The presence of water (H₂O/OH) on the lunar surface has been a topic of significant research. Initially believed to be extremely dry, the Moon has been shown to contain water in various locations, particularly at the poles and in permanently shadowed regions. This discovery was initially made using neutron spectrometers, followed by numerous spectroscopic observations from missions like Chandrayaan-1, Deep Impact/EPOXI, Cassini, and OSIRIS-REx. Analysis of Apollo samples also provided direct evidence of water in various mineral forms. The origin of this water is multifaceted, with proposed sources including indigenous water, solar wind implantation, and meteorite or comet impacts. Recent suggestions include contributions from Earth’s wind and exospheric dust impacts. While most remote sensing data supports solar wind implantation as the primary source, some studies have detected evidence of indigenous water. The debate on the relative contributions of these sources and the distribution of water across the lunar surface remains an open question. The Chang'E-5 mission, with its in-situ measurements and returned samples, provides a unique opportunity to address this.

A considerable body of research exists concerning the presence and origin of lunar water. Early studies based on Apollo samples suggested an extremely dry Moon. However, subsequent observations using neutron spectrometers revealed global hydrogen distribution, particularly enriched at the poles. Various space-based missions, including Chandrayaan-1, Deep Impact/EPOXI, Cassini, and OSIRIS-REx, provided further spectral evidence of water. The LCROSS mission directly impacted the lunar surface and confirmed the presence of water ice in permanently shadowed craters. Studies examining Apollo samples showed water trapped in volcanic glass beads, melt inclusions, apatites, and plagioclase. Diverse hypotheses regarding the origin of lunar water have been put forward. Solar wind implantation is widely considered a significant contributor, with hydrogen ions from the solar wind reacting with oxygen on the lunar surface to form hydroxyl. However, the presence of indigenous water within the lunar mantle and its potential outgassing has also been proposed. Meteorite impacts and cometary delivery are further possible sources. More recently, contributions from the Earth’s wind and exospheric dust impacts have been suggested, highlighting the complexity of this phenomenon. The discrepancies in the findings necessitate further investigation to determine the relative contributions of these potential sources.

This study utilizes data from the Chang'E-5 mission, which included both in-situ spectral measurements and the return of lunar samples. The in-situ data were acquired using the Lunar Mineralogical Spectrometer (LMS) on the Chang'E-5 lander. The LMS collected 11 hyperspectral datasets of lunar soil and rock targets. These data underwent rigorous processing, including radiometric and thermal corrections (using three different models: Clark et al., Li et al., and Groussin et al.) and photometric correction using the Lommel-Seeliger model. Hydroxyl content was estimated from the 2.85 µm absorption feature in the spectra. Laboratory analysis of the returned samples involved several techniques including: LMS-EQM (engineering qualification module of the LMS), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, and electron-probe micro-analysis (EPMA). The XRD analysis was performed on three lunar soil samples to determine mineral composition and abundance. Raman spectroscopy was used to assess the presence of hydroxyl within apatite grains. EPMA data were used to estimate the relative proportions of OH, F, and Cl in the apatite, allowing for calculation of hydroxyl content. The results from the in-situ spectral analysis and laboratory analysis were then compared with existing data from the Moon Mineralogy Mapper (M³) to determine the broader context of hydration at the Chang'E-5 landing site. The contribution of solar wind implantation was assessed by comparing the agglutinate glass content in the Chang'E-5 samples to that in Apollo samples.

In-situ spectral analysis using the Chang'E-5 LMS revealed a mean hydroxyl content of 28.5 ppm in lunar soils at the landing site. This value is on the lower end of observed lunar hydration features. The LMS data showed varying degrees of absorption at 2.85 µm, with the strongest absorption observed for a rock sample. Laboratory analysis confirmed the presence of hydroxyl in the returned samples, primarily within apatite grains. The apatite content in the samples varied from 0.1 to 1.4 wt%, resulting in an estimated hydroxyl content range of 0 to 179 ± 13 ppm. The relatively low agglutinate glass content in Chang'E-5 samples (11.5-20 wt%) suggests a weak contribution from solar wind implantation compared to Apollo samples. The comparison of Chang'E-5 LMS data with M³ remote sensing data highlighted the relatively weak hydration observed at the Chang'E-5 landing site. Several factors contribute to this, including the high surface temperature during data acquisition, the shielding of the lunar surface from the solar wind due to the Moon's location within the Earth's magnetosphere, and the nature of the mare basalts at the landing site. The excess hydroxyl content observed in some soil samples compared to the expected solar wind contribution is attributed to indigenous hydroxyl, with hydroxyapatite identified as a likely significant source.

The findings address the ongoing debate on the origin and distribution of lunar water by providing in-situ data from a young mare region. The observed low hydroxyl content in the Chang'E-5 samples supports existing remote sensing and telescopic observations. The relatively low contribution from solar wind implantation is significant, underscoring the importance of indigenous sources. The identification of hydroxyl-bearing apatite as a potential major source of hydroxyl at the landing site challenges the previous dominance of solar wind as the primary source in mare regions. This finding highlights the spatial heterogeneity of lunar hydration, with variations potentially influenced by factors such as mineral composition, temperature, and solar wind exposure. The results are particularly significant in the context of future lunar exploration, as understanding the distribution and sources of water is critical for resource utilization.

This study provides compelling evidence for the presence of water on the lunar surface, with a focus on the Chang'E-5 landing site. The low overall hydroxyl content is explained primarily by the high temperatures and Earth’s magnetosphere shielding. Hydroxyl-containing apatite emerges as a significant potential indigenous source, challenging the prevalence of solar wind as the main contributor. This research emphasizes the complexity of lunar water distribution and the need to consider multiple sources and influencing factors for a complete understanding of lunar hydration. Further studies using broader spatial and temporal sampling should be undertaken to confirm these findings and to explore the distribution of hydroxyapatite across the lunar surface.

The study focuses on a single landing site, limiting the generalizability of the findings to the entire lunar surface. While multiple thermal correction models were employed, inherent uncertainties associated with thermal emission correction remain. The analysis of apatite relies on laboratory measurements, which may not completely reflect the in-situ conditions. Additionally, the investigation of indigenous sources is primarily inferential, requiring further research to fully quantify their contribution.