In the Moon's polar craters, where sunlight never reaches, regolith may accumulate electric charge for decades, buried deep below the surface. It's like a bank account where deposits and balance don't match: a particle delivers energy, but the charge doesn't always stay put.
The Moon lacks an atmosphere and a global magnetic field, so its surface is directly bombarded by the solar wind, flares, cosmic rays, and micrometeorites. The classic picture describes surface charging as a balance of currents: photoelectrons leave, while plasma electrons and ions arrive. But in permanently shadowed regions at the poles, there is no sunlight, temperatures drop to tens of kelvins, and the regolith's conductivity becomes vanishingly small. There, charge brought in by energetic particles can persist for weeks and accumulate at depths of millimeters to centimeters.
The memorandum's authors didn't run experiments; instead, they gathered and reinterpreted scattered data, from laboratory measurements of secondary electron emission to observations from the Surveyor, Apollo, Lunar Prospector, Kaguya, and ARTEMIS missions. They traced the path of an energetic particle through regolith step by step: elastic and inelastic scattering, generation of secondary electrons, their recombination, trapping, and escape across grain boundaries. For each step, they compared known models and data to pinpoint where the link between a microscopic event and macroscopic charge accumulation breaks down.
What sets this work apart from neighboring studies is that it doesn't add another model or observation; it exposes a structural gap: how to go from a single fallen particle to a volumetric charge source. Related papers describe either surface effects (dust, electric wind) or individual mechanisms (radiolysis of Europa's ice), but no one has assembled the full chain from collision to accumulated field. This memorandum sets a common language for comparing different models and planning experiments.
The next logical step is to measure what's missing: how much charge remains after a particle stops, implants, backscatters, or passes through; what fraction of generated electrons recombine, get recaptured, fall into traps, or escape. This requires cryogenic vacuum chambers with regolith irradiation simulating polar soil, plus Monte Carlo simulations with grain-level resolution. The resulting response functions could then be fed into macroscopic models to test whether fields in permanently shadowed regions indeed reach breakdown values.
The charge dissipation time in a permanently shadowed crater can reach 20 days, nearly one lunar day. For comparison, at the equator at noon, charge disappears within minutes. This contrast underscores how crucial it is to understand deep processes: they determine whether dust sticks to spacesuits, whether sparks appear near a lander, and how long electronics can operate in eternal shadow.
The charge dissipation time in a permanently shadowed crater can reach 20 days, almost one lunar day. For comparison, at the equator at noon, charge disappears within minutes.
Arthur C. Clarke's novel A Fall of Moondust (1961) describes a sea of dust that behaves like a liquid due to electrostatic forces. Although the real Moon isn't covered by such a sea, the idea that charge can make dust flow and accumulate in low-lying areas resonates with modern models of electrostatic transport of lunar regolith.