Gravity Anomaly Features And 3D Density Imaging Of The Moon

One of the objectives in "Chang'E project" is to study the origination and evolution of the Moon, especially the interior layered structures. Current satellite gravity and topography of the Moon, in presence of high resolution and accuracy, and other data from lunar physical or chemical investigation have provided useful information to interpreate the lunar internal structure. Gravity field is currently the best data set to study the planetary interior structures. The lunar gravity anomaly reveals the lateral heterogeneities of surface and interior substance. Gravity inversion is a valid method to investigate the density structure in the lunar interior. In this thesis, the lunar Bouguer gravity anomaly was calculated by using a method of terrain correction in spherical coordinates system. Inversion of lunar gravity data can be used to analyze the density distribution and origination of the mascons, and then to study the evolution of lunar impact basins and the crust-mantle structures.By applying the geophysical inversion theory to solve the planetary science problem, I firstly studied and discussed the inner structure of the Moon based on the Chang'E-1 topography model, CLTM-s01, and the Japanese SELENE's newly gravity field model, SGM100h. Thesis proposed the different views of the mascons formation from the lunar Bouguer gravity anomaly and the classification of mascons. Secondary, in order to quantitatively interpret the Bouguer anomaly, I constructed a new density imaging method used in spherical coordinates. The synthetic models demonstrated that this method is feasible to the inversion of the gravity data in small area or in global scale. Then, I applied it to lunar gravity data and retrieved the global 3-D density structures of the Moon.I proposed that the masons should be identified from Bouguer gravity anomaly. In this study, the terrain correction for lunar free-air gravity anomaly is calculated in spherical coordinates based on the global topography data detected by the laser altimeter on Chang'E-1. According to the Bouguer gravity anomaly of the Moon, two impact basins, Hertzsprung and Korolev are lack of mascon features. The mascon basins seem to be classified into two types, Type Highland and Type Plain. For the mascon basins of Type Highland the dense materials mainly come from the shallow crust, which are associated with the basalt deposits. The other type, Type Plain, includes mascon basins whose major dense materials may be located deep at the litho-sphere, corresponding to the uplifted mantle. Further, the South Pole-Aitken (SPA) basin is considered the largest mascon basin on the Moon, and the feature of BGA in the basin implies the impacting direction.This study addressed the inversion issues in spherical coordinates system, especially the key one of weighting function. I derived a new model objective function in the light of the Backus-Gilbert model appraisal theory in geophysical inversion. Furthermore, the correct depth weighting function was also derived because the previous function was not applicable to spherical coordinates system. The new method can directly solve the inversion problem of global satellite gravity data. Reliable results of density imaging were obtained and the non-uniqueness was suppressed by using model constraints and Tikhonov curves.Lunar interior density distribution was retrieved by applying the new 3-D density imaging method. On the one hand, results from three mascons'gravity inversion indicated that the high density anomalies concentrated at the depth of 20-50km beneath the mascon basins. Their residual densities were larger than 0.3g/cm3 which are close to the density difference between lunar mantle and crust. Density structures along radial direction also demonstrated that the uplifted mantle dominates the origin of mascons'gravity anomalies. On the other hand, the global density imaging down to 100km depth of the Moon showed the similar features, which illustrated again that the lateral heterogeneities of interior density structures are mainly located above the depth of 50km.