Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0403b&link_type=abstract
American Astronomical Society, DPS meeting #28, #04.03; Bulletin of the American Astronomical Society, Vol. 28, p.1070
Astronomy and Astrophysics
Astronomy
Scientific paper
The radar properties of the icy Galilean satellites at cm wavelengths are dramatically different from those of most inner solar system objects. Data obtained with the Goldstone and Arecibo systems at wavelengths of 3.5cm and 13cm yield total radar cross sections for Europa, Ganymede, and Callisto of ~ 2.5, ~ 1.5 and ~ 0.7 respectively, and circular polarization ratios in the range 1.2-1.5 [Ostro et al. JGR 97, 18227 (1992)]. However, Arecibo measurements made at 70cm wavelength yield cross sections substantially lower, while the circular polarization ratios are generally consistent with the 3.5cm and 13cm wavelength values [Black et al. LPSC XXVII 1, 143 (1996)]. The radar scattering properties of Europa, Ganymede, and Callisto are thought to be due to the low absorption of cm wavelength radiation in ice. A low-loss medium permits long path lengths, allowing volume scattering from subsurface structures. Hapke [Icarus 88, 407 (1990)] first suggested that a coherent backscatter effect can explain the radar observations. This effect results in an enhanced backscatter cross section and a greater than unity circular polarization ratio; i.e. the incident sense of polarization is largely preserved in the backscatter direction. We fit a model of the coherent backscatter effect from Peters [Phys. Rev. B 46, 801 (1992)] to the measured cross sections at the three different wavelengths for each object in order to constrain surface properties. The model parameterizes the scatterers in terms of a power law size distribution and a mean angle for individual scattering events. The medium containing the scatterers is parameterized by an absorption path length and thickness. Preliminary results indicate that the scatterer size distribution can be described by a power law (number density of scatterers with a given radius n(r)~ r(-alpha ) ) with alpha falling in the range 2-3, and that the scattering layer may be extremely thin, on the order of a few meters. Absorption in the medium is not well constrained by the model.
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