Coherent backscatter and the radar characteristics of outer planet satellites

Abstract

The radar properties of Europa, Ganymede, and Callisto are characterized by high reflectivities dominated by a diffuse component, and a large amount of polarization in the opposite sense of that expected if the waves were specularly reflected once. It is pointed out that these are the properties that might plausibly be expected when a collimated source illuminates a weakly absorbing, particulate medium in which wavelength-sized scatterers are separated by distances somewhat larger than the wavelength, such as a regolith consisting of voids and/or silicate rocks imbedded in an icy matrix. No specialized structures are required. Portions of the wave front that are multiply scattered within the medium and that traverse the same path in opposite directions combine coherently in the backscatter direction to produce an increased intensity. The enhancement is different for the two components of polarized reflected radiation, so that this model explains the observed linear polarization ratio and may be able to account for the observed circular polarization ratio. The coherent effects are confined to a peak centered on the backscatter direction of angular half-width λ 2πD , where λ is the wavelength and D is the photon diffusion length in the medium. Bistatic radar observations would test this model and give information on the structures of the regoliths. The same phenomenon may play a role in radar scattering from Saturn's rings and also in the sharp opposition effects observed on outer planet satellites at optical frequencies.