Modeling Lunar Radar Scattering From Icy Regoliths

For lunar orbital synthetic aperture radars, such as the Chandrayaan Mini-RF operating at S-Band (13cm) wavelength and the Lunar Reconnaissance Orbiter Mini-RF operating at S-Band and X-Band (3-cm) wavelengths, it is important to understand and model the radar backscattering characteristics of the icy regoliths. If ices in the permanently shadowed areas of the lunar poles backscatters like the ices on Mercury, Mars and the Galilean satellites, then it will have a substantial radar enhancement characterized by a Circular Polarization Ratio (CPR) greater than unity. We examine the possibilities that these distinct signatures may be diminished by factors such as a thin regolith covering and/or the ice occupies small patches within a larger radar pixel. Our first model for scattering from lunar surface assumes a simple mixing model consisting of diffuse and quasi- specular components. The quasi-specular component results from the surface and sub-surface layers that are oriented perpendicular to the radar's line-of-sight. The diffuse component associated with either rocks or ice is assumed to be uniformly bright, where backscatter is proportional to the cosine of the incidence angle. Rocks are assumed to have CPRs of unity while ices are assumed to have CPRs of 2 like those observed on Mercury, Mars and Galilean Satellites. This first model shows that radar signatures for ice and rocks are separable if the depolarized (SC, same sense circular) enhancements are larger than about 4 and are indistinguishable for smaller depolarized (SC) enhancements. Our second model addresses CPR changes for ice filling the pores of the regolith. Here only the quasi-specular backscatter from the surface and the diffuse backscattering from sub-surface rocks will change with increased abundances of ice in the regolith. This model indicates that only small indistinguishable changes in CPRs would occur.