MORE THAN CRATERS AND DUST: THE MOON AS A LYNCHPIN IN UNDERSTANDING TERRESTRIAL PLANET EVOLUTION (Invited Presentation)

The Moon occupies a unique position in planetary science. It is large enough and was once hot enough to have fully differentiated and separated into a core, mantle, and primary crust, and to have sustained magmatism for billions of years. Yet unlike Earth, is small enough and cooled quickly enough to retain the materials and chemical heterogeneities produced by primary differentiation. Furthermore, it is close enough to Earth to make it the first exploration target for new technologies and space-faring nations and companies, yet there remain numerous unanswered big-picture questions about the origin and evolution of the Moon. Fortunately, a fleet of international and commercial missions, including the return of new samples, has already begun the next phase of exploration. Here I summarize a few outstanding questions.

Age of the Moon: There has been heavy community focus in the past ~15 years on determining the age of the Moon, and thus the Moon forming giant impact, given the importance of this to Solar System and early Earth models. However, despite more high-precision ages, this question has not been resolved. There remain fundamental discrepancies between ages determined from Sr-Nd-Hf-Pb isochron and model ages for lunar reservoirs, which tend to point to lunar differentiation around ~4.38-4.35 Ga, and the oldest ages determined from zircons which, although few in number, reach as old as ~4.46 Ga. These age determinations are seemingly incompatible.

Nearside-Farside Asymmetry: The Earth-referenced asymmetric nature of lunar crustal thickness, volcanism, and KREEP distribution has been known since the first images of the farside, but still lacks a holistic formation mechanism that account of all the observed features. Models invoking large impacts lack convincing physical evidence, asymmetric magma ocean models lack a forcing mechanism, and post-LMO overturn models cannot explain crustal thickness and compositional variations.

Mantle Dynamics and Prolonged Magmatism: The connections between mantle dynamics such as the scale and timing of initial cumulate overturn, the duration and vigor of subsequent convection, and the connections to magmatism are still unclear. Lunar magmatism continued to as late as 1 Ga, and possibly as recently as 14 myr ago on a small scale, but the connections between the state of the mantle and heat sources are unknown.