The Role of Crustal Magnetic Anomalies and Topography in Shaping Lunar South Polar Water Ice

Lunar magnetic anomalies are abundant near the south pole, where several moderate-strength anomalies spatially overlap permanently shadowed regions. This environment provides a unique setting to assess how crustal magnetic fields and complex topography regulate plasma–surface interactions and, in turn, the stability and distribution of surface water ice.A global fully kinetic electromagnetic particle-in-cell numerical model is used to simulate proton and electron surface fluxes near the south pole, averaged over a full lunar rotation. The simulations incorporate a regional crustal magnetic field model based on Kaguya and Lunar Prospector magnetometer measurements, together with high-resolution surface topography from the Lunar Reconnaissance Orbiter Laser Altimeter. This approach enables a self-consistent evaluation of how terrain and crustal magnetic fields jointly influence plasma access to the surface.The simulations show that topography strongly structures the surface plasma environment, enhancing fluxes on crater walls while partially shielding crater floors. The inclusion of crustal magnetic fields further modulates plasma access, producing relatively modest proton and electron flux variations relative to simulations without magnetic anomalies.Using the modelled fluxes, plasma-driven production, sputtering, and electron-stimulated desorption rates are evaluated alongside thermally driven sublimation. While the absolute balance depends on laboratory-derived yield assumptions, the results indicate that permanently shadowed regions consistently exhibit a positive net surface water ice balance rate, which closely coincides with inferred surface water ice exposures and highlights the importance of including realistic crustal magnetic fields and topography when assessing plasma-surface interactions and volatile evolution at the lunar poles.