Other
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2201h&link_type=abstract
American Astronomical Society, DPS meeting #42, #22.01; Bulletin of the American Astronomical Society, Vol. 42, p.987
Other
Scientific paper
Numerous compact, optically-thick clumps in the F ring have been detected in both occultation and imaging data returned by the Cassini spacecraft (Esposito et al. 2008, Beurle et al. 2010). Such features are particularly interesting because they may represent sites of active particle aggregation. Stellar occultations observed with the Visual and Infrared Mapping Spectrometer (VIMS) reveal that some of these clumps have distinctive infrared transmission spectra, which may provide new insights into their structure and dynamics.
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The transmission spectra of the dense main rings are featureless between 1 and 5 microns, consistent with the expected extinction by large (centimeter-to-meter-sized) particles. However, dusty structures such as the F ring (and Encke Gap ringlets) show a reduced optical depth at wavelengths around 2.9 microns. This opacity dip occurs where the real index of refraction of water ice nearly equals that of free space due to anomalous dispersion close to the fundamental absorption band at 3.1 microns. Mie-scattering calculations indicate that the dip arises due to a reduction in surface scattering from small (1-10 micron across) ice-rich particles. However, several compact, optically-thick features in the F ring exhibit much weaker opacity dips than other parts of the F ring with comparable optical depths, and therefore probably contain a smaller fraction of 1-10 micron grains than the rest of the ring. We identify these spectrally-distinct regions with the F-ring clumps seen by other Cassini instruments, and we suggest that their distinctive particle size distributions are due to unusually low local velocity dispersions that enable small grains to stick to the surfaces of larger particles. Furthermore, by using spectral decomposition techniques we are able to isolate these clumps from the rest of the F ring in order to determine their radial extent and their spatial distribution within the ring.
Cassini Vims Team
Hedman Matthew M.
Nicholson Philip D.
Showalter Robert M.
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