Unveiling the lunar magma ascent process with indigenous volatiles via radar data analysis

Lunar indigenous volatiles provide key constraints on both Earth–Moon formation and the evolution of lunar volcanism. Recent analyses of Apollo samples and remote-sensing observations suggest that the lunar mantle is richer in volatiles than previously assumed. To obtain additional information on indigenous volatiles, we attempt to detect subsurface gas voids formed by volatile exsolution from intrusive magmatic bodies using the Lunar Radar Sounder (LRS). We improve the lunar digital elevation model (DEM) using a generative adversarial network (GAN) and perform surface scattering simulations to suppress topography-driven radar clutter, enabling the identification of subsurface echo candidates (SECs) in Mare Tranquillitatis. SECs are more abundant in the eastern part of Mare Tranquillitatis, where the crust is thicker, and less common in the thinner-crust western part. Radar equation analysis of SECs’ echo intensity and shape indicates that they require either voids or highly porous structures. We then conduct magma ascent simulations to evaluate eruption feasibility. Volatile-free magma cannot erupt a 40-km-thick crust representative of eastern Mare Tranquillitatis. Furthermore, magma ascent simulations that include volatiles show that 686–1976 ppm CO is required for the eruption in eastern Mare Tranquillitatis. We therefore propose that volatile-bearing magma rose beneath there, but a fraction stalled and exsolved volatiles, leaving subsurface voids at the tips of stalled intrusions. The inferred CO content (686–1976 ppm) exceeds the maximum (140 ppm CO) estimated for parent magmas from Apollo samples. If we consider that H2 exsolved at depth under reducing mantle conditions replaces the CO required for eruptions, the required H2 content would be 39–141 ppm. The implied mantle hydrogen abundance (5.9–21 ppm) is difficult to reconcile with late accretion alone, suggesting that volatile-retention processes operated during Moon formation.