Astronomy and Astrophysics – Astronomy
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000dda....31.0801s&link_type=abstract
American Astronomical Society, DDA Meeting #31, #08.01; Bulletin of the American Astronomical Society, Vol. 32, p.863
Astronomy and Astrophysics
Astronomy
1
Scientific paper
Dynamical N-body simulations suggest the formation of transient, recurrent particle chains in the outer portions of Saturn's rings, due to local gravitational instabilities. The scale of these ``wakes'' is in the range of few tens to few hundreds meters. Due to differential rotation these formations trail on the average by about 20o with respect to the tangential direction. The amplitude of the wakes depends, besides the ring surface density and the planetocentric distance, on the local velocity dispersion determined by the elastic properties and the size distribution of the particles. Such wakes seem to offer a natural qualitative explanation for the observed azimuthal brightness variation in Saturn's A-ring (Dones et al. 1993, Icarus 105, 84): the bi-symmetric brightness minima can be intepreted to correspond to the longitudes where the wakes are seen along their long-axis (about 20o before the ansae). To study more quantitatively the photometric implications of such wakes, Monte Carlo simulations including multiple scattering have been performed. These simulations yield the scattered intensity from the particle field as a function of the illumination and viewing directions, and thus a model for the ring brightness along the longitude. These photometric simulation fully confirm the expectation of the locations of the brightness minima. We have started exploring models where the different parameters affecting the dynamical amplitude of the wakes are varied, with the goal of eventually matching the observed amplitude of brightness variations as a function of Saturnocentric distance. Also, photometric simulations are performed for different observing geometries (the Sun-observer phase angle and the ring tilt angle). So far, simulations indicate a strong increase in the brightness amplitude with the surface density and distance. Also, the elasticity of particles can not greatly exceed that implied by Bridges et al. (1984, Nature 309, 333) model. Simulations with power-law size distribution imply a reduction in amplitude by about one half as compared to models with single-sized particles, in qualitative agreement with the recent HST (French et al., this meeting) and radar (Nicholson et al., this meeting) observations.
French Richard G.
Karjalainen Raine
Salo Heikki
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