Cassini–VIMS observations of Saturn’s main rings: I. Spectral properties and temperature radial profiles variability with phase angle and elevation

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2014-10, Vol.241, p.45-65
• Main Authors: Filacchione, G., Ciarniello, M., Capaccioni, F., Clark, R.N., Nicholson, P.D., Hedman, M.M., Cuzzi, J.N., Cruikshank, D.P., Dalle Ore, C.M., Brown, R.H., Cerroni, P., Altobelli, N., Spilker, L.J.
• Format: Article
• Language: English
• Subjects: Aliphatic compounds; Cassini mission; Grain size; Ices; Infrared; Pollution abatement; Saturn, rings; Spectra; Spectroscopy; Thermal properties; Wavelengths
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2014.06.001

•VIS–IR spectrograms are built from ten Cassini–VIMS ring mosaics.•Spectral properties are inferred using VIS slopes, water ice band depths and I/F ratios.•Characterization of the spectral properties at different viewing and illumination geometries.•Position of the 3.6μm continuum maximum is used to infer rings temperature.•Detection of the aliphatic feature at 3.42μm on C ring and Cassini division spectra. The spectral properties and thermal behavior of Saturn’s rings are determined from a dataset of ten radial mosaics acquired by Cassini–VIMS (Visual and Infrared Mapping Spectrometer) between October 29th 2004 and January 27th 2010 with phase angle ranging between 5.7° and 132.4° and elevation angles between −23.5° and 2.6°. These observations, after reduction to spectrograms, e.g. 2D arrays containing the VIS–IR (0.35–5.1μm) spectral information versus radial distance from Saturn (from 73.500 to 141.375km, 400km/bin), allow us to compare the derived spectral and thermal properties of the ring particles on a common reference. Spectral properties: rings spectra are characterized by an intense reddening at visible wavelengths while they maintain a strong similarity with water ice in the infrared domain. Significant changes in VIS reddening, water ice abundance and grain sizes are observed across different radial regions resulting in correlation with optical depth and local structures. The availability of observations taken at very different phase angles allows us to examine spectrophotometric properties of the ring’s particles. When observed at high phase angles, a remarkable increase of visible reddening and water ice band depths is found, probably as a consequence of the presence of a red-colored contaminant intimately mixed within water ice grains and of multiple scattering. At low phases the analysis of the 3.2–3.6μm range shows faint spectral signatures at 3.42–3.52μm which are compatible with the CH2 aliphatic stretch. The 3.29μm PAH aromatic stretch absorption is not clearly detectable on this dataset. VIMS results indicate that ring particles contain about 90–95% water ice while the remaining 5–10% is consistent with different contaminants like amorphous carbon or tholins. However, we cannot exclude the presence of nanophase iron or hematite produced by iron oxidation in the rings tenuous oxygen atmosphere, intimately mixed with the ice grains. Greater pollution caused by meteoritic material is seen in the C ring and Cassini division while the low