Physics – Geophysics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004dda....35.0906n&link_type=abstract
American Astronomical Society, DDA meeting #35, #09.06; Bulletin of the American Astronomical Society, Vol. 36, p.864
Physics
Geophysics
Scientific paper
Starting in 1999, annual delay-Doppler images of Saturn's rings have been made using the Arecibo S-band (12.6 cm wavelength) planetary radar facility. With a frequency resolution corresponding to a radial resolution at the ring ansae of 2000 km, we easily resolve the classical A, B and C rings. To date we have measured the radar cross-section and depolarization ratio of the A and B rings at ring opening angles |B| = 20.1, 23.5, 25.8 and 26.7 deg. No echoes have been detected from the C ring or the Cassini Division. Images from all four years in both circular polarizations show a pronounced m=2 azimuthal asymmetry in the reflectivity of the A ring. The analogous phenomenon at visual wavelengths is ascribed to gravitational `wakes' generated by individual large ring particles or arising from internal instabilities, which are distorted by keplerian shear into elongated structures trailing at angles of 70 deg from the radial direction (Franklin and Colombo 1978). Such wakes are expected to have characteristic wavelengths of 30 to 100 m in the A ring. To the radar, the rings appear brighter when the wakes are seen sideways, and fainter when they are viewed end-on. When compared with a numerical model fitted successfully to Voyager and HST data (Salo et al 2004; French et al 2004) the phase of the radar asymmetry is found to match that of the model to within a few degrees, whereas the amplitude is found to be larger by almost a factor of two. The model is based on a local dynamical simulation employing a realistic ring particle size and elasticity used as input to a Monte Carlo light scattering code (Salo et al. 1995; 2003). Further experiments indicate that the unexpectedly large radar asymmetry amplitude may be due to the strongly forward-scattering phase function of meter-size ice particles at radio wavelengths, which enhances the sensitivity to optical depth variations. This work was supported by NASA's Planetary Geology and Geophysics program.
Campbell Don B.
French Richard G.
Nicholson Philip D.
Salo Heikki J.
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