The lunar moho and the internal structure of the Moon: A geophysical perspective

Extraterrestrial seismology saw its advent with the deployment of seismometers during the Apollo missions that were undertaken from July 1969 to December 1972. The Apollo lunar seismic data constitute a unique resource being the only seismic data set which can be used to infer the interior structure of a planetary body besides the Earth.

On-going analysis and interpretation of the seismic data continues to provide constraints that help refine lunar origin and evolution. In addition to this, lateral variations in crustal thickness (~ 0–80 km) are being mapped out at increasing resolution from gravity and topography data that have and continue to be collected with a series of recent lunar orbiter missions.

Many of these also carry onboard multi-spectral imaging equipment that is able to map out major-element concentration and surface mineralogy to high precision. These results coupled with improved laboratory-based petrological studies of lunar samples provide important constraints on models for lunar magma ocean evolution, which ultimately determines internal structure.

Whereas existing constraints on initial depth of melting and differentiation from quantitative modeling suggested only partial Moon involvement (< 500 km depth), more recent models tend to favor a completely molten Moon, although the former cannot be ruled out sensu stricto. Recent geophysical analysis coupled with thermodynamical computations of phase equilibria and physical properties of mantle minerals suggest that the Earth and Moon are compositionally distinct.

Continued analysis of ground-based laser ranging data and recent discovery of possible core reflected phases in the Apollo lunar seismic data strengthens the case for a small dense lunar core with a radius of < 400 km corresponding to 1–3% of lunar mass.