Deep dielectric and surface charging of regolith in the Moon's permanently shadowed regions

Galactic cosmic rays (GCRs) and solar energetic particles (SEPs) can penetrate into the lunar regolith, causing deep dielectric (or subsurface) charging. The regolith discharges with a timescale that depends on temperature. In permanently shadowed regions (PSRs), for example, this timescale is ~20 days. To estimate the effects of subsurface charging, Jordan et al. [2014] created a data-driven, deep dielectric charging model. The model used GCR data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) and SEP data from the Electron, Proton, and Alpha Monitor (EPAM) on the Advanced Composition Explorer (ACE). Data-driven runs of the model indicated that GCRs create, throughout the top tens of centimeters of regolith, a persistent electric field of up to ~700 V/m. They also indicated that large SEP events could create episodic subsurface electric fields of ≥106 V/m, possibly inducing dielectric breakdown. Over time, such SEP events may drive "breakdown weathering," preferentially enhancing the fraction of fine and monomineralic grains within PSRs. This model assumed that no other charging processes exist, i.e., that the environment is a vacuum. Solar wind plasma, however, can enter PSRs and charge the regolith's surface, so we revise the model to include this charging. While it cannot simultaneously nullify or decrease all the subsurface electric fields found using the model's previous iteration, adding this process does change the depth profile and maxima of the electric fields. Using the data-driven results of the revised model, we estimate the subsurface electric fields due to the solar wind, GCRs, and SEPs. We also show how including solar wind charging affects the possibility of dielectric breakdown and the importance of breakdown weathering to the regolith in PSRs. Reference: Jordan, A. P., T. J. Stubbs, J. K. Wilson, N. A. Schwadron, H. E. Spence, and C. J. Joyce (2014), Deep dielectric charging of regolith within the Moon's permanently shadowed regions, J. Geophys. Res.-Planet, DOI: 10.1002/2014JE004648.