Structure in the upper lunar crust

Abstract

Estimates are made of the degree of lithification and of structure densities which are compatible with lunar in situ seismic profiles in the top 30 km of the Moon. Estimates are based on comparison of results of passive and active lunar seismic experiments with the pressure dependence of elastic moduli for various classes of lunar samples. Competent rock, such as igneous rock or recrystallized breccias with crack porosity of not more than about 0.5 % are required to satisfy velocity profiles in the depth range 1-30 km. Velocity profiles in the upper 1 km are best satisfied by comminuted material to highly fractured lithic units. These estimates constrain those thermal and shock histories which are compatible with lunar seismic results. After crystallization, or recrystallization, rock below 1 km cannot have been exposed to more than moderate shock levels. In the uppermost 1 km, an unannealed and broken rock layer would imply low thermal conductivity resulting in possible temperatures at 1 km depth of several hundred kelvins.