The Opposition Effect On Saturn's A And B Rings: Observations from Cassini-Vims

Details The Opposition Effect On Saturn's A And B Rings: Observations from Cassini-Vims The Opposition Effect On Saturn's A And B Rings: Observations from Cassini-Vims

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.2910c&link_type=abstract

American Astronomical Society, DPS meeting #40, #29.10; Bulletin of the American Astronomical Society, Vol. 40, p.444

Astronomy and Astrophysics

Astronomy

Scientific paper

The opposition effect on Saturn's A and B rings is investigated using the observations of the VIMS instrument onboard Cassini. We have analyzed the visible channel data coming from sequence S26 acquired December 2006. After the reconstruction of the observations geometric parameters (incidence, emission and phase angles) and the projection of the hyperspectral images over the rings plane we have extracted reflectance profiles at fixed radial distances, studying different regions of A and B rings.
We have modeled the profiles using Hapke's equation for the bidirectional reflectance (Hapke, 1983) and using a SIMPLEX procedure to fit the parameters. The analysis of the profiles at three different wavelengths (440, 550 and 700 nm) and the independence of the shape of the profiles from the wavelength enable us to exclude the CBOE (Coherent Backscattering Opposition Effect) in the phase angle range studied.
The fit procedure (at 550 nm) applied to all the 59 images of the sequence determines the values, at various radial distances from Saturn's centre, of the four parameters in the bidirectional reflectance equation: single scattering albedo, asymmetry parameter of the phase function, amplitude of the Shadow Hiding Opposition Effect and width of SHOE. Mean values of these parameters are calculated both for A and B rings, obtaining in particular for the single scattering albedo A = 0.632, B = 0.8656 and for the SHOE width A = 0.00172, B = 0.00137.
Values of the width of SHOE indicate that the effect is due mainly to agglomerates of particles rather than to single particles. An analysis of the asymmetry factor between different images obtained at the same radial distance but at different times points out the presence of gravitational wakes in A ring.
This work is supported by an Italian Space Agency (ASI) grant.

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