Overview: A New Clue About the Moon’s Largest Crater
For decades, scientists have studied the South Pole-Aitken (SPA) basin, the Moon’s most expansive impact feature. A new study, published in Nature, upends a long-standing assumption about how this colossal crater formed. By analyzing the basin’s asymmetry and the distribution of radioactive materials, researchers argue that a northward incoming asteroid created a down-range ejecta pattern, with the basin narrowing toward the south. This reinterpretation not only reshapes our understanding of lunar history but also points to prime science targets for future missions, including NASA’s Artemis program.
What the New Model Proposes
Traditionally, the SPA basin was viewed as a consequence of a massive impact striking the Moon from the south. The latest research suggests otherwise: the basin’s shape leans toward the south, indicating the impact came from the north. Dr. Jeffrey Andrews-Hanna of the University of Arizona explains that the “down-range” end of the basin should bear a thick layer of material excavated from deep within the Moon, while the up-range end would lack such a deposit. In short, the asymmetry is a diagnostic clue about the direction of the original impact and the interior makeup of the Moon at the time of formation.
KREEP and the Lunar Magma Ocean
A central theme of the paper is the story of KREEP—potassium (K), rare earth elements (REE), and phosphorus (P)—leftover materials that resisted full incorporation into the solid mantle and crust as the lunar magma ocean crystallized. This last, enshrined liquid pockets around the Moon’s interior became enriched in heat-producing elements and, crucially, biased toward the near side of the Moon. The researchers compare this to leaving a soda can in the freezer: as freezing progresses, the last remaining liquid concentrates the heavier, residual materials. The result was a KREEP-rich mantle beneath the near side that energized prolonged volcanism and the dark volcanic plains visible from Earth.
Over millions of years, as the magma ocean solidified, the Moon’s crust thickened unevenly—far-side crust grew thicker than the near side. The study suggests that continued squeezing of the deep magma ocean toward the near side left behind a patchy, thinner crust on the far side, with remnants of molten material trapped below. This dynamic set the stage for the SPA impact: when a large body collided with the Moon, the ejecta and melted rock would reflect this internal distribution, producing the distinctive ejecta patterns now observed around the basin’s western and eastern margins.
Evidence of Asymmetry in Ejecta and Radioactivity
The researchers found a striking asymmetry around SPA: the western ejecta blanket is rich in radioactive thorium, while the eastern flank shows comparatively less. This distribution aligns with a model in which the last portions of the magma ocean at depth—rich in KREEP and heat-producing elements—concentrated on the near side. The impact would have broken through this boundary, leaving a “window” through the Moon’s crust at the boundary region. The combination of directional ejecta, crustal thickness differences, and the near-side concentration of radioactive materials supports a northward impact and a magma-ocean legacy that still echoes in the basin’s geology today.
Why This Matters for Artemis and Lunar Science
The study has practical implications for upcoming missions. The team notes that the down-range rim of SPA—the side toward which ejecta piled up—offers among the best sites to study the Moon’s oldest, largest impact basin. Artemis missions planning to explore this region could access material excavated from deep within the lunar interior, providing a rare window into the Moon’s formative years and the end stages of the magma ocean. By combining crater geology with the distribution of KREEP and radioactive elements, scientists can refine models of the Moon’s thermal and tectonic evolution, improving our broader understanding of terrestrial bodies in the inner Solar System.
What to Look For Next
Future lunar missions and sample-return efforts will be able to test the northward-impact hypothesis and the proposed distribution of KREEP and thorium. Analyzing rock samples from the basin’s rim and surrounding terrain will help confirm whether the ejecta layering matches the predicted excavation of material from the Moon’s interior. The integration of remote sensing data, in situ measurements, and precise dating promises to shed more light on the magma ocean’s final chapters and the Moon’s asymmetric growth.
Conclusion
The new interpretation of the South Pole-Aitken basin adds a compelling chapter to the Moon’s early history. By linking the basin’s distinctive shape to a northward impact and the last vestiges of a magma ocean concentrated on the near side, scientists are weaving a narrative that connects deep interior processes with surface features. As humanity returns to the Moon, the down-range rim of SPA may become one of the most informative laboratories for understanding planetary formation and evolution.
Reference: J.C. Andrews-Hanna et al. 2025. Southward impact excavated magma ocean at the lunar South Pole-Aitken basin. Nature 646, 297-302; doi: 10.1038/s41586-025-09582-y
