Volatile elements and compounds, such as water, are crucial to geological processes and will be vital for future in-situ resource utilization on the Moon. Thus, it is important to understand the abundance of these elements, their location (i.e., locked in minerals or at depth as ice), and evolution through time on the Moon. Spacecraft missions such as the Lunar Reconnaissance Orbiter have demonstrated that the lunar surface harbors water in at least some form. However, the origin of this water and its distribution across the lunar surface remain largely unknown. Compounding this is the fact that volatile elements likely migrate around the surface of the Moon and can be lost to space. An as-yet-undiscovered reservoir in the near-surface is required to replenish surface water and maintain the lunar water cycle.

In recent work, Huicun He and colleagues from the Chinese Academy of Sciences analyzed 117 glass beads from a lunar soil sample collected by the Chang’e 5 robotic arm and returned to Earth on December 16, 2020. These beads showed water-poor cores but had elevated water abundances in their rims. This trend correlated with changing hydrogen isotope composition (the ratio of hydrogen to deuterium) from core to rim. This can be explained by the inward diffusion of water derived from the solar wind (primarily composed of pure hydrogen). The researchers also noted that one bead showed water loss at the outermost rim and suggested that this records water loss during daily surface temperature fluctuations.

Modeling-based estimates indicate that the amount of water that these glass beads could contribute to the lunar regolith is as much as 2.7 × 10¹⁴ kg. Thus, lunar glass beads could be the reservoir required to maintain the lunar water cycle and act as a buffer for the global and daily variations of water abundance on the lunar surface. READ MORE