The SPA basin, one of the largest and oldest impact basins in the solar system, has challenged scientists for decades due to its complex morphology and irregular outline. Unlike typical basins with clear topographic rims, the SPA’s boundary is heavily modified by subsequent impacts, volcanic activity, and tectonic processes. To overcome these challenges, the researchers innovatively utilized gravity gradients derived from Bouguer gravity anomalies, which reflect subsurface density variations corrected for topographic influences, to delineate an accurate basin outline. This approach allowed them to circumvent the uncertainties introduced by surface alterations and confidently identify the basin’s true dimensions and asymmetric tapering.

Fitting the SPA basin’s outline with a tapered elliptical model incorporated a sophisticated Markov chain Monte Carlo algorithm, which iteratively refined basin parameters such as major and minor axis lengths and directional tapering. This advanced statistical modeling accounted for azimuthal asymmetries and positional offsets, facilitating a nuanced understanding of the SPA basin’s shape. Interestingly, the tapering factor revealed a distinct northward elongation, consistent with an oblique impact hypothesis, implying that the impactor struck with a trajectory favoring one hemisphere. Such geometric characterization of the basin provides deeper insight into the impact dynamics that formed this colossal lunar structure.

The examination of elemental surface composition using Lunar Prospector Gamma Ray Spectrometer data further reinforced the hypothesis of excavated magma ocean remnants. Elevated thorium (Th) concentrations within the western ejecta blankets of the SPA basin contrasted with surrounding far-side highlands, suggesting exposure of deeper, KREEP-enriched late-stage magma ocean liquids. These anomalies were mapped in greater detail by smoothing titanium (Ti) data through spherical harmonic filtering, revealing broader compositional variations consistent with subsurface excavations. Notably, localized Th-rich spots, unrelated to basin processes, were carefully excluded to focus on the signature directly attributable to the SPA impact.

The Moon’s crustal asymmetry, much thicker on the far side than the near side, has long puzzled scientists. By assuming vertical hydrostatic equilibrium between the crust and mantle, the study models the lateral distribution of residual magma ocean thickness tied to crustal thinning beneath the SPA basin. This model elegantly explains why late-stage magma ocean liquids were concentrated beneath the thinner crust of the basin’s southwesterly region. It also reconciles these findings with the well-known Procellarum KREEP Terrane (PKT) on the near side, where the magma ocean remnants were likewise trapped but concentrated differently due to the contrasting crustal thickness distribution.

To build this model, the researchers reconstructed a pre-impact crustal thickness map by interpolating and smoothing across existing basins to remove ejecta-related heterogeneities. The underlying assumption posits that the basin’s crustal thinners in the southwest and thickens toward the northeast, matching observed Bouguer gravity anomalies. Iterative adjustments accounted for the influence of cumulative geological processes, ensuring that the model represents the Moon’s early crust prior to the SPA impact. This meticulous approach provides a credible baseline for understanding magma ocean distributions before such a titanic impact altered the lunar surface.

One of the study’s most compelling aspects lies in estimating thorium concentration within the excavated magma ocean reservoir. By analyzing FeO excesses—which influence reflectance and spectral signatures—alongside thorium abundances in the ejecta, the team inferred that about 9% of the western ejecta constitutes late magma ocean liquids mixed with crustal material. This mixing corresponds to a roughly 4-kilometer layer excavated beneath the crust, with optimal thorium concentrations estimated around 9 ppm. This is notably lower than the near-side PKT concentrations but still provides a critical window into the residual magma ocean’s composition, suggesting substantial heterogeneity in lunar mantle reservoirs.

The temporal aspect of magma ocean crystallization and basin formation is also addressed. Thorium partitioning behavior during crystallization, influenced by trapped melt fractions and diffusion-limited percolation, shapes elemental distributions through time. The study models Th concentration as a function of progressive crystallization stages, considering trapped melt percentages consistent with existing petrological constraints. The onset and completion of the KREEP reservoir, marked between 4.34 and 4.37 billion years ago, anchors the timing of the SPA basin-forming impact. Intriguingly, zircon geochronology hints that residual magma oceans persisted beneath the near side as late as 3.9 billion years ago, though this extended lifespan does not conflict with the SPA basin findings.

The utilization of gravity gradients as a diagnostic for basin rim delineation represents a methodological leap in planetary geophysics. Unlike traditional thresholding methods sensitive to topographic noise and local anomalies, the calculation of maximum eigenvalue gravity gradients directly captures concurring arcuate discontinuities circumferential to the basin. This enables tracing of the basin outline with unprecedented accuracy despite the structural complexities of the SPA basin’s western and northeastern quadrants. Such precision paves the way for refined models of other heavily modified planetary basins.

In addition to geophysical delineation, the study incorporated a refined statistical framework to handle irregular basin outlines by fitting tapered ellipse models, allowing for the quantification of asymmetric elongation and “tapering” factors. This quantification is crucial to understanding the mechanics of oblique impacts and subsequent basin collapse evolution. Moreover, the best-fit parameters emerged from comprehensive posterior distributions produced during thousands of Monte Carlo iterations, ensuring robust confidence intervals. This statistical rigor enables confident interpretation of basin formation dynamics that influence lunar geology.

The insights garnered extend beyond the SPA basin, touching on fundamental questions about the Moon’s asymmetric crustal evolution and the spatial distribution of incompatible elements. By correlating elevated thorium with localized residual magma ocean thickness, the study links geochemistry and geophysics in a compelling narrative of early lunar differentiation. The exclusion of external geochemical anomalies further strengthens the localized association, setting a new standard for integrated planetary analyses.

Finally, the study’s broader implications resonate within planetary science, as it highlights the intricate interplay of impact cratering, crustal asymmetry, and mantle differentiation. Understanding how the SPA basin’s formation excavated and exposed magma ocean remnants not only teaches us about the Moon’s geological past but also informs models of crust formation and impact processes on terrestrial planets more broadly. The amalgamation of high-resolution gravity data, elemental mapping, and rigorous statistical modeling sets a precedent for future lunar and planetary exploration, potentially guiding mission planning and sample return strategies.

Subject of Research: Lunar geology and geophysics, specifically the South Pole–Aitken impact basin and residual magma ocean.

Article Title: Southward impact excavated magma ocean at the lunar South Pole–Aitken basin.

Article References:
Andrews-Hanna, J.C., Bottke, W.F., Broquet, A. et al. Southward impact excavated magma ocean at the lunar South Pole–Aitken basin. Nature 646, 297–302 (2025). https://doi.org/10.1038/s41586-025-09582-y

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41586-025-09582-y

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