Moon’s origin and evolution is the primary scientific objectives of lunar explorations. One of the most critical scientific questions is to investigate the lunar interior structure, especially the formation and evolution of the lunar crust. From a geophysical perspective, orbiting data of high-resolution gravity and topography give important constraints on the interior structure of the Moon. Combined these two data set with geochemistry information and reasonable geologic assumptions, it is possible to invert for some important geophysical parameters, such as crustal thickness, elastic thickness, and crustal and mantle density. These model parameters can then be used to address questions concerning the differentiation, crust formation and thermal evolution of the Moon. The Clementine and Lunar Prospector missions have significantly improved our knowledge of the Moon’s gravity and topography. Unfortunately, the resolution of the gravity field varies dramatically between the near and farside hemispheres and the topography data is sparse without poleward observation. Several lunar explorations have been implemented since the year of 2007, and it opens an era of lunar gravity and topography detections. In China’s Chang’E-1 lunar program, a laser altimeter has been employed to measure the vertical ranges between the satellite and the lunar surface, and it is planned to use those ranges to construct a global topographic map of the Moon. The present thesis focuses on constructing a high resolution topographic model from Chang’E-1 laser altimetry measurements, and applying topography and gravity joint-analysis on resolving some geophysical questions of the lunar crust evolution.
In the part of lunar topographic model construction and optimization, more than 3 million effective range measurements from Chang’E-1 laser altimeter (LAM) are used to construct a global topographic model of the Moon with both improved vertical accuracy and lateral resolution. Pre-processing with instrumental calibrations is applied on raw laser ranges. Combined those data with precise orbit determinations and attitude measurements with an along-track elevation filtering, we obtain a set of lunar surface elevation values. This topographic filed, referenced to a mean radius of 1738 km, has an absolute vertical accuracy of approximately 31 m and a spatial resolution of 0.25 degrees (~7.5 km along the equator). Our topographic model, a 360th degree and order spherical expansion of the lunar radii, is designated as Chang’E-1 Lunar Topographic Model s01 (CLTM-s01). This new model has greatly improved previous models in spatial coverage, accuracy and spatial resolution, especially of the lunar poleward regions. From CLTM-s01, the mean equatorial and polar radii of the Moon are 1737,013±2 m, 1737,646±4 m and 1735,843±4 m, respectively. In the lunar-fixed coordinate system, this model shows a center of mass (COM) and center of figure (COF) offset to be (-1.777, -0.730, 0.237) km along the x, y and z directions, respectively. Model optimizations are implemented by comparing CLTM-s01 model with other high-resolution topographic model from Japanese Kaguya/Selene and NASA Lunar Reconnaissance Orbiter (LRO). An improved new model of CLTM-s02 is obtained by reprocessing the precise satellite orbit, correcting the instrumental validation error and time flags drift of the laser ranges. A reasonable strategy of elevation combination between CCD camera and CLTM-s01 has been proposed and executed, with particular application to China’s current and future missions.
In the part of new identified impact basins of the Moon from CLTM-s01, 3 new impact structures has been proposed and identified, and 1 mountain feature is suggested as a shield volcano. Lunar craters, especially large-scale craters namely impact basins play an important role on the surface dichotomy evolutions of the Moon. We identified and verified ~57 impact basins proposed by previous work, and re-divided them to different basin grade. Using high-resolution lunar topography CLTM-s01 and assisted with global gravity information from Kaguya, it reveals that there still exist 4 unknown features: Namely Sternfeld-Lewis (20S, 232E), Fitzgerald-jackson (25N, 191E), Wugang (13N, 189E) and Yutu (14N, 308E), with the first 3 as impact features, and Yutu as a volcanic deposited highland or similarly with a shield volcano like Hawaii islands. Furthermore, we analyses and identify about 11 large-scaled impact basins that have been proposed since 1994, and re-classify those impact basins according to their circular characteristics on the topographic map.
In the part of joint-analysis of gravity and topography data, a localized spectral admittance technique is applied to various crustal regions by windowing the free-air gravity and surface topography with band-limited localization windows. These admittances are interpreted using a geophysical model that includes both surface and subsurface loads that are supported by a thin elastic lithosphere. By varying the crustal density, elastic thickness and loading ratio in certain ranges, the best fitting bulk densities for a number of homogeneous regions are constrained to vary laterally from 2590 kg/m3 to 3010 kg/m3, with a mean value of 2700 kg/m3. Assuming that composition of the upper crust is uniform, the porosity of the upper crust could be determined if the pore-free surface density is know. Based on the known compositions of lunar rocks and estimated mineralogical norms, it finds that the pore-free densities of lunar rocks are highly correlated with FeO and TiO2 abundances. The rock density can vary from 2884 to 3038 kg/m3 in estimated regions by using the iron and titanium abundances from Lunar Prospector gamma-ray spectrometer. The porosity of each region is calculated with a mean value of ~7.4±3.4%, with permissible values from 0 to 14%. Results are consistent with applying the same methods on previous gravity models from Lunar Prospector mission. An improved error estimation method is used by considering noises from the observed gravity field. Comparing with the magnitude of surface loads, subsurface loads are negligible in most of the analysis regions. Elastic thickness is less constrained with any value lager than 0 km could fit our model. As the sparse gravity data coverage on South Pole Aitken (SPA) basin, densities uncertainties among this region are normally lager than other highland regions. Furthermore, we take into account the vertical variation of crust density, and develop a novel technique that the density profile of the crust could be inverted using higher resolution gravity models. However, since all these analyses are challenging by using the recent Kaguya gravity models, higher resolution gravity data expected from NASA’ Gravity Recovery and Interior Laboratory (GRAIL) mission would place tight constraints on both the lateral and vertical density variations of the lunar crust.