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S. Pilorz, L. Spilker, B. Wallis, S. Brooks (JPL/CalTech), J. Pearl, F. M. Flasar (GSFC), CIRS Team
Observations of Saturn's rings taken with the Cassini Infrared Spectrometer (CIRS) reveal that thermal emission from the rings often has a pronounced directional variation, as well as variation with local hour angle and radius. Spectra taken with the CIRS Focal Plane 1 (FP1) detector in the thermal-infrared region (10 to 500 cm-1) resemble a scaled Planck function with a well resolved peak indicative of temperatures near those expected for the ring particles, between approximately 80 and 110 K.
The size of the FP1 footprint on the rings varies from hundreds to thousands of kilometers, and the spectra are representative of an ensemble of particles, containing valuable information about the ring structure and particle properties. Simple inversion techniques are of limited use for extracting this information, so we take a forward modelling approach to the inverse problem, investigating the properties of the mapping from physical parameters onto predicted thermal emission.
We model ring emission using a radiative transfer calculation that contains a thermal model for particle heating and emission, including spin effects. Given illumination conditions and a vector of parameters describing a vertically varying particle size distribution, spin distribution, thermal inertia, albedo and optical depth, it calculates a self consistent emission spectrum accounting for mutual shading, interparticle heating, and directionally varying emission. For a ringlet at a given radius, each parameter vector generates a set of predicted emission spectra varying with hour angle and direction, providing a mapping from the parameter space onto a high dimensional space of observation vectors.
We present analyses of the dimensionality of some sets of Cassini observations, evaluate the agreement of some simple mono- and multi-layer toy models with these, and present sensitivity studies highlighting the interplay of parameters. Limitations of the model are discussed.
This research was carried out at JPL/Caltech, under contract with NASA.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.