Coherent backscatter model for the unusual radar reflectivity of icy satellites

Abstract

RADAR is a powerful technique for probing the surfaces and subsurfaces of Solar System bodies. Inner Solar System bodies reflect radar in an almost specular fashion, with low reflectivity and little polarization. The icy satellites of Jupiter, by contrast, show high reflectivities, diffuse scattering laws and unusual polarization properties¹: compared with what would be expected for specular reflection, there is 1.5 times as much power reflected in the unexpected sense of circularly polarized radar as in the expected sense, and half as much power in the unexpected as in the expected sense of linear polarization. According to the coherent backscatter model², most of the received power from icy satellites is multiply reflected by particles about a wavelength in size located randomly under the surface of the regolith. We have constructed a laboratory analogue of this model by reflecting laser light off polystyrene beads suspended in water, and find that this model reproduces the unusual polarization ratios observed in the radar data. This implies that the regoliths of icy satellites are weakly absorbing matrices of small refractive index containing imbedded scatterers separated by distances of the order of a wavelength. No structures of special shape or other geometrical or optical properties are required.