Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13b1668b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13B-1668
Physics
[5759] Planetary Sciences: Fluid Planets / Rings And Dust
Scientific paper
We use data from Cassini's Composite Infrared Spectrometer to characterize ring shadow cooling and to document radial variations in ring thermal inertia. CIRS records infrared radiation between 7 and 1000 microns. Far infrared radiation (16.7 - 1000 microns) is recorded at focal plane 1 (FP1). Thermal emission from Saturn's rings peaks at FP1 wavelengths. As shown in Spilker et al. (2005, 2006), ring thermal emission is well characterized as blackbody emission multiplied by a scalar factor related to the emissivity of the rings. Ring temperatures are generally warmer and the rings show significantly more thermal contrast at larger solar elevations. The thermal budget of the rings is dominated by incident solar radiation. When ring particles enter Saturn's shadow this source of energy is abruptly cut off with a consequential decrease in ring temperature. In order to quantify this cooling, FP1 scanned the main rings repeatedly with a constant offset from the ingress shadow boundary. From such shadow observations we create cooling curves at specific locations. By resampling the FP1 scans onto a consistent radial grid, we can document the cooling of the ensemble of ring particles as they enter Saturn's shadow. Observations of the lit side of the rings reveal that shadow cooling is most significant in the C ring and Cassini Division. More modest cooling occurs in the B and A rings. More importantly, we can show that cooling rates vary with location within each ring. These cooling rates reflect local reactions to the short-term change in thermal forcing as the ring particles enter Saturn's shadow. Observations of the unlit side of the rings tell a different tale. The optically thin rings show some shadow cooling, but this cooling is not as pronounced as on the lit side. No evidence for shadow cooling exists for the unlit A and B rings. Temperature variations on the unlit sides of these optically thick rings appear to reflect thermal forcings that vary with Saturn season and likely probe deeper into the rings and the ring particles themselves. We will map out these cooling trends in an effort to help determine what controls this dichotomy between lit and unlit cooling behavior. Why do the optically thick rings cool on the lit side, as the optically thin rings do, yet show no cooling on their unlit sides? At what optical depth does the transition in cooling behavior take place? And what role does the vertical structure of the ring play? This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Copyright 2011 California Institute of Technology. Government sponsorship acknowledged.
Brooks Shawn M.
Morishima Ryuji
Pilorz Stu
Spilker Linda J.
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