Saturn’s icy satellites investigated by Cassini - VIMS. V. Spectrophotometry

Abstract

Albedo, spectral slopes, and water ice band depths maps for the five midsized saturnian satellites Mimas, Enceladus, Tethys, Dione, and Rhea have been derived from Cassini-Visual and Infrared Mapping Spectrometer (VIMS) data. The maps are systematically built from photometric corrected data by applying the Kaasalainen-Shkuratov model (Kaasalainen et al., 2001; Shkuratov et al., 2011). In this work a quadratic function is used to fit phase curves built by filtering observations taken with incidence angle i≤70∘, emission angle e≤70∘, phase angle 10∘≤g≤120∘, and Cassini-satellite distance D≤100.000 km. This procedure is systematically repeated for a subset of 65 VIMS visible and near-infrared wavelengths for each satellite. The average photometric parameters are used to compare satellites’ properties and to study their variability with illumination conditions changes. We derive equigonal albedo, extrapolated at g = 0∘, not including the opposition effect, equal to 0.63 ± 0.02 for Mimas, 0.89 ± 0.03 for Enceladus, 0.74 ± 0.03 for Tethys, 0.65 ± 0.03 for Dione, 0.60 ± 0.05 for Rhea at 0.55μm. The knowledge of photometric spectral response allows to correct individual VIMS spectra used to build maps through geolocation. Maps are rendered at a fixed resolution corresponding to a 0.5∘×0.5∘ bin on a longitude by latitude grid resulting in spatial resolutions of 1.7 km/bin for Mimas, 2.2 km/bin for Enceladus; 4.7 km/bin for Tethys; 4.5 km/bin for Dione; 6.7 km/bin for Rhea. These spectral maps allow establishing relationships with morphological features and with endogenic and exogenic processes capable to alter satellites’ surface properties through several mechanisms. The hemispheric dichotomies in albedo and spectral indicators between leading and trailing hemispheres are common properties of all midsized satellites: the accumulation of fine E ring grains is responsible for the higher albedo measured across the leading hemispheres of Tethys, Dione, Rhea, and the trailing side of Mimas. Conversely, the dark and red-colored material visible across the trailing hemispheres of Tethys, Dione, and Rhea is associated with the implantation of cold plasma particles. The thermal anomaly lenses located on the leading equatorial regions of Mimas and Tethys have been resolved and mapped. VIMS data evidence that the distribution of water ice band depths on Mimas lens is biased by the presence of the large Herschel impact crater pointing to a different regolith size distribution with respect to Tethys. Maps show local changes of albedo and spectral indicators in correspondence of recent impact craters (Inktomi on Rhea, Creusa on Dione) and on Dione’s wispy terrains are caused by the exposure of pristine water ice. Enceladus’ tiger stripes, the active sources of plumes in the southern polar region, despite being partially resolved on VIMS maps allow measuring an exceedingly high band depths in comparison with the rest of the satellite’s surface. Moreover, Enceladus’ smooth terrains located on the leading hemisphere around (lon,lat)=(90∘,30∘) show peculiar properties (low infrared albedo, positive 0.35-0.55μm slope and maximum band depth) possibly associated with the presence of a buried diapir in this area. The variability of average spectral albedos and indicators induced by phase angle is investigated and exploited to establish a comparison among satellites’ properties as a function of orbital distance from Saturn.