Icy Galilean Satellites: Modeling Radar Reflectivities as a Coherent Backscatter Effect

Abstract

The icy moons of Jupiter, Europa, Ganymede, and Callisto, are efficient backscatterers of centimeter-wavelength radiation. At 3.5-cm and 13-cm wavelengths their specific radar cross sections can exceed unity (Campbell et al. 1978, Icarus 34, 254–267, Ostro et al. 1992, J. Geophys. Res. 97, 18227–18244), while by 70-cm wavelength their cross sections have dropped considerably (Black et al. 2001) though are still high compared to terrestrial planets and the Moon. Their polarization ratios are also high, indicating effects other than single scattering from the vacuum–surface interface. We model the unusual radar properties of these objects as a coherent backscatter effect which could result from scatterers embedded in the weakly absorbing water ice surfaces of these moons. A vector model of the coherent backscatter effect (Peters 1992, Phys. Rev. B 46, 801–812) has been extended to provide absolute cross sections and, hence, the volume density of scatterers and is applied to the observed wavelength variations of the radar properties to derive generalized properties of the scattering layer. This model can reproduce the data with scatterers following fairly steep power-law size distributions with exponents of −3.5 to −4.0 and maximum sizes of 0.5−1.0 m. The model suggests that on Ganymede and Callisto the scatterers occupy only several percent of the volume, whereas the volume density of scatterers on Europa must be much higher. Efficient scatterers are required, and partially absorbing scatterers such as silicates cannot reproduce the high cross sections observed. The model is less sensitive to absorption in the medium containing the scatterers or to its thickness, but the fits suggest that the radar wave need penetrate only to a depth of several meters for this mechanism to produce the observed scattering properties.