The Apollo seismic experiments on the Moon in the 1960s-1970s allowed for the first determination of the internal structure of a body outside Earth. While decreased seismic wave velocities indicated a core-mantle boundary, these passive experiments, using meteorite impacts and deep moonquakes as seismic sources, provided insufficient resolution to detect the core structure. Reanalyses of the Apollo-area seismic data in 2011 revealed that the outer core was fluid; however, the data did not unambiguously determine whether the lunar inner core was solid like the Earth’s. Understanding the nature of the core is critical to constraining models of early lunar evolution. The lunar overturn scenario, a model of magma ocean solidification, which is invoked to explain observed concentrations of uranium, thorium, and potassium in basaltic source regions, occurs when the gravitationally unstable iron-titanium-bearing mineral ilmenite sinks into the interior.  

A team led by Arthur Briaud (Université Côte d’Azur) used observational constraints of the total mass of the Moon, its moment of inertia, and its tidal deformation (from lunar ranging and altimetry determination), as well as the crustal structure inferred by the GRAIL mission, to determine best-fit models of the lunar interior. They found that only models with a low-viscosity zone near the core-mantle boundary, a fluid outer core, and a solid inner core could match the observed tidal deformations. These results strengthen the evidence of an early lunar overturn by confirming the existence of a low-viscosity zone consistent with ilmenite-bearing material near the core-mantle boundary. The formation of an inner core further helps explain the remanent magnetization of lunar rocks of 4.25–3.19 Gyr in age, similar to how the nucleation of an inner core on the Earth is thought to have increased the power of its dynamo. This work brings insights into the timeline of the lunar bombardment in the first billion years of the solar system. READ MORE