Source: Journal of Geophysical Research: Planets
The structure of a planet’s deep interior can only be constrained indirectly using chemical tracers and remote geophysical information. The lowermost mantle is a particularly intriguing region of the Moon’s interior as it contains clues to the early evolution of our satellite. The lowermost 150 kilometers of the mantle has been described as “weak” based on a low seismic velocity and the way it dissipates tidal energy. Previous studies have proposed that this weak layer contains melts. However, it is hard to explain how the Moon’s mantle can be hot enough to melt unless it contains significant amounts of water. Alternatively, this layer may be enriched in the mineral ilmenite. Either interpretation has important implications for the formation and early evolution of the Moon. However, Walterová et al.  suggest that there may be no such partially molten layer at the base of the lunar mantle after all.
One constraint on the deep interior of the Moon comes from precise measurements of the shape and motion of the Moon, especially tides. Surprisingly, the lunar interior dissipates tidal energy more efficiently at periods of one month than one year. Classical viscoelastic models of the lunar interior can explain this observation only if there is a distinct layer at the base of the mantle. However, these models may not be the best description of the complex behavior of real rocks.
Walterová et al.  show that a dissipation model that Sundberg and Cooper  developed can explain the higher dissipation at one-month period without requiring a separate deep mantle layer. If this arguably more realistic mechanical model adequately represents the interior of the Moon, the lunar mantle could be made of a single material and be solid throughout. At this point, though, both the uniform and the layered model remain admissible, given the limited data available. This situation can only be remedied by collecting new geophysical data, especially on the far side of the Moon. Several teams have proposed missions to the Moon with a geophysical focus. These missions, as well as improvements in satellite-tracking technologies, would be crucial for better understanding the interior structure and early history of our natural satellite.
Citation: Walterová, M., Běhounková, M., & Efroimsky, M. (2023). Is there a semi-molten layer at the base of the lunar mantle? Journal of Geophysical Research: Planets, 128, e2022JE007652. https://doi.org/10.1029/2022JE007652
—Laurent G. J. Montési, Editor in Chief, JGR: Planets