What do the FUV Phase Curves of Saturn’s Rings Tell Us About Their Composition?

When determining the composition of Saturn’s rings via remote sensing observations, we must consider the morphology of the rings. The observed spectra of the rings are influenced by a variety of factors, such as regolith grain size, material mixing ratios, and macroscopic roughness or texture. The later case can be determined by observation of the rings over many different viewing geometries. This is important to determine because a rougher surface can darken the rings which makes it appear as if it is more polluted than it really is. We present FUV reflectance phase curves of Saturn’s A, B and C rings, as measured by Cassini UVIS over a wide range of phase angles. For each ring, we then calculate a best fit to a Hapke reflectance model to determine the mean slope angle of the ring surface. A higher mean slope angle indicates a macroscopically rough surface.

But how can surface roughness give an indication about the composition of the rings? For that, we again utilize a best fit to Hapke’s model for bidirectional reflectance. We use the mean slope angle, calculated in the previous step, to constrain the model and reduce the number of free parameters. We then fit in the spectral dimension with a single free parameter, which is the fractional pollution of the rings. We then perform this fit for different end member candidate materials to determine the most likely composition of the rings.