Observations and Modeling of the Opposition Surges of the Icy Moons of Saturn Based on Cassini Visual Infrared Mapping Spectrometer Data

Abstract

Observations of the opposition surges on the main moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus) during Cassini's prime and extended missions are reduced and analyzed. The main data set comes from the Visual Infrared Mapping Spectrometer (VIMS) with wavelength coverage in the visible and near-infrared out to 3.60 μm, covering 99% of the solar spectrum. Imaging Science Subsystem images and Ultraviolet Imaging Spectrometer data augment the VIMS data set. Hapke models are fit to Dione and Rhea, and Irvine's simpler shadowing model is fit to the sparser data sets of Enceladus, Dione, and Rhea. The high porosities (∼95% void space in the optically active portion of the regolith) and forward-scattering properties of the surfaces are similar to other icy moons and to Pluto. A change in the character of their opposition surge at 3.60 μm can be attributed largely to the noninteraction of long-wavelength photons with small particles on these moons’ surfaces. The opposition surge on the low- and high-albedo regions of Dione are similar. However, the low-albedo terrain of Iapetus exhibits a less robust surge than the high-albedo regions, which we attribute to differences in surface texture. The low-albedo hemisphere of Iapetus seems to be akin to the small number of bodies in the solar system that lack an appreciable opposition surge, possibly due to the accumulation of low-albedo dust. With observations over the range of the solar spectrum, we compute new values for the bolometric Bond albedos of these moons.