How surface charging affects the photoelectron energy distribution on the moon

As sunlight reaches the moon, it creates a sheath of photoelectrons near the lunar surface. The electric properties of this region – which cause the lunar horizon glow due to light scattering from floating charged particles – are an important consideration for lunar missions. To investigate the relationship between the moon’s electrostatic potential and photoelectron distribution, Sanjay Mishra developed an analytical description of the photoelectron energy distribution in the moon’s sunlit portions, specifically focusing on the role of surface charging, which has been ignored in the past.

“The manifestation of charged fine dust and solar wind with photoelectron sheath features creates a complex picture of plasma environment in the vicinity of the lunar regolith,” Mishra said. “Such a complex electrostatic plasma environment is certainly of significance and technological interest to efficient instrument operation on lunar modules.”

Mishra found high-energy photons from the sun are the predominant contribution to the moon’s photoelectron distribution, altering the electrostatic potential in sunlit areas. The surface potential also varies with location, since the incoming photon flux is dependent on the latitude. The high energy solar photon contribution leads to a large disparity between these results and black body approximations.

To derive the lunar photoelectron distribution, Mishra considered a wide variety of previously neglected parameters, such as the temperature of the surface and the presence of solar wind. This enabled the determination of the rate and magnitude of photoemission, and hence the photoelectron distribution function.

For an exact understanding of the lunar photoelectric surface, the theory presented here must be complemented by data. Mishra hopes potential experimental schemes can be applied upon a future return to the moon.

Source: “Photoelectron distribution on sunlit surface of the Moon: A formalism,” by S. K. Mishra, Physics of Plasmas (2020). The article can be accessed at https://doi.org/10.1063/5.0016411 .