Hemispheric Thermal Dichotomy in the Lunar Mantle

The Moon is thought to have solidified from a global lunar magma ocean (LMO) through fractional crystallization. Well-documented hemispheric asymmetries in topography, crustal thickness, surface abundances of radiogenic elements, and volcanic history suggest that the nearside and farside underwent distinct evolutionary pathways. These differences likely reflect variations in the deep interior, particularly in the distribution of radiogenic heat-producing elements (HPEs) capable of sustaining long-lived temperature contrasts. However, direct geophysical evidence for such a dichotomy has been limited. A recent study based on tidal response by Park et al. (2025) reveals a 2-3% difference in shear modulus between the nearside and farside mantle, implying that the nearside mantle remains ~200 K warmer today. Similarly, He et al. (2025), using Chang’e-6 farside basalt samples combined with remote-sensing-based geochemical modeling, report farside mantle temperatures at least ~100°C cooler than those of the nearside. In this study, we employ numerical modeling to investigate whether a hemispheric thermal contrast of several hundred kelvins in the lunar mantle can persist throughout lunar history and to assess how degree-1 mantle convection and HPE distributions influence the maintenance of this dichotomy. We further explore the role of dense ilmenite-bearing cumulates (IBCs), initially crystallized beneath the crust during the final stages of LMO solidification, and later overturned and settled near the core-mantle boundary due to gravitational instability, to shape the Moon’s long-term thermochemical and dynamical evolution.