Lunar Interior Structure: Current Understanding, Unresolved Questions and Future Advances

Lunar missions to date have resulted in the identification of geographical variations in tectonism, volcanism and composition that range in spatial scale from sub-sample dimensions to hemispheric. Basin-scale and larger variations in surface properties are of interest because they may reflect the structure and dynamics of the deep lunar interior. Constraints on crust, mantle and core structure are provided by spacecraft observations of topography, gravity, and magnetic fields, and Earth-based Lunar Laser Ranging measurements (LLR). In addition, seismic stations at Apollo sites 12, 14, 15 and 16 transmitted data from 1969-1977, and 12,558 seismic events – moonquakes and impacts - were originally identified. Crustal thickness models, based on gravity and topography data, indicate thick crust beneath the lunar highlands (and hence a near-side / far side asymmetry) and thinning beneath large basins. However, the absence of far-side gravity data prior to the Kaguya mission, limits global characterization of the structure and volume of the crust. Models for seismic velocity as a function of depth show a discontinuity interpreted as the crust-mantle boundary, and allow investigation of mantle structure to about 1200 km depth. The presence or absence of a mantle seismic discontinuity at about 500 km depth has been hotly debated, since proposed explanations for its presence include the depth extent of an early magma ocean, and/or the maximum depth of the source region for mare basalts. The lack of seismic imaging of depths below 1200 km means that core size and composition are poorly constrained, although LLR data suggest a fluid, or partly fluid state. I will review current understanding of lunar interior structure and expected advances from new missions. In particular, I will highlight unresolved questions that are critical to our understanding of lunar evolution.