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A close look at Saturn's rings with Cassini VIMS |
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| Authors: | Philip D Nicholson Matthew M Hedman Mark R Showalter Jeffrey N Cuzzi Fabrizio Capaccioni Gary B Hansen Pierre Drossart Bonnie J Buratti Angioletta Coradini |
| Institution: | a Department of Astronomy, Cornell University, Ithaca, NY 14853, USA b USGS, Mail Stop 964, PO Box 25046, Federal Center, Denver, CO 80225, USA c SETI Institute, 515 North Whisman Road, Mountain View, CA 94043, USA d NASA Ames Research Center, Moffett Field, CA 94305, USA e Istituto de Astrofisica Spaziale e Fisica Cosmica, Via del Fosso del Cavaliere 100, 00133 Rome, Italy f Department of Earth & Space Sciences, Box 351310, University of Washington, Seattle, WA 98195, USA g Observatoire de Paris, 5 Place Jules Janssen, F-92195, Meudon cedex, France h Lunar & Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA i Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA j Istituto de Astrofisica de Fisica dello Spazio Interplanetario, Via del Fosso del Cavaliere 100, 00133 Rome, Italy |
| Abstract: | Soon after the Cassini-Huygens spacecraft entered orbit about Saturn on 1 July 2004, its Visual and Infrared Mapping Spectrometer obtained two continuous spectral scans across the rings, covering the wavelength range 0.35-5.1 μm, at a spatial resolution of 15-25 km. The first scan covers the outer C and inner B rings, while the second covers the Cassini Division and the entire A ring. Comparisons of the VIMS radial reflectance profile at 1.08 μm with similar profiles at a wavelength of 0.45 μm assembled from Voyager images show very little change in ring structure over the intervening 24 years, with the exception of a few features already known to be noncircular. A model for single-scattering by a classical, many-particle-thick slab of material with normal optical depths derived from the Voyager photopolarimeter stellar occultation is found to provide an excellent fit to the observed VIMS reflectance profiles for the C ring and Cassini Division, and an acceptable fit for the inner B ring. The A ring deviates significantly from such a model, consistent with previous suggestions that this region may be closer to a monolayer. An additional complication here is the azimuthally-variable average optical depth associated with “self-gravity wakes” in this region and the fact that much of the A ring may be a mixture of almost opaque wakes and relatively transparent interwake zones. Consistently with previous studies, we find that the near-infrared spectra of all main ring regions are dominated by water ice, with a typical regolith grain radius of 5-20 μm, while the steep decrease in visual reflectance shortward of 0.6 μm is suggestive of an organic contaminant, perhaps tholin-like. Although no materials other than H2O ice have been identified with any certainty in the VIMS spectra of the rings, significant radial variations are seen in the strength of the water-ice absorption bands. Across the boundary between the C and B rings, over a radial range of ∼7000 km, the near-IR band depths strengthen considerably. A very similar pattern is seen across the outer half of the Cassini Division and into the inner A ring, accompanied by a steepening of the red slope in the visible spectrum shortward of 0.55 μm. We attribute these trends—as well as smaller-scale variations associated with strong density waves in the A ring—to differing grain sizes in the tholin-contaminated icy regolith that covers the surfaces of the decimeter-to-meter sized ring particles. On the largest scale, the spectral variations seen by VIMS suggest that the rings may be divided into two larger ‘ring complexes,’ with similar internal variations in structure, optical depth, particle size, regolith texture and composition. The inner complex comprises the C and B rings, while the outer comprises the Cassini Division and A ring. |
| Keywords: | Planetary rings Saturn rings Infrared observations |
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