A fresh perspective on the Moon’s biggest crater
For decades, the South Pole-Aitken (SPA) basin has stood as a cornerstone of lunar geology. Spanning more than 1,200 miles across, it is the Moon’s largest known impact feature and a key target for NASA’s Artemis program. A new paper published in Nature challenges a long-held assumption: that SPA formed from a direct, head-on collision. Instead, researchers argue that a glancing, southward blow carved the basin, leaving a distinctive teardrop shape that reshaped our understanding of the Moon’s early days.
How the teardrop shape changes the story
The team compared SPA’s geometry with other giant basins across the solar system. The elongated, asymmetrical form points to an oblique impact—striking from the south and gouging through the Moon’s crust. This is more than a curious detail. It implies core differences in how material from deep inside the Moon was exposed and redistributed during the planet’s formative years.
Revealing deeper materials
When a projectile strikes at an angle, it can excavate deeper crustal layers and reveal heavier minerals near the impact site. The SPA basin’s off-center shape thus supports a scenario in which deeper, mantle-like materials emerged at the rim, potentially altering our map of the Moon’s interior structure. This has implications for how scientists interpret past volcanism, crust formation, and the distribution of elements inside the Moon.
The KREEP puzzle and lunar asymmetry
A longstanding mystery in lunar science is the stark contrast between the Moon’s near side and far side. The near side is peppered with volcanic maria and high concentrations of radioactive elements, while the far side is more rugged and pale. The latest work refines a part of that story by tying the SPA’s formation to the Moon’s initial magma ocean and subsequent crustal dynamics.
As the lava ocean cooled, heavier minerals settled toward the interior while lighter materials formed the crust. Potassium, rare earth elements, and phosphorus—collectively known as KREEP—tended to accumulate in the remaining melt. The study suggests that KREEP-rich material ended up concentrated more on the near side, a pattern that may have been amplified by the southward impact that created SPA. The result is a compelling narrative: a thickened far-side crust with a magma ocean that shrank and pooled toward the near side, leaving a detective trail of radioactive elements across the SPA basin.
Why this matters for Artemis and lunar exploration
Artemis missions plan to land on the down-range rim of the SPA basin—precisely where ejecta from the basin would have accumulated and where clues about the Moon’s interior are likely to be preserved. The new interpretation increases the potential scientific yield of those landings. By returning samples that capture material from the Moon’s deep interior and from the last stages of the magma ocean, astronauts and researchers alike could gain unprecedented insight into how the Moon evolved and why its sides differ so markedly today.
Looking ahead: what the future may hold
As NASA prepares to reestablish a human presence on the Moon, studies like this remind us that our knowledge remains dynamic. The SPA basin is not just a scar on the Moon’s surface; it is a key to understanding the planet’s early evolution. With Artemis, scientists will have the opportunity to analyze true moon rocks on Earth and to test hypotheses about crust formation, interior structure, and volcanic history under controlled conditions.
Conclusion
The South Pole-Aitken basin’s teardrop symmetry invites a reconsideration of how the Moon filled in its earliest years. This fresh angle does more than solve a crustal mystery—it guides the next phase of human exploration by highlighting where the Moon’s ancient stories are most likely written in rocks and minerals. As researchers continue to test this oblique-impact idea, the Moon remains not just a destination, but a preserved archive of the early solar system.
