First High Solar Phase Angle Observations of Rhea Using Cassini VIMS: Upper Limits on Water Vapor and Geologic Activity

Using radiances acquired with Cassini's Visual and Infrared Mapping Spectrometer (VIMS), we construct high solar phase angle curves for Saturn's second largest moon, Rhea. Ground-based studies of Rhea and Saturnian icy satellites are focussing on low phase angles; to our knowledge, these are the first solar phase curve data for Rhea on phase angles >70°. We compare these data to similar phase curves for Enceladus at near-infrared wavelengths to estimate the amount of water vapor that could possibly be generated and thus set an upper limit on the amount of geologic activity that may be occurring on Rhea. We find that Enceladus's plume manifests itself in the VIMS solar phase curve at a phase angle near 160° and peaks most strongly for λ = 2.017810 μm. No such peak can be found in the Rhea VIMS phase curve. Absence of a forward scattering peak supports the recent determination that particles in Rhea's surrounding ring are not small. We calculate that the maximum water vapor column density that could be supplied from Rhea ranges from 1.52 × 10¹⁴ to 1.91 × 10¹⁵ cm⁻², 2 orders of magnitude less than what is calculated by Cassini UVIS for Enceladus. This implies that for Rhea, the level of active internal (endogenic) processes is exceedingly small, if any.