In a groundbreaking study published in Nature, researchers have unveiled new insights into the Moon’s South Pole–Aitken (SPA) basin, revealing that its formation excavated a remnant of the ancient lunar magma ocean. This finding offers compelling evidence about the early evolution of the Moon’s crust and mantle and provides critical clues about the asymmetric distribution of lunar materials. By harnessing state-of-the-art gravitational and compositional data, the team reconstructed the basin’s shape and composition, shedding light on the monumental processes that shaped our closest celestial neighbor.
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
