Constraining the distribution of radiogenics on the Moon from global geodynamic models

The predominant concentration of volcanic activity and surface enrichment of heat producing elements (HPE) on the lunar nearside suggest an asymmetry in interior properties and thermal history of the Moon. The distribution of HPE beneath the surface and the processes that led to their enrichment on the nearside surface remain poorly understood (Gaffney et al., 2023). Interior radiogenic heating directly affects surface heat fluxes measured in situ during the Apollo program (Langseth et al., 1976), and estimated from orbit at the Compton-Belkovich (Siegler et al., 2023) and Region 5 locations (Paige & Siegler, 2016). 

Here, we link the subsurface distribution of HPE to the present-day surface heat flux using 3D thermal evolution models. We investigate the interior dynamics of the Moon from the post magma ocean crystallization phase to present-day using the mantle convection code GAIA (Hüttig et al., 2013). Similar to Plesa et al. (2016), we use a spatially variable crustal thickness model as input (Broquet & Andrews-Hanna, 2024). We investigate the structure of a putative HPE-rich layer underneath the PKT (Procellarum KREEP Terrane, Jolliff et al., 2000) assuming a circular geometry. We vary its location, size, depth and HPE enrichment compared to the mantle and anorthositic crust. Our models consider the HPE concentrations as constrained from magma ocean crystallization studies, but assume that additional mechanisms may have led to a migration of heat sources below the PKT region. 

Similar to Laneuville et al. (2013), we find that an enriched layer placed below the crust can efficiently heat up the mantle and contribute to explaining prolonged lunar magmatism. We show that the prominent gravity anomaly associated with the warm mantle beneath the PKT (Laneuville et al., 2013; Grimm, 2013) can be used to construct updated crustal thickness models, which display a substantially thinner nearside crust.

Our models show that a large HPE anomaly underneath PKT (~1500 km radius) allows sufficient surface heat flux variability to account for the low Region 5 value (Paige & Siegler, 2016) and the Apollo 15 & 17 measurements (Langseth et al., 1976). Conversely, a smaller anomaly (<1200 km) fails to produce any significant difference in surface heat flux between Apollo 17 measurement and Region 5 estimate. However, this geometry may help explain the absence of Imbrium’s western ring (Broquet & Andrews-Hanna, 2024) and the spatial variability in the relaxation state of lunar basins (Ding & Zhu, 2022).

Our models provide an important baseline for the interpretation of upcoming heat flux measurements within Mare Crisium (TO19D, Nagihara et al., 2023) and Schrödinger Crater (CP-12, Nagihara et al., 2023), predicting that the heat flux at Crisium and Schrödinger should be comparable to that measured at Apollo 17 and estimated at Region 5, respectively. Significantly different heat flux measurements would have profound implications for our understanding of the distribution of radiogenic elements within the lunar interior. Lastly, quantifying the subsurface thermal state and the distribution of HPE on the Moon will prove crucial for infrastructure development in the framework of the Artemis program.