value is fairly high if compared to surface structures (e.g. reliefs or craters), which supports the hypotesis of a correlation with size scales on the order of particle clumping. This research is fully supported by an Italian Space Agency grant.