Other
Scientific paper
Aug 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007epsc.conf..774n&link_type=abstract
European Planetary Science Congress 2007, Proceedings of a conference held 20-24 August, 2007 in Potsdam, Germany. Online at ht
Other
Scientific paper
On several occasions during its orbital tour the Cassini spacecraft has flown between the sun and Saturn in such fashion that the zero phase point passed through the rings. The Visual and Infrared Mapping Spectrometer (VIMS) recorded spectral image cubes (0.4< <5.2μm) that showed the opposition effect (OE) at zero phase. The OE is a spike in the reflected light observed near 0o phase when it is displayed as a function of phase angle. This is the first time the OE has been resolved for small areas on the rings. Previous work has shown that the OE arises from two distinct processes, shadow hiding (SHOE) and coherent backscattering (CBOE). The SHOE process causes an OE by the elimination of shadows cast by regolith grains upon one another as phase angle decreases. The CBOE process causes an OE by constructive interference between photons traveling in opposite directions along the same path within the medium. SHOE is expected to dominate the contribution to the OE in absorbing media where multiple scattering of photons is not significant. CBOE is expected to dominate the contribution to the OE in highly reflective media with much multiple scattering. We have made spectral scans the VIMS images that traverse the zero phase point. We selected narrow spectral bands that reflected a variety of wavelengths and reflectance levels. In this way, phase curves of the ringlet were obtained for each band. We have compared these data to data we acquired in the laboratory using the JPL long arm goniometer where we measured the phase curve of particulate materials that simulate the surface of Saturn's ring particles. We argue here that the OE is due to coherent backscattering because: 1) The theoretical CBOE function fit to the data is excellent. 2) The OE width is extremely narrow 3) The angular width of the peak increases with wavelength. CBOE theory also predicts that the width depends on the transport mean free path (TMFP) in the medium. We find that the OE is caused by coherent interactions between sub-particles in the outer layers of the ring particles, and that these sub-particles are of the order of 20-40 μm in size. A portion of this work was performed at JPL under contract with NASA
Baines Kevin Hays
Bellucci Giancarlo
Bibring Jean-Pierre
Boryta M.
Brown Harvey R.
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