New Insights into Lunar Halogen Distribution Reveal Secrets of the Moon’s Formation and Evolution
The Moon, Earth’s steadfast celestial companion, has long been a subject of intense scientific investigation. As the brightest object in our night sky, it has fascinated humanity for centuries, yet many mysteries about its formation and geological evolution persist. A recent groundbreaking study, published in Nature Communications, sheds new light on one such enigma – the distribution and behavior of halogen elements, particularly chlorine, within lunar rocks. This research, led by an international collaboration among scientists from the University of Münster, Ehime University in Japan, and Vrije Universiteit Amsterdam, brings a remarkable advancement in our understanding of the Moon’s geological past and chemical development.
Halogens, such as chlorine and fluorine, are volatile elements whose presence and abundance in planetary bodies are crucial indicators of geological processes and histories. On the Moon, these elements offer invaluable clues about the formation of its crust and the chemical transformations it underwent through the eons. However, unraveling the precise role of halogens on the lunar surface has been a persistent challenge due to uncertainties about how these elements become incorporated and how they are spatially distributed among lunar minerals and magmatic melts.
The study employed a series of highly controlled laboratory experiments designed to simulate the conditions inside the Moon during its earliest molten phase, often referred to as the "lunar magma ocean." This period, occurring shortly after the Moon’s formation, was marked by widespread melting that ultimately solidified into the initial crust. By synthesizing lunar rock analogs under high-pressure and high-temperature conditions, researchers mimicked natural lunar crystallization processes, enabling them to track the behavior of chlorine and other halogens within evolving mineral phases and residual melts.
One of the key revelations from these experiments is that lunar rocks formed on the Earth-facing side of the Moon harbor unexpectedly high concentrations of chlorine. This finding was somewhat counterintuitive because volatile elements like chlorine traditionally are considered to be depleted on airless bodies due to loss during volcanic degassing and lack of atmospheric retention. The elevated chlorine levels imply that the near side of the Moon experienced extensive volcanic activity and degassing events that enriched its crust and near-surface materials with chlorine-bearing gases.
Further analysis revealed spatial heterogeneity in halogen distribution linked to distinct lunar geological regions. Notably, samples emerging from the so-called KREEP terrain—an area rich in potassium (K), rare earth elements (REE), and phosphorus (P)—displayed characteristic elevated concentrations of incompatible elements but did not exhibit the same chlorine enrichment seen elsewhere. This suggests that the processes governing chlorine incorporation are more complex and regionally variable than previously thought, potentially tied to localized magmatic histories or metasomatic events.
The Moon’s crust consists mainly of two contrasting rock types: the light-colored highland anorthosites and the darker basaltic lavas. These lithologies differ significantly in age and spatial distribution. The anorthosites, which mainly constitute the lunar highlands, are older and believed to have crystallized early from the lunar magma ocean, while the basalts are relatively younger and primarily occupy the lunar maria on the near side. This dichotomy has puzzled scientists for decades, but the new evidence regarding halogen distribution provides an intriguing piece of this puzzle.
Professor Stephan Klemme of the University of Münster’s Institute of Mineralogy emphasizes the importance of this discovery, remarking, “Our results demonstrate an unusual partitioning of chlorine during lunar rock melting processes. The unexpectedly high chlorine content on the Moon’s near side points to extensive volcanic volcanism and outgassing that shaped the chemical landscape of the lunar crust.” His statement underlines the dynamic and evolving nature of lunar geology, overturning earlier assumptions of a relatively static and chemically uniform lunar surface.
To simulate the lunar interior conditions accurately, the team melted specially prepared chemical mixtures mimicking primitive lunar compositions and introduced chlorine as a variable component. Using high-pressure presses capable of reproducing pressures and temperatures found deep within the early Moon, they crystallized rock-forming minerals such as plagioclase and pyroxene from these melts. Careful measurement of halogen partition coefficients between minerals and melt allowed the team to build a comprehensive model of chlorine behavior during the crustal formation stage.
The implications of this work extend far beyond laboratory curiosities. The chemical fingerprints unearthed have the potential to refine models of the Moon’s geological past, including how the early magma ocean crystallized and how subsequent volcanic episodes altered the surface. Such knowledge is vital for interpreting lunar rock samples from past Apollo missions and for preparing to analyze fresh samples expected from upcoming exploratory missions.
China’s Chang’e-6 mission, scheduled for June 2024, aims to return lunar rock samples from the Moon’s far side—a region previously unsampled and poorly understood compared to the near side. The researchers anticipate that analysis of these new samples will provide critical tests of their halogen distribution model and offer unprecedented insight into whether the far side’s crust experienced similar chemical metasomatism or preserves an older, chemically primordial signature.
Dr. Jasper Berndt, also affiliated with the University of Münster, highlights the value of these future studies, stating, “Rocks sourced beyond the KREEP region do not show chlorine enrichment, indicating they may retain pristine characteristics from the Moon’s nascent stages. Such samples are essential to reconstructing the thermal and chemical evolution of our closest neighbor.” This perspective captures the dynamic dialogue between experimental petrology and planetary exploration driving contemporary lunar science.
Overall, this research represents a pivotal advancement in planetary geochemistry by elucidating how volatile elements like chlorine interact with magmatic processes on the Moon. It bridges a critical gap in our understanding of lunar crustal formation and metamorphism, illustrating that volatile recycling and metasomatism have played more prominent roles in lunar history than traditionally recognized. Importantly, it challenges long-standing perceptions of the Moon as a chemically simple and volcanically inert body.
As humanity stands on the brink of a new era of lunar exploration, fueled by international missions and renewed scientific curiosity, these findings provide a robust foundation for interpreting forthcoming lunar material. They emphasize the Moon’s geological complexity and encourage a re-evaluation of volatile element cycles on airless rocky bodies. Such insights not only deepen our grasp of lunar science but also inform broader theories about planetary formation and differentiation across the solar system.
In conclusion, the study’s meticulous experimental approach, combined with observational data from Apollo samples, presents compelling evidence that chlorine’s behavior and abundance on the Moon reveal critical chapters of lunar history. The Moon continues to surprise and teach us, hinting at a chemical past as dynamic and multifaceted as its role in the story of Earth and humanity. With forthcoming lunar sample returns, this evolving narrative is poised for an exciting new chapter.
Subject of Research: Halogen distribution and abundance in lunar rocks; lunar crust formation and metasomatism
Article Title: Halogen abundance evidence for the formation and metasomatism of the primary lunar crust
News Publication Date: 20-Jun-2025
Web References: 10.1038/s41467-025-60849-4
Image Credits: Jasper Berndt
Keywords
Moon, lunar geology, halogens, chlorine, lunar crust, lunar magma ocean, lunar rock formation, volcanism, experimental petrology, lunar volcanism, metasomatism, KREEP, Chang’e-6 mission
