Thermal Studies of Saturn's Rings

Details Thermal Studies of Saturn's Rings Thermal Studies of Saturn's Rings

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p53e..02p&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P53E-02

Physics

0545 Modeling (4255), 5465 Rings And Dust, 6275 Saturn

Scientific paper

The observed thermal emission from Saturn's rings has a locational and directional variation that ultimately results from the time dependent response of individual ring particles to the quasi-periodic radiation forcing they experience as they orbit Saturn. The observed thermal emission from any radial region of the rings is representative of a large ensemble of particles or structures which are subject to statistically similar conditions as they orbit. Near any particular radius, the thermal forcing and response are quasi-periodic around an orbit, varying secularly with Saturn's twenty-nine year seasonal cycle. This talk will discuss the coupled thermal and radiative transfer processes within the rings as determined by the interplay between individual particle properties and those of the ensemble (i.e., the ring structure), and the constraints that are placed on those by the most comprehensive thermal observations of the rings to date, taken with the Cassini Infrared Spectrometer (CIRS). Over one hundred thousand thermal-infrared spectra of the rings, between 10 and 500 cm-1, have been taken with the CIRS Focal Plane 1 (FP1) detector since Cassini orbit insertion in July, 2004. They resemble scaled Planck functions with well resolved peaks indicative of temperatures between approximately 60 and 120 K. We investigate the properties of a mapping from a space of physical parameters describing ring particles and their distribution onto predicted time-dependent spectral thermal emission. Ring emission is modeled using a radiative transfer code augmented with ray tracing calculations; a thermal model is embedded within the calculation to model the particles as a thermal source, and statistical averaging is incorporated. The model is specified by a vector of parameters describing a vertically varying particle size distribution, spin distribution, thermal inertia, albedo and optical depth, and when driven by radiation calculated from ephemeris parameters it produces a self-consistent, time- and direction-dependent emission spectrum accounting for mutual shading, interparticle heating, and directionally varying emission. Any parameter vector generates a set of emission spectra that vary with orbital location and direction, and perturbations of the parameters allow investigation of the mapping represented by the model. 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 and limitations of the model. This research was carried out at JPL/Caltech, under contract with NASA.

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