CPR Modeling for Mini-RF Data Study on Lunar PSR Region

Searching for water ice in the lunar media has been a key issue in the Moon explorations. Missions of Mini-SAR and Mini-RF SAR made compact-pol(polarization) measurements on lunar polar permanent shadowed region (PSR). High circular polarization ratio (CPR) data and two-layer model were applied to studying if water-ice in PSR could be retrieved. However, it has been studied that high CPR is not simply due to the presence of water-ice. We point out that the Campbell model is a degenerated half-space model, which confused dielectric inversion of Mini-RF data. In this paper, our numerical BART (Bi-directional Analytic Ray Tracing) is applied to simulate high-order scattering from the lunar rough surface with- and without-stone-scatters, the CPR is numerically simulated and analyzed. Furthermore, a two-layer model of KA surface (Kirchhoff-approximation)/SPA subsurface (small perturbation approximation) subsurface is developed to take account of all these topographic factors for PSR dielectric inversion. The Mini-RF data on the PSR of Hermite-A crater in the North pole (see Fig. 1) are employed for the inversion. Our results do not support previous conclusions based on the half space model, suggesting that the inversion of Mini-RF are not so straight forward in proving water-ice existence in the PSR media. It points out that double and high-order scattering of random volumetric scatters on the lunar surface can play a certain role, and high-resolution measurements and more elaborate modeling are needed for further studies.