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S.G. Edgington, L.J. Spilker, S.H. Pilorz, B.D. Wallis (Jet Propulsion Laboratory), R. Carlson, J.C. Pearl, F.M. Flasar (Goddard Space Flight Center), S.M. Brooks (Jet Propulsion Laboratory/N.R.C.), M.R. Showalter (Stanford University), C. Ferrari (CEA Saclay/University Paris 7), C.A. Nixon (University of Maryland), R.K. Achterberg (SSAI), A.A. Simon-Miller (Goddard Space Flight Center), CIRS Investigation Team
The Cassini Composite Infrared Spectrometer (CIRS) obtained thermal spectra of the rings during the approach to Saturn. CIRS is a Fourier-transform spectrometer that measures radiation in the thermal infrared from 7 microns to 1 millimeter (1400 to 10 cm-1). For 7.5 hours on June 22, 2004, CIRS obtained a sit-and-stare observation of the lit afternoon ansa of the B ring (centered at 105568.0 km) using its Focal Plane 1 (FP1) detector with a spectral resolution of 1.0 cm-1. At this time, the spacecraft was at a ring range of ~118 Rs, resulting in a 27800 km diameter footprint filling the entire B-ring ansa, with a phase angle of ~67 degrees.
The resulting FP1 spectrum, covering 10 cm-1 to 600 cm-1, appears relatively featureless and is characteristic of a Planck curve peaking near 180 cm-1 (corresponding to an effective temperature of 89 K) scaled by a function of ring optical depth, ring inclination, and particle phase function. One exception is the presence of a roll-off in the intensity shortward of 50-60 cm-1 relative to the 89K blackbody curve. This is thought to be indicative of material properties of the ring particles. We explore the implications of this characteristic in conjunction with the absorption and reflective properties of several ices, e.g. water, ammonia, and carbon dioxide, along with those of silicates and carbonaceous materials. In this analysis, we will examine lower limits on particle sizes observable at these wavelengths. In addition, we will assess the degree to which Saturn’s own thermal emission appears in the FP1 spectrum.
This work was performed at JPL under contract with NASA.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.