Water ice abundance and CO2 band strength on the saturnian satellite Phoebe from Cassini/VIMS observations

Abstract

We have studied the near-infrared spectrum of the Saturn satellite Phoebe, a distant satellite observed before Cassini’s Saturn orbit insertion, using data from the Visual and Infrared Mapping Spectrometer (VIMS) on the Cassini orbiter. We have done a critical calibration of the dataset that involves careful correction of dark artifacts. We model areally mixed water ice and non-ice (assumed segregated because of the low ∼3% albedo of the non-ice material) for several high and medium resolution observations of Phoebe made near closest approach. Using a Hapke roughness factor of 15°, we find ice abundances from ∼0.1% to over 4%. The ice grain radii vary from 1 to 10 μm. These are displayed on a projected map of Phoebe with about 50% coverage (about 33% coverage at better than 5 km spatial resolution). Detailed looks at the water ice spectral fits shows that the weak 1.05 and 1.25-μm bands are missing in most of the spectra, implying that the ice endmember is not pure ice, but has a dark material mixed with it that lowers the albedo and suppresses these bands. We made a model of ice contaminated with Phoebe-like dark material showing that a few percent of dark material lowers the albedo to ∼50% and suppresses the bands. The dirty ice model produces better fits to the spectra and implies that the amount of dirty ice is about 1.5 times the amount of pure ice. We have also calculated the CO2 band depth for these same observations and projected the results. The CO2 band depth varies inversely with water ice abundance.