Regolith properties and Stokes vector radar polarimetry models for the moon

Abstract

Earth-based and orbital radar systems collect images of the Moon in a polarimetric format that allows derivation of the Stokes vector, which represents the echo power and relative phase of signals in two orthogonal polarizations. Backscattering analyses for geologic studies often use two observable parameters, such as the opposite-sense circular echo power with the circular polarization ratio (CPR or μ_c), or the three components from a decomposition model such as the m-χ approach. The m-χ model provides a composite of power values assumed to correlate with the properties of “polarized” single-bounce and double-bounce reflections, and a random or “unpolarized” component. This model can be written equivalently in terms of m (degree of polarization) and the circular polarization ratio, and there is a similar correlation between the m-χ components and the CPR in data for the lunar maria (and superposed craters) at 12.6-cm and 70-cm wavelengths. These correlations suggest that the fully polarized mare echo is primarily produced by scattering from subsurface rocks, with an approximately constant degree of linear polarization of 30–40 % at the two wavelengths. Most of the surface echo is unpolarized until roughness becomes extreme in areas like crater ejecta or impact melt where the CPR is >1. Because of the substantial subsurface component, the m-χ model cannot uniquely separate single-bounce and double-bounce surface returns, consistent with similar ambiguities in interpreting the CPR. Understanding these general properties of lunar regolith scattering can be used to highlight areas with anomalous enhancements and suggests new directions for modeling of radio-frequency scattering by regolith surfaces, particles, and rocks. Future work can produce Stokes-vector values from existing Earth-based data, and these products can provide a larger wavelength and incidence angle range to complement images from orbital radar systems.