Modeling Simulation and Inversion for Microwave Active and Passive Remote Sensing of the Lunar Surface

Abstract

Passive and active microwave remote sensing for the lunar regolith media are studied.Based on currently available digital elevation mapping(DEM) and some measurements of lunar regolith layer thickness at Apollo landing sites,a correspondence of the lunar regolith layer thickness to the lunar DEM is proposed to construct the global distribution of lunar regolith layer thickness.Using Clementine UVVIS multispectral data,the global spatial distribution of FeO+ TiO2 content on the lunar regolith layer is calculated.Thus,dielectric permittivity of global lunar regolith layer can be obtained.Based on some measurements of physical temperature of the lunar surface,an empirical formula of physical temperature distribution over the lunar surface is presented.Based on aforementioned works,brightness temperature of lunar regolith layer in passive microwave remote sensing,which is planned for China's Chang-E lunar project,is numerically simulated by a three layer model using fluctuation dissipation theorem.When these simulations as observations are obtained,an inversion approach of the lunar regolith layer thickness is developed.Because the penetration depth is small for the areas with high FeO+TiO2 content,the emission contribution from underlying lunar rock becomes negligible at high frequency channels,19.35 GHz and 37.0 GHz.Under this situation,the temperature of the top layer and the lower regolith can be inverted by the brightness temperature at these two channels using a two-layer model.Taking those points with high FeO+TiO2 content along each latitude as the reference points,the temperature variation with the latitude of top layer and the lower regolith layer can be inverted.Then,taking the inverted temperature as a known parameter,the regolith layer thickness can be inverted by the brightness temperature at the channel 3.0 GHz.Numerical simulation and inversion approach in this paper make an evaluation of the performance for lunar passive microwave remote sensing,and for future data calibration and validation.To explore the potential utilities of lower frequency radar pulse for lunar exploration,a theoretical model of lunar regolith layer and numerical simulation of polarimetric radar pulse echoes are developed in this study.The lunar regolith layer consist of the regolith layer with the randomly rough interfaces at the top and bittom of the regolith layer,and a layer of random discrete stones is laid over the lunar rock media.The time domain Mueller matrix solution is derived from vector radiative transfer,and is applied to numerical simulation of polarimetric radar pulse echoes from the stratified random media.The Mueller matrix solution contains seven scattering mechanisms of the stratified media: surface scattering from the rough interfaces,volumetric scattering from random stone scatterers(non-spherical scatterers are assumed),and their multi-interactions.As the radar pulse at L band is penetrating through the random surface,attenuation through the low loss regolith medium,polarimetric scattering through the stone scatter medium and rough surfaces are formulated in our Mueller matrix solution.Model parameters are set according to the study of the lunar regolith structure.Temporal characteristics and structure of the polarimetric echo profile are analyzed.It consists of a prominent peak due to the top boundary scattering and a complex tail due to the bottom boundary and stone volumetric scattering.Contributions of different scattering mechanisms,i.e.surface,volumetric scattering and interactions,are analyzed,and their dependence on model parameters such as the layer thickness and the content of FeO+TiO2,are also discussed.Simulation of pulse echoes might display a image of underneath structures.It reveals information of the depth and other properties of the lunar regolith layer,and demonstrates a potential new way to explore moon surface in future.