The Inversion Of Lunar Regolith Layer Thickness By Using The Data Obtained From Microwave Radiometer On CE-1Satellite

Lunar regolith layer contains a large number of important materials and energy information of theMoon and solar system. During the billions of year's formation and evolution process, lunarregolith not only records the origin and evolution history of the Earth-Moon system, the impacts ofmeteorites and micro-meteorite on the Moon, the bombardments of solar wind and cosmic ray tothe Moon, but reflects the composition and distribution of the lunar material. Plenty of scientificinformation can be obtained by the systematic study of lunar regolith. Study on the thickness of thelunar regolith has very important significance for the future lunar exploration, such as the mannedlanding on the Moon, the lunar base site selection, the exploitation and the further utilization oflunar resources. In the previous lunar exploration activities, the thickness of lunar regolith is allalong concerned by scientists. One of the scientific objectives of China's first lunar probe "Chang-e1"(CE-1) is to investigate the distribution of the lunar regolith thickness and then to estimate thecontent of3He resource on the Moon by using the microwave brightness temperature data of thelunar surface which were acquired by the carried microwave radiometer. It is the first time usingthe passive microwave remote method on lunar orbit to explore the lunar regolith thickness.The microwave brightness temperature of lunar surface contains plenty information on the Moonand the outer space. It directly reflects the physical and chemical properties of the lunar surfacematerial, including the chemical composition, the density, the dielectric properties, the thicknessof lunar regolith, and the temperature of lunar surface layer. It is the first time that the microwavebrightness temperature data obtained from the lunar orbit are used to inverse the lunar regolith. It isa complex process. There are some remained theoretical problems to be systematically solved. Themain research work and contributions of this paper are shown as follows:1. The influence of various factors on the lunar surface microwave brightness temperature such asthe content of FeO and TiO2in lunar regolith, the density and dielectric permittivity distributionof lunar regolith, and the roughness of the lunar surface are studied, resulting in a basis for thefurther establishment of a reasonable microwave radiative transfer model in lunar regolith.2. The temperature distributions of lunar surface and regolith are studied. In all lunar regolithparameters influencing the microwave brightness temperature, the change of temperature playsa dominant role. Study on temperature distribution in the lunar regolith is very important in theboth processes to establish microwave radiation transfer model and to inverse thickness of lunar regolith. On the basis of previous studies, the establishment of a one-dimensional heattransfer model with variable coefficient for lunar surface layer, the simulation of thetemperature variations of lunar surface and regolith with time, lunar regolith depth, andlocation, and then, comprehensive analysis of the influence of all kinds of thermal physicalparameters on calculated results were carried out in the paper, providing the prerequisite toestablish a reasonable lunar regolith microwave radiative transfer model and to simulate thelunar surface brightness temperature obtained by the microwave radio meter on CE-1satellite.3. Based on the above research results and the variations of density, dielectric permittivity, andtemperature with lunar regolith depth, an inhomogeneous multilayer model is presented.Furthermore, the function relationship between the microwave brightness temperature ofinhomogeneous multi-layered medium surface and the medium thickness, temperature, andother parameters is deduced respectively by applying the fluctuation dissipation theory andnon-coherent method. A uniform formula for calculating the brightness temperature is obtained bycomparing and simplifying process. The changes of lunar surface microwave brightnesstemperatures with time, location and thickness of lunar regolith obtained from four frequencychannels of the microwave radiometer on CE-1satellite are simulated. Compared to the simplylayered model in the literature, the model presented in this paper can more accurately reflectthe relationship between brightness temperature and the thickness of lunar depth. Finally, thestability of this inhomogeneous multilayer model is testified by simulating results based on afine layered model with407layers.4. The data processing method for the2C-level brightness temperature data obtained by themicrowave radio meter on CE-1is studied. A regression and analysis method for the2C-levelbrightness temperature data which is based on Support Vector Machine (SVM) and ParticleSwarm Optimization (PSO) is presented. Then, the distribution of microwave brightnesstemperatures from four frequency channels (3GHz,7.8GHz,19.35GHz,37GHz) on full lunarsurface on specific moments of a lunar day is obtained by using this method. This workprovides the foundation for quick inversion of lunar regolith thickness. Meanwhile, theregression results can also be used as useful references to correct obvious errors in2C levelbrightness temperature data.5. According to the inhomogeneous multi-layer microwave radiative transfer model presented inthis paper, the corresponding inversion method for lunar regolith thickness is put forward, andthen, the full lunar surface regolith thickness distribution is inversed by using the brightness temperature data obtained from3GHz frequency channel. The analysis indicates that thereversion results presented in this paper display the higher accuracy. Additionally the resultsalso indicate that there are a few very "cold sites" lay on the north and south poles of the moon.The lunar regolith thickness of these sites is not inversed because the brightness temperaturesat these sites are significantly smaller than that at the same latitude area. However, thefinding of these very "cold sites" provides the assumptions precondition for the further studieson the existence of water ice at these sites.6. The SVM method inversing of lunar regolith layer thickness is tried firstly in this paper.According to the relationship between the TiO2+FeO content of lunar regolith, lunar latitude,sampling time, and the lunar surface brightness temperature obtained at certain time from fourfrequency channels, the predictive models for lunar regolith thickness has been trained. Then,by using the brightness temperature data obtained from four frequency channels and thecorresponding sample information, the distribution of lunar regolith thickness on the full moonsurface is inversed. The results show that the preliminary results of the inversion of lunarregolith layer thickness based on the SVM method is closed to the inversion result based on theinhomogeneous multi-layer model. It indicts that the method for lunar regolith inversion basedon SVM method is feasible.