月球表层及月壳物质密度分布特征

The characteristics of rock-density distributions on the surface and in the crust of the Moon

月球表层与月壳岩石密度的横向与径向的变化,反映了月表及内部成分以及月球演化等特征.本文利用月球勘探者号伽马射线谱仪探测的月表Fe,Th与Mg元素分布数据,依据前人给出的元素含量与岩石类型的关系,对月球表层进行了岩性填图,并结合岩石样品与陨石的密度测试数据建立初始密度模型,采用铁元素与岩石密度的关系对其进行修正,从而建立了月表物质密度分布模型.基于嫦娥一号激光测高数据和日本SELENE计划发布的月球重力模型,计算出月球布格重力异常,进而反演得到月壳0～40 km深度范围内岩石平均密度分布模型.分析表明,大部分区域上,月壳至少月壳上部岩石成分主要以轻质的富含铝、钙、镁质的硅酸盐类岩石为主.由此推测,原始月壳极有可能是由轻质的、富含钙、镁质硅酸盐类岩石构成的全球性月壳.现今的玄武岩与克里普岩只是覆盖于原始的月壳之上的岩层,且厚度不大.

The variations of rock-density on the lunar surface and inside the crust indicate the component of lunar crust and even helpful in comprehending lunar tectonics and evolution. Based on the distributions of metal elements Fe, Th and Mg on the lunar surface derived from the measurement of Lunar Prospector mission, the global petrologic distributions of plagioclase, Mgrich rock, basalt and KREEP rock can be interpreted mineralogically and geochemically. In this study, the rock-density distribution on the lunar surface was calculated according to petrologic distributions and densities of the rock samples, meteorites and estimations from recent achievements as well. Values of density in the near surface were adjusted with the relationship between the contents of Fe and rock-density. The density model of lunar crust was determined using separated lunar Bouguer gravity anomaly data, which were calculated from topography model CLTM-s01 and gravity model SGM100h. This model represents the lateral variations of density within the depth of 40 km. The distributions of density in the lunar crust indicate that the lunar crust, at least the upper crust, in most area, is dominated by light silicate rock with rich aluminium, calcium and magnesium elements. It is speculated that the primary lunar crust is composed globally of light and calcium magnesium silicate rocks. The basalt and KREEP rock, as a thin layer, covers the original lunar crust.