The Lunar Dust Puzzle
Authors: Zakharov Alexander V. 
Published: November 2020

The Moon was the first extraterrestrial body to attract the attention of space pioneers. It has been about half a century since an active lunar exploration campaign was carried out. At that time, a series of Russian and American automatic landing vehicles and the American manned Apollo Program carried out an unprecedented program of lunar exploration in terms of its saturation and volume. Unique breakthrough data on the lunar regolith and plasma environment were obtained, a large number of experiments were carried out using automated and manned expeditions, and more than 300 kg of lunar regolith and rock samples were delivered to Earth for laboratory research. A wealth of experience has been accumulated by performing direct human activities on the lunar surface. At the same time, the most unexpected result of the studies was the detection of a glow above the surface, recorded by television cameras installed on several lunar landers. The interpretation of this phenomenon led to the conclusion that sunlight is scattered by dust particles levitating above the surface of the Moon. When the Apollo manned lunar exploration program was being prepared, this fact was already known, and it was taken into account when developing a program for astronauts’ extravehicular activities on the lunar surface, conducting scientific research, and ground tests. However, despite preparations for possible problems associated with lunar dust, according to American astronauts working on the lunar surface, the lunar dust factor turned out to be the most unpleasant in terms of the degree of impact on the lander and its systems, on the activities of astronauts on the surface, and on their health.

Over the past decades, theoretical and experimental model studies have been carried out aimed at understanding the nature of the lunar horizon glow. It turned out that this phenomenon is associated with the complex effect of external factors on the lunar regolith, as a result of which there are a constant processing and grinding of the lunar regolith to particles of micron and even submicron sizes. Particles of lunar regolith that are less than a millimeter in size are commonly called lunar dust. As a result of the influence of external factors, the upper surface of the regolith acquires an electric charge, and a cloud of photoelectrons and a double layer are formed above the illuminated surface. Coulomb forces in the electric field of this layer, acting on microparticles of lunar dust, under certain conditions are capable of tearing microparticles from the surface of the regolith. These dust particles, near-surface plasma, and electrostatic fields form the near-surface dusty plasma exosphere of the Moon. The processes leading to the formation of regolith and microparticles on the Moon, their separation from the surface, and further dynamics above the surface include many external factors affecting the Moon and physical processes on the surface and near-surface dusty plasma exosphere. As a result of the research carried out, a lot has been understood, but many unsolved problems remain. Recently, since the space agencies of the leading space powers have been turning their attention to intensive research and subsequent exploration of the Moon, interest in the processes associated with the dynamics of lunar dust and its influence on landing vehicles and their engineering systems is increasing, and significant attention is being paid to reducing and mitigating the negative impact of lunar dust on the activities of astronauts and their health.