Lunar Geophysical Parameters Inversion With The Application Of Gravity/Topography Admittance And Research In The Thermal Evolution Of The Moon

Gravity coefficients of most lunar gravity field models are becoming more valid globally. And the lunar topography model derived from LOLA data is even expressed to degree and order720. This makes lunar geophysical research thus become possible. Because of the selenographical limited extent of network, lunar geophysical parameters cannot be obtained globally just from limited seismic data. Another powerful method that can be used to probe the lunar subsurface structure globally is through the collaborative analysis of gravity and topography data. Then the lunar crustal thickness, lithosphere elastic thickness and surface heat flux can be obtained from gravity/topography admittance. Besides, LLR data indicates that the distance between the moon and the earth is increasing. This shows that the distance might be smaller in the early stage when the moon suffered a larger tidal force exerted by the earth. Tidal force should therefore be considered in the lunar thermal evolution. Using heat flux derived from elastic thickness as a constraint, the tidal effect on the thermal evolution can then be predicted. So this work in this thesis is as follows:1. We analyzed degree RMS of various gravity field models by using localized spherical harmonic functions. The contribution of various orbits in the lunar gravity field model was also investigated through the orbit determination software GEODYN-Ⅱ. Results indicate that CEGM02takes an advantage in the lunar geophysical parameters estimation, but SGM150j is more suitable for the precision orbit determination. With the joint of data from Chang’E-1, Chang’E-2, SELENE, LP, Clementine, Lunar Orbiters and GRAIL, we can solve a more accurate lunar gravity field model, when considering new optimization methods and Kaula constraint. This research can provide a good reference for the production of new gravity field model and the estimation of lunar geophysical parameters.2. Because of the limited lunar seismic data, the limited selenographical extent of network and the traditional methods failing in the best inversion solution, some lunar geophysical parameters such as load ratio, crustal thickness, crustal density and lithosphere elastic thickness can be simultaneously calculated with the joint of lunar gravity/topography admittance and nonlinear inversion method Particle Swarm Optimization namely PSO. Using this method, we also analyzed the admittance and correlation difference of various gravity field models. Results show that all the models are appropriate for gravity/topography admittance and correlation analysis on the nearside of the moon. But as to the farside, CEGM02and SGM150j show a superior advantage in such analysis. Moreover, through the algorithm PSO, all the parameters of the best-fit model can be calculated with a reducing calculation time. It can be concluded that multi-parameters inversion can be successful with the joint admittance and PSO, which is unquestionably beneficial for lunar internal structure investigation.3. Separate data and approach often lead to a discrepancy in the size of the lunar core and its composition. And the lack of information about the interior structure often results in uncertainty. Up to now, the lunar average density and its inertia moment factor Ⅰ/MR2are widely used to place constraints on its interior structure. We modeled the moon as a sphere with six layers by using such method. Results show that the size of the lunar core is about410km and its corresponding density is approximately5.768g/cm3. This indicates that the core may be composed of Fe-FeS alloy, which is similar with other findings. Consequences concluded here could provide a useful reference for further studies.4. The distance between the moon and the earth may be slight in the early stage, when the moon might suffer a larger tidal force exerted by the earth. This is why tidal force should be considered in the lunar thermal evolution, especially in its initial stage. We converted the governing equation of the thermal evolution into dimensionless weak form. Using the finite element method analysis, the thermal evolution of the moon was investigated with different initial temperatures when considering tidal effect on different stages. Results indicate that tidal force can speed up the generation of convection, and cause large-scale convection column in center-to-center direction from the moon to the earth, which accelerates the interior heat release. Consequences here could provide some interpretation for lunar asymmetric thermal evolution, crust dichotomy, basalt dichotomy and remanence in the rock with the combination of other physical mechanism.