Unearthing the Moon’s deep secrets: multi-site geomagnetic depth sounding evidence for mantle heterogeneity and partial melt

Abstract The structure and thermal state of the Moon’s deep interior remain incompletely understood. Here, we apply geomagnetic depth sounding (GDS) to Apollo 12, 15, and 16 surface magnetometer data—yielding the first comparative, multi-site resistivity models of the lunar mantle. Using transfer function analysis during magnetotail quiet periods, we invert apparent resistivity profiles via one-dimensional Occam methods. All three sites reveal a resistive upper mantle (> 10 3 Ω·m) extending to ~ 300 km. However, Apollo 15 and 16 exhibit sharp resistivity reductions between 300 and 500 km, likely associated with thermochemical heterogeneity caused by ilmenite-bearing cumulate (IBC) descent. More strikingly, a deep conductive zone spanning 900–1100 km depth is consistently detected, coinciding with the location of deep moonquakes (DMQs). This zone’s enhanced conductivity, unexplained by seismic velocity models, supports the presence of low-degree partial melt insufficient to eliminate brittle failure. These findings redefine our understanding of the Moon’s thermal evolution and provide a new explanation for DMQ generation, demonstrating that surface-based GDS techniques offer critical insight into planetary interiors. Graphical abstract