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J. E. Baron, G. L. Tyler, R. A. Simpson (Stanford Univ.)
Several scattering models have been proposed to explain the unusual radar properties of the icy Galilean satellites and certain other icy surfaces. Among these models are the coherent backscatter effect (Hapke 1990, {\it Icarus} \textbf{88}, 407--417), retrorefractive permittivity gradients or ``lenses'' (Hagfors 1985, {\it Nature} \textbf{315}, 637--640), and radar glory from buried craters (Eshleman 1986, {\it Science} \textbf{234}, 755--757), the latter two of which have been analyzed in the geometric optics limit for large scatterers. Radar data currently available are insufficient to distinguish among the models. In this paper we use numerical methods to calculate scattering in both monostatic and bistatic geometries from wavelength-scale refractive lenses and buried craters, which---if they exist---are more likely to be found in large numbers at sizes for which geometric optics may not apply.
We model single scattering from hemispherical craters and spherical refractive lenses with effective size parameters as large as ka ~16. For the lenses we choose a refractivity profile corresponding to that of an Eaton-Lippman lens, or ``cat's eye.'' The (hemi)spherical shapes are chosen for convenience, but can inhibit the same-sense circular (SC) or orthogonal linear (OL) responses due to coherent cancellations from annular apertures. We introduce randomly-distributed, sub-wavelength-scale dielectric inhomogeneities (corresponding to rocky debris, ice, or air pockets) into the background crater/lens model as a means of simulating irregularities within the scatterering centers.
We do not observe any unusual backscatter enhancements in the circular polarization ratio, \mu\textrm{\scriptsize C} = \sigma\textrm{\scriptsize SC} / \sigma\textrm{\scriptsize OC}, or the linear polarization ratio, \mu\textrm{\scriptsize L} = \sigma\textrm{\scriptsize OL} / \sigma\textrm{\scriptsize SL}, for small buried craters. We have assumed that small craters are bowl-shaped, but deviations from spherical symmetry could act as an additional depolarizing mechanism. The bistatic angle at which \sigma\textrm{\scriptsize SC} exceeds \sigma\textrm{\scriptsize OC} ranges from 10\circ to 20\circ for ka \approx 16. Refractive lenses have stronger depolarized backscatter responses. For example, a lens with ~15% of its volume filled with rocky debris gives an average \mu\textrm{\scriptsize C} \approx 1.6 and \mu\textrm{\scriptsize L} \approx 0.8, slightly higher than values measured for the Galilean satellites (Ostro {\it et al.\/} 1992, {\it JGR Planets} \textbf{97}, 18227--18244).
The author(s) of this abstract have provided an email address for comments about the abstract: johnb@nova.stanford.edu