VIS-IR spectrophotometric indicators for Saturn's rings and spokes

Details VIS-IR spectrophotometric indicators for Saturn's rings and spokes VIS-IR spectrophotometric indicators for Saturn's rings and spokes

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p51b1128f&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P51B-1128

Other

[6265] Planetary Sciences: Solar System Objects / Planetary Rings

Scientific paper

The composition of Saturn's main rings can be inferred through the application of VIS-IR spectral indicators applied to Cassini-VIMS observations and comparison of these properties with those of other icy bodies in the Saturn system and in the outer solar system. We have selected several rings mosaics taken at observation geometries at which the contamination of the rings spectra by saturnshine in negligible. Main ring particles are composed of almost pure crystalline water ice: at VIS wavelengths <0.52 µm the rings have a distinctive reddish color indicative of a non-icy component that must not exceed (based on microwave observations) a few percent. No other IR absorption feature of CO2, CH4, silicates or C-H organics (i.e. tholins) are detected in VIMS spectra. By using several spectral indicators (visible colors, water ice band depths and particle albedo) we map the radial variation of the rings composition, showing how it correlates with the local surface mass density and/or optical depth: C ring and Cassini Division are more contaminated by non-icy material than the A and B rings. The degree of visual “redness” is highly correlated with ice band strengths, suggesting the presence of a UV absorber, still unidentified, intimately mixed within the icy component. We have also applied the spectral indicators to several observations of the B ring in which spokes are visible. These shadowy radial structures, first discovered by Voyager, were almost absent during the Cassini nominal mission (2004-2008) but have become more and more frequent as Saturn approaches equinox. When observed in reflectance in the 0.35 to 3 µm range, the region of the B ring intersected by the spoke has a lower reflectance than adjacent points at the same radial distance, as seen by Voyager and HST, and interpreted as local overabundance of submicron-sized grains. The presence of the spoke don't influence the rings spectra in the 3-5 µm range nor introduce variations in the distribution of the 1.25, 1.5 and 2.0 µm water ice band depths. These characteristics are compatible with spoke's particles having a radius of about 1 µm or less.

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