Modeling polarimetric radar scattering from the lunar surface: Study on the effect of physical properties of the regolith layer

A theoretical model for radar scattering from the lunar regolith using the vector radiative transfer theory for random media has been developed in order to aid in the interpretation of Mini-SAR data from the Chandrayaan-1 and Lunar Reconnaissance Orbiter missions. The lunar regolith is represented as a homogeneous fine-grained layer with rough upper and lower parallel interfaces that possesses embedded inclusions with a different dielectric constant. Our model considers five scattering mechanisms in the regolith layer: diffuse scattering from both the surface and subsurface, volume scattering from buried inclusions, and the interactions of scattering between buried inclusions and the rough interfaces (both the lunar surface and subsurface). Multiple scattering between buried inclusions and coherent backscatter opposite effect are not considered in the current model. The modeled radar scattering coefficients are validated using numerical finite difference time domain simulations and are compared with incident angle-averaged Earth-based radar observations of the Moon. Both polarized and depolarized radar backscattering coefficients and the circular polarization ratio (CPR) are calculated as a function of incidence angle, regolith thickness, surface and subsurface roughness, surface slope, abundance and shape of buried rocks, and the FeO+TiO2 content of the regolith. Simulation results show that the polarized (opposite sense) radar echo strength at S and X bands is mostly dominated by scattering from the rough surface and buried rocks, while the depolarized (same sense) radar echo strength is dominated by scattering from buried rocks or ice inclusions. Finally, to explore the expected polarimetric signature of ice in the polar permanently shadowed areas, four parametric regolith models are considered and the possibility of detecting diffuse ice inclusions by the CPR is addressed. Our study suggests that detection of ice inclusions at the lunar poles using solely the CPR will be difficult given the small dielectric contrast between the regolith and ice.