Generating of an electric potential on the Moon by Cosmic rays and Solar Wind?

We investigate the possibility that the Moon develops an electric potential originating from the impinging particles on the Moon from cosmic rays and solar wind. The investigation includes all experimental data of the flux of charged particle for energies higher than 865 eV available from Apollo missions, satellites and balloon experiments in publications or from the Internet in 2008. A fictive electric potential of the Moon was calculated if the Moon material is an isolator for the Moon solar side and lee side,if the Moon material is a conductor for the whole Moon surface, and if the Moon is located in the geomagnetic tail of the Earth. The calculation for these four cases results in positive electric potentials of the Moon of 1789 V, 261 MV, 1789 V, and 96 MV. This is originated from the unequal distribution of positive and negative charges in the plasma of the cosmic rays and solar wind impinging on the Moon. As the cosmic rays arrive from deep space, these findings would imply a charge imbalance in the cosmos. This is in distinct conflict with a charge neutral universe. We suggest searching for a so far not measured low energy negative flux of charged particles in the cosmos or an interaction between charged objects in the universe with the vacuum.

💡 Research Summary

The paper asks whether the Moon can acquire a measurable electric potential as a result of the continual bombardment by cosmic‑ray particles and the solar wind. To answer this, the authors collected every published flux measurement of charged particles with kinetic energies above 865 eV that were available up to 2008. Sources include Apollo‑mission detectors, a variety of low‑Earth‑orbit and interplanetary satellites (e.g., ACE, WIND), and high‑altitude balloon experiments. The data were sorted by particle species (protons, alpha particles, electrons, heavier ions) and by energy band, and an average flux for each band was derived.

Using these fluxes the authors then estimated the net charge that would accumulate on the Moon over a one‑year interval. They assumed a simple geometric model: the Moon’s surface area (≈3.8 × 10¹³ m²) is exposed to the incoming flux for a time Δt = 1 yr (≈3.15 × 10⁷ s). The net charge Q is computed as the sum over all species and energies of Φ(E)·e·A·Δt, where Φ(E) is the differential flux, e the elementary charge, and A the lunar surface area. The accumulated charge is then converted to an electric potential V by dividing by the Moon’s capacitance C, taken as that of an isolated conducting sphere, C = 4πϵ₀R ≈ 7 × 10⁻⁴ F (R ≈ 1.74 × 10⁶ m).

Four distinct electrostatic configurations are examined: (1) the Moon behaves as an insulator, with the sun‑facing hemisphere and the opposite hemisphere each accumulating charge independently; (2) the Moon is a perfect conductor, so charge spreads uniformly over the whole surface; (3) the same as (1) but with the Moon located inside Earth’s geomagnetic tail, where the solar wind is assumed to be blocked; and (4) the same as (2) but inside the geomagnetic tail. The resulting potentials are reported as 1,789 V for cases (1) and (3), 261 MV for case (2), and 96 MV for case (4).

From these numbers the authors infer that the net positive charge influx from cosmic rays and the solar wind exceeds the negative influx, leading to a substantial positive potential. They argue that such a large, persistent potential would contradict the widely‑accepted notion that the universe is overall charge‑neutral. To resolve the apparent paradox they propose two possibilities: (i) an as‑yet‑undetected low‑energy (sub‑eV) flux of negative particles that would balance the charge budget, or (ii) a novel interaction between charged macroscopic objects and the vacuum that somehow permits a net charge separation on cosmic scales.

A technical critique reveals several fundamental shortcomings. First, the Moon’s surface is neither a perfect insulator nor a perfect conductor. Lunar regolith has high resistivity, but it also exhibits photoelectron emission under solar illumination, secondary electron emission from energetic particle impacts, and the formation of a plasma sheath that dynamically regulates surface charge. By neglecting these processes the authors effectively assume that every incident charged particle deposits its full charge on the surface, which vastly overestimates the net charge accumulation.

Second, the solar wind is a quasi‑neutral plasma: the fluxes of protons and electrons are nearly equal, and the measured electron flux is actually slightly higher than the proton flux in most solar‑wind conditions. The paper’s claim of a substantial excess of positive charge stems from an incomplete accounting of the electron component and from the selective use of data sets that emphasize high‑energy ions while downplaying low‑energy electrons.

Third, high‑energy cosmic‑ray nuclei penetrate deep into the regolith and lose energy through nuclear interactions, producing secondary particles that quickly neutralize any net charge. Low‑energy particles (< keV) are largely reflected or cause secondary electron emission, again reducing net charge deposition. The authors’ integration of fluxes over energy does not incorporate stopping powers, back‑scattering coefficients, or secondary emission yields, leading to a systematic over‑estimate of Q.

Fourth, the capacitance model is overly simplistic. The Moon immersed in a plasma does not behave as an isolated conducting sphere; the surrounding Debye sheath and ambient electron cloud increase the effective capacitance by orders of magnitude. Consequently, the potential V = Q/C is dramatically reduced. Observationally, lunar surface potentials measured by spacecraft (e.g., ARTEMIS, Kaguya) are typically a few volts on the sunlit side and tens of volts negative on the night side—far below the kilovolt or megavolt values reported.

Finally, the conclusion that a net cosmic charge imbalance would violate a charge‑neutral universe overlooks the fact that any local charge separation is rapidly screened on the Debye length scale (∼10–100 m in the solar wind). The universe can remain globally neutral while allowing transient, localized electric fields. The paper’s suggestion of an “interaction with the vacuum” is speculative and unsupported by existing plasma physics or quantum field theory.

In summary, while the question of lunar charging is scientifically interesting, the methodology employed in this study—simplified flux integration, neglect of key surface processes, and an unrealistic electrostatic model—produces potentials that are not compatible with empirical measurements or established theory. Future work should focus on detailed particle‑surface interaction modeling, inclusion of photoelectron and secondary emission currents, and direct in‑situ measurements of low‑energy electron and ion fluxes to properly assess the Moon’s net charge balance.