Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p54a..10p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P54A-10
Mathematics
Logic
[5759] Planetary Sciences: Fluid Planets / Rings And Dust, [6265] Planetary Sciences: Solar System Objects / Planetary Rings, [6275] Planetary Sciences: Solar System Objects / Saturn
Scientific paper
The Cassini spacecraft has been in orbit about Saturn collecting data from July, 2004 to the present. During this time, Saturn has progressed through almost a quarter of its year, and the incident solar illumination angle on the rings has dropped from an elevation of 25 degrees-South to 0 degrees, obtained at Saturn equinox on August 11, 2009. The Composite Infrared Spectrometer (CIRS) instrument has collected data during 480 dedicated observations of the rings and several hundred rider observations, resulting in a total of more than 350,000 ring spectra taken at different viewing geometries and epochs. The CIRS spectra extend from <10 to >700cm^-1, amply covering the spectral location of the Wien peak for Planck radiators at the temperatures seen in the rings of 40 to 120K. The vast majority of the ring spectra produce excellent fits to a Planck function multiplied by a scalar. The location of the peak provides a robust indicator of physical temperatures within the instrument field of view, and an accompanying scalar 'filling factor' contains information about the optical cross section, thermal heterogeneity, and emissivity. Previous modeling of the ring thermal spectra has focussed on fitting small subsets of the data, approaching the constraint of thermal inertia, possible particle spin, low phase behavior, and packing structure. Although in each area plausible model fits have been obtained using reasonable hypotheses about the rings, the models used are not unique and generally contain enough parameters such that for any model, a good fit to data implies more about phenomenological properties than about the actual physical state of ring particles. We present the first comprehensive and systematic fitting to date using all spectra for various radial regions, in the context of a forcing/response picture for individual ringlets. In previous Cassini CIRS investigations, approximate normal optical depths for the rings have been used to analyze the scalar filling factor derived from the spectra. We present a new reduced data set using angle-dependent cross sections for each footprint derived from high resolution UVIS occultations, enabling investigation of small scale variations in temperature and particle emissivity in different ring regions. The simple forcing/response model assumes independently orbiting spherical particles, treated consistently by classical radiative transfer, approximate thermal heating solutions, and orbital elements assigned for consistency with observations. The model is not sufficient to fit all data for any ringlet, and points to further avenues of research. Early indications are that the flux available to particles is highly dependent on local small scale structure near their radial location in the rings. As an exercise in evaluating models, we formally examine the null hypothesis that ring particles are independent rotators with randomly oriented spin. This research was carried out at the SETI Institute, under contract with NASA.
Altobelli Nicolas
Colwell Joshua E.
Pilorz Stu
Showalter Mark
Spilker Linda J.
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