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

From Earth, the Moon always reveals the same face – a stark landscape dominated by dark basaltic plains called lunar maria and bright highland regions. The near hemisphere, perpetually facing Earth due to tidal locking, is richly embroidered with these extensive basaltic seas, formed by volcanic eruptions approximately 3.5 billion years ago. Contrastingly, the lunar farside appears notably different, featuring an absence of these maria and instead a terrain dominated by rugged, bright highlands marked by an almost complete lack of basaltic volcanism. This hemispherical asymmetry, or lunar dichotomy, has long intrigued planetary geologists and demands an explanation rooted in the early Moon’s formation and evolution.

Recent groundbreaking research conducted by an international collaboration including the Geodynamics Research Center at Ehime University, Universität Münster, and Vrije Universiteit Amsterdam offers crucial insights into the Moon’s evolutionary pathways. Focusing on the behavior of halogen elements, particularly chlorine (Cl), within lunar minerals and magmatic melts, the team sought to elucidate the underlying cause of this hemispherical disparity. Halogens such as fluorine and chlorine, despite their trace abundances, bear significant geochemical importance because of their volatility and sensitivity to magmatic processes, effectively recording chemical conditions and events during lunar crust formation.

The researchers employed sophisticated high-pressure and high-temperature laboratory experiments designed to simulate the extreme conditions of lunar magma oceans and subsequent crust formation. These experiments allowed precise determination of chlorine’s partitioning behavior between solid lunar minerals and coexisting melts under variable conditions. Incorporating these experimental partitioning data into geochemical models that incorporate physical and chemical lunar mantle evolution, the researchers then compared predicted halogen abundances with those measured from authentic lunar crustal specimens, derived primarily from Apollo mission samples and lunar meteorites.

Their analyses uncovered a striking enrichment of chlorine in the samples obtained from the lunar nearside crust – a signature conspicuously missing from farside samples. This differential Cl concentration implies the widespread presence of a volatile chloride-rich vapor phase that was likely confined to the nearside environment during and after the formation of the lunar crust. Given chlorine’s known behavior as a highly incompatible and volatile element during magmatic differentiation, the presence of such a vapor phase suggests a distinct geochemical process influencing the nearside’s crustal evolution that did not affect the farside in the same manner.

This vapor phase metasomatism – a process whereby gases chemically alter the solid rock composition – likely originated from extensive degassing episodes associated with prolonged volcanism on the lunar nearside, particularly in the Procellarum KREEP Terrane, a geochemically enriched region known for its unique potassium (K), rare earth elements (REE), and phosphorus (P) signatures. The release of metal chlorides into a vapor phase during volcanic eruptions or impact-induced evaporation could have chemically modified the lunar nearside crust substantially, imparting it with its observed chlorine enrichment and contributing to its distinct mineralogical and physical characteristics.

Meanwhile, the lunar farside, devoid of this chloride vapor interaction, retains a more primordial geochemical signature. The crustal rocks here are consistent with magmatic products derived from deeper mantle sources dating back to approximately 4.3 billion years ago, closer to the initial magma ocean crystallization epoch. This preserved geochemical heritage offers a rare window into the Moon’s earliest differentiation events, relatively undisturbed by later resurfacing or volatile-mediated modification processes. In particular, the identification of Mg-suite lithologies within the farside crust, associated via F/Cl ratio modeling to deep mantle residues, highlights the importance of examining these pristine regions to reconstruct lunar magma ocean dynamics.

These discoveries carry significant implications for our understanding of lunar formation and the key processes that sculpted its hemispheric asymmetry. Chlorine-rich vapors, acting as geochemical agents during volcanic plumes and impact vaporization events, emerge as important drivers in modifying the lunar nearside’s crustal composition and structure. This volatile-driven alteration adds an essential piece to the puzzle of how the Moon’s two hemispheres diverged so markedly in both appearance and internal composition, tying chemical evolution tightly to volcanic history and surface processes.

Beyond advancing lunar petrology, this research underscores the value of halogen geochemistry as a sensitive tracer for volatile histories in planetary bodies. The strong dichotomy in halogen signatures between the nearside and farside rocks suggests that localized volatile cycles and degassing regimes can heavily influence planetary crust formation and preservation, a concept applicable not only to the Moon but also potentially other terrestrial planets and satellites.

The findings lend noteworthy support to the scientific imperative behind recent and future lunar missions targeting the farside of the Moon. Missions such as China’s Chang’e program and NASA’s Artemis initiative owe part of their scientific motivation to the farside’s status as a relatively untouched archive of the Solar System’s early history. Comprehensive sampling and in situ analyses from these regions promise to validate and extend the nuances of lunar volatile evolution now illuminated by halogen geochemistry.

In summation, this research constructs a compelling narrative that clarifies the lunar dichotomy through the lens of volatile element behavior, particularly chlorine, bridging laboratory experimentation with planetary-scale geochemical modeling and actual lunar sample data. It presents the Moon not as a homogeneous relic frozen in time, but as a dynamic world whose surface and interior have been chemically and physically sculpted by processes far more intricate than mere cooling and solidification of a molten sphere.

Future inquiries targeting the interplay between volatile species, high-temperature magmatic processes, and impact-induced vaporization are poised to unravel additional secrets concerning the Moon’s evolution and its stark hemispherical contrasts. These endeavors will continue to enrich our knowledge not only of our nearest celestial neighbor but also of the fundamental planetary formation and differentiation mechanisms operating throughout the cosmos.

Subject of Research: Lunar geochemistry, halogen behavior in lunar minerals, lunar crust evolution, lunar dichotomy

Article Title: Not directly provided

Web References: http://dx.doi.org/10.1038/s41467-025-60849-4

Image Credits: Jiejun Jing

Keywords: Planetary science, Chemistry, Earth sciences