Theoretical Modeling And Parameters Inversion For Microwave Remote Sensing Of Lunar Surface

THE lunar regolith layer, the uppermost layer of the lunar surface, preserves the geological history of the Moon, and knowledge of its structure, composition and distribution might provide important information concerning lunar geology and resources for future lunar exploration. Owing to the low dielectric property of the lunar regolith, microwaves at appropriate frequencies should penetrate through the lunar surface to great depth and hence provide a complementary view about subsurface geologic structure to the observations that have been obtained in the visible, infrared, and thermal infrared regimes. In China's current Chang-E 1 lunar program, a multi-channel microwave radiometer is being employed to measure the brightness temperature of the lunar surface for the retrieval of regolith layer properties, and it is planned that a radar sounding technique will be utilized for characterizing the lunar surface and subsurface structures in the future Chang-E 2 lunar program. The present thesis focuses on theoretical modeling of microwave radiometer and radar observations of the lunar surface and the potential information retrieval from those observations from the viewpoint of electromagnetic wave interactions with lunar surface layers, with particular application to China's current and future Chang-E lunar program.In passive microwave remote sensing, the brightness temperature of the lunar surface mainly depends on the thermal and dielectric properties of the regolith and the possible variation of these properties with depth. Using a global distribution of FeO and TiO₂ content as determined from the Clementine UVVIS multispectral data, the distribution of regolith dielectric permittivity over the lunar surface is obtained on the basis of a relationship between dielectric permittivity, bulk density and FeO+TiO₂ content. The physical temperature distribution over the lunar surface and its dependence on regolith depth is investigated based on the previous study results of regolith thermal properties from both direct measurements and theoretical calculation, and an empirical formula of physical temperature distribution over the lunar surface is presented. A correlation of the lunar regolith layer thickness with lunar topography is proposed in order to tentatively construct a global map of lunar regolith layer thickness. On the basis of the aforementioned work, the brightness temperature of the lunar regolith layer in passive microwave remote sensing is numerically simulated using a three-layer model (the layering dust, regolith and underlying rock media) using fluctuation dissipation theorem. Taking the simulated brightness temperature with random noise as observations, the potential inversion of regolith layer properties is studied. Because the penetration depth is small for those areas with high FeO+TiO₂ content and at high frequencies, e.g. 19.35GHz and 37.0GHz in the Chang-E project, the physical temperature of the top dust layer and the regolith layer can be inverted by use of the brightness temperature at these high frequency channels using a two layer model. Utilizing the statistics of those points with high FeO+TiO₂ content along each latitude band as a reference, temperature variations as a function of latitude can be retrieved. Finally, the regolith layer thickness can be inverted from the brightness temperature at lower frequency channels, such as 1.4GHz or 3.0GHz.On the basis of the regolith layer thickness that will be inverted from the Chang-E radiometer data, the global distribution of lunar resources, in particular ³He (Helium-3), is studied using a combination of both optical and microwave remote sensing data. ³He in the lunar regolith implanted by the solar wind is one of the most valuable resources because of its potential as a fusion fuel. The abundance of ³He in the lunar regolith is related to many factors, such as the solar wind flux, lunar surface maturity and TiO₂ content. A model of the solar wind flux, which takes into account variations due to shielding of the lunar nearside when the Moon is in the Earth's magnetotail, is used to determine the global distribution of relative solar wind flux over the Moon's surface. The global distribution of lunar surface optical maturity (OMAT) and TiO₂ content in the lunar regolith are calculated from Clementine UVVIS multispectral data. On the basis of the Apollo regolith samples, a linear relation between the ³He abundance and normalized solar wind flux, optical maturity and TiO₂ content is presented in order to produce a global map of regolith ³He abundances. The total amount of ³He per unit area in the lunar regolith layer and the global inventory of ³He will be able to be calculated after an inversion for regolith layer thickness from the actual Chang-E radiometer data.To explore the potential utility of lower frequency (L band) radar pulse penetration for future lunar exploration, a theoretical model of a stratified lunar regolith and a numerical simulation of polarized radar pulse echoes are developed. The lunar regolith layer consists of the low lossy layer with randomly rough top and bottom interfaces, and a layer of random stone-scatterers overlaying the underlying rock media. The time-domain Mueller matrix solution derived from vector radiative transfer formulations contains several scattering mechanisms of the stratified media, including surface scattering from the rough top and bottom interfaces, volumetric scattering from random stone scatterers, and multiple scattering interactions. Temporal characteristics and structure of the polarimetric echo profile as a function of model parameters such as the regolith layer thickness and FeO and TiO₂ content are numerically simulated, and well display an image of the regolith's structure. Simulated results show that polarimetric pulse echoes could reveal rich information about the lunar regolith layer depth and other structural properties, and it demonstrates a potential new way to explore the lunar surface in China's future Chang-E program.Compared with radio waves at microwave frequencies, high frequency (HF) radar sounding penetrates much deeper into the lunar regolith due to its lower frequency. The main strategy for HF radar sounding in lunar exploration is to detect weak nadir subsurface echoes, which in general suffers attenuation during subsurface propagation as well as interference by strong off-nadir surface echoes (i.e., clutter). Thus, it is important to study the surface echo properties in order to detect weak subsurface echoes. High resolution lunar surface topography for both the relatively smooth mare surface and the cratered highland terrain are numerically generated on the basis of statistical study results of the lunar surface terrain. A triangulated irregular network (TIN) is utilized to divide the undulating surface into discrete triangular meshes as a digital elevation model. The mesh dimensions are variable, with large dimension for areas with small slope variations and small dimensions for areas with large slope variations. This model takes into account the inhomogeneous lateral variations in the surface slopes, and makes the number of meshes as small as possible. Finally, Kirchhoff approximation (KA) for rough surface scattering is applied to compute radar echoes from the surface meshes. Radar echoes from both mare and cratered highland surface areas are simulated and their variation with various surface parameters such as surface roughness, crater population and size are discussed. Furthermore, using the simulated off-nadir lunar surface echoes on the basis of a digital elevation model and the Clementine UVVIS optical data for the Apollo 15 landing site, an synthetic aperture radar (SAR) image is simulated using Back Projection (BP) algorithm, which would also provide a tool for data and image evaluation, feature identification and information extraction in future lunar exploration such as the Mini-SAR on Chandrayaan-1 and Lunar Reconnaissance Orbiter (LRO).In order to meet the development of the Chinese Mars exploration program, a brief review of Mars exploration from the viewpoint of microwave remote sensing (e.g., available research results such as the distribution, structure and dielectric properties of Martian subsurface layers and the possibility of water existence) is presented. Some potential approaches, such as the detection and mapping of fluvial features beneath the Martian dust layer using synthetic aperture radar and the possible detection of water in the Martian subsurface layer using orbital radar sounder and ground penetrating radar are discussed. Finally, some examples and technical key points are presented and discussed.Some primary issues for investigating the lunar surface and subsurface properties from microwave remote sensing and HF radar sounding are studied using both forward simulation and inverse studies. However, many aspects and issues still remain for further study, not mentioning the rapidly developing microwave and radar remote sensing techniques as well as the increasing needs of lunar and other planetary exploration.