Abstract:
Charged particles constitute one of the essential physical elements within the lunar space environment. Their origins, compositions and energy spectra are remarkably complex, and their spatial distribution and temporal dynamics are intricately coupled with localized electromagnetic field structures, rendering the lunar space environment fundamentally distinct from that of Earth or other magnetized planetary bodies. Compared to Earth, our methods for probing the lunar space environment remain relatively limited, both in terms of observational techniques and detection frequency. Notably, in situ measurements within the critical region encompassing the lunar surface and the low-altitude zone below 20 kilometers are exceptionally sparse. These observational gaps have collectively led to a fragmented and incomplete understanding of the fundamental characteristics of lunar charged particles, including their genesis, density fluctuations, dynamic behavior, as well as their potential spatial effects and associated hazards for exploration infrastructure and human activity. This review aims to synthesize and systematically organize the research findings from observations of lunar charged particles dating back to the Apollo era, endeavoring to present a coherent picture of the current state of knowledge and the trajectory of exploration. Recognizing that our comprehension of extraterrestrial space is profoundly shaped by the execution of national and international deep-space exploration programs, Section 1 begins by cataloging the key lunar missions undertaken by various nations, along with their primary scientific payloads and objectives. This historical overview is intended to illustrate the shifting priorities and evolving interests in lunar exploration over the decades. Given the inextricable coupling between charged particles and ambient electromagnetic fields, Section 2 provides a concise summary of the current understanding of lunar electromagnetic fields, encompassing both crustal magnetic anomalies and global fields, either transiently induced or permenantly intrinsic. The core of this review is structured along two parallel analytical threads. The first, covered in Sections 3 through 5, focuses on the "Background space environment along lunar orbit and its interactions with the Moon." This part examines the solar wind and geomagnetospheric plasma populations as they encounter and interact with the Moon, leading to phenomena such as absorption, reflection, and the formation of wake structures. The second thread, spanning Sections 6 to 11, is organized by "Types of charged particles in lunar space." Here, we systematically review observational results and theoretical models pertaining to specific particle populations, including photoelectrons, secondary electrons, exospheric neutrals, lunar dust, high-energy rays, and particles generated by human activities. Throughout this synthesis, it becomes evident that significant discrepancies and even outright contradictions exist, not only between theoretical predictions and observational data but also among different observational datasets themselves. These inconsistencies highlight the challenges inherent in remote and in situ space physics measurements and point to critical gaps in our current knowledge. Such uncertainties and unresolved questions likely define the most promising and necessary directions for future targeted investigations, requiring more advanced instrumentation, coordinated multi-point measurements, and sustained observation campaigns to unravel the complex electrodynamic environment of our closest celestial neighbor.