Hidden History beneath the Moon’s Largest Crater
Scientists have long studied the South Pole-Aitken basin (SPA) as a window into the Moon’s formative years. New research published in Nature suggests the basin’s distinctive shape and composition tell a story of a magma ocean at the Moon’s endgame and of an impact that came from the north rather than the south. This discovery reshapes our understanding of how the Moon cooled, how its crust and mantle settled, and where future Artemis missions should land to study deep lunar material.
Rethinking the Basin’s Shape and Origin
The SPA is the Moon’s most expansive crater, spanning roughly 1,200 miles from north to south and about 1,000 miles east to west. Its elongated form had been interpreted as the result of a glancing collision. The latest analysis, however, shows the basin narrows toward the south, implying the impactor approached from the north. Dr. Jeffrey Andrews-Hanna of the University of Arizona explains, “The down-range end of the basin should be covered by a thick layer of material excavated from the lunar interior by the impact, while the up-range end should not.”
Why This Matters for Future Missions
This new perspective is timely for NASA’s Artemis program. The down-range rim, where most ejecta accumulated, becomes a key site for studying the Moon’s oldest impact and the hidden interior. “Artemis missions will be landing on the down-range rim of the basin — the best place to study the largest and oldest impact basin on the Moon,” the study notes. The evidence suggests the most informative samples and geologic records lie in this region.
From Magma Ocean to KREEP: Tracing the Lunar Heat Engine
Beyond the crater, the study revisits a classic idea: the Moon once hosted a global magma ocean that gradually solidified into crust and mantle. As crystallization proceeded, heavy minerals sank while lighter ones floated, but some elements—potassium, rare earth elements, and phosphorus—remained concentrated in the last pockets of liquid. This trio is collectively known as KREEP.
Andrews-Hanna and colleagues propose a refined sequence: during cooling, the last remnants of the magma ocean—the KREEP-rich liquids—became concentrated on the Moon’s near side. This left a pronounced hemispheric asymmetry, with the far side crust thicker and the near side (facing Earth) slightly thinner. The result? An interior heat source and volcanism that favored near-side regions, forming the familiar lunar maria and dark plains visible from Earth.
The Puzzle of Asymmetry and the Basin’s Ejecta
A striking finding from SPA’s study is the distinct distribution of radioactive thorium: the western ejecta are thorium-rich, while the eastern flank shows a contrasting pattern. Such an asymmetry supports a model in which the remaining magma ocean materials and heat-producing elements concentrated toward the near side while the far side preserved a patchier, thinner mantle. The excavation at SPA may thus reveal a boundary where crust meets the last throes of the magma ocean, acting as a window into the side-by-side evolution of lunar layers.
Putting It All Together
The researchers argue that the Moon’s crustal thickness, the flow of magma in its final days, and the mechanics of the SPA-forming impact all align with a north-to-south story of impact and exhumed interior. The last patches of magma ocean beneath the far side created a unique asymmetry in ejecta and thorium distribution, offering a coherent explanation for both the basin’s shape and the global distribution of KREEP-derived materials.
The full findings, including the proposed mechanism for the crustal and mantle asymmetry and its implications for Artemis landings, appear in Nature: J.C. Andrews-Hanna et al., 2025. Southward impact excavated magma ocean at the lunar South Pole-Aitken basin.
