Charge Retention in Lunar Regolith: Microscopic Processes and Constraints on Subsurface Charging

Abstract

Energetic-particle irradiation can produce persistent charge separation within lunar regolith, but the internal fields predicted by subsurface-charging models depend on how particle interactions are translated into retained charge. This critical review examines the evidence and assumptions connecting microscopic charge formation to macroscopic charging. We distinguish energy deposition, net charge transfer across a specified boundary, and the spatial distribution of signed excess charge. We organize these distinctions using an average retained-charge response defined for specified material, irradiation, electrical-state, and evaluation conditions. Its use as a volumetric source requires consistent charge accounting, compatible incident-flux definitions, and an adequate separation between response formation and subsequent transport. Applying these criteria to existing models, emission studies, and silicate irradiation experiments yields three assessments. First, emission measurements constrain electron loss across the measured boundary but do not locate internally redistributed charge. Second, evidence of internal reabsorption does not by itself establish that the charge separation sustaining subsurface fields is reduced; an illustrative calculation shows that the same net charge supply can produce different peak fields depending on transfer depths. Third, laboratory electrical damage in silicate crystals does not determine breakdown rates in cryogenic, porous regolith, which additionally require constraints on local fields, failure properties, and discharge recurrence. Joint constraints on boundary charge transfer, spatial charge separation, and post-irradiation relaxation are central to evaluating lunar subsurface-charging predictions.