New View of the Moon’s Largest Crater
New research published in Nature is prompting scientists to rethink how the Moon’s largest known impact basin, the South Pole-Aitken (SPA) basin, formed. Measuring more than 1,200 miles across, the SPA has long been a focal point for understanding the Moon’s early history. The latest analysis argues that the basin’s distinctive teardrop shape likely arose from a southward, glancing collision rather than a direct, head-on strike. This reinterpretation could shed light on why the Moon’s far side bears more large craters while the near side remains comparatively smooth.
What the Study Found
Led by planetary scientist Jeffrey Andrews-Hanna of the University of Arizona, the team compared the SPA’s geometry with other giant impact basins across the solar system. The oblong, teardrop profile of the SPA suggests a shallow, oblique impact that gouged into the Moon’s crust and exposed heavier minerals beneath. Such a scenario contrasts with previous models that favored a straight-ahead collision as the cradle of the basin.
Implications for lunar interior and crustal evolution
The implications extend beyond the basin’s shape. The Moon’s early history likely involved a magma ocean during formation, with heavier materials sinking to form the mantle and lighter ones rising to form the crust. A key complication in this narrative is KREEP—potassium, rare earth elements, and phosphorus—that concentrated in remaining magma as cooling progressed. The new interpretation helps explain a striking asymmetry: KREEP-rich material is far more evident on the near side than on the far side.
KREEP, the Crust, and the Far Side Mystery
Andrews-Hanna explains an analogy to a freezer-full soda: as water solidifies, sweet syrup resists freezing and concentrates in the last liquid pockets. A similar process is thought to have occurred on the Moon, leading to a concentration of KREEP in the near-side crust. The study suggests the far side crust thickened, while magma from below was squeezed outward, ending up largely on the near side. This would create a boundary where the SPA sits—between the KREEP-rich crust and the more typical crust that dominates the far side.
Why This Matters for Artemis and the Next Moon Visit
The findings arrive at a time when NASA is preparing to return humans to the Moon through the Artemis program. Artemis missions will land near the SPA’s down-range rim, a region that could hold critical clues about the Moon’s evolution and interior structure, thanks to ejecta and deeper material exposed by the basin-forming impact. The new model underscores that samples collected near the SPA could be rich with information, potentially reshaping what we expect to learn from in-situ study and Earth-based analyses.
Looking Ahead: What We Might Learn Next
With Artemis enabling hands-on study of the Moon’s south pole, researchers anticipate a treasure trove of data that could confirm or refine the proposed oblique-impact scenario. The study’s authors are optimistic that future sample analysis will reveal more about the end stages of the lunar magma ocean and how its residual material ended up distributed across the Moon. As Andrews-Hanna notes, the returned samples may reveal more about the Moon’s early evolution than previously imagined.
In sum, the SPA basin—a structure once thought to be the product of a straightforward collision—may narrate a more complex story of the Moon’s infancy. The revised view aligns with broader questions about asymmetry between the Moon’s near and far sides and sharpens the scientific case for examining the South Pole-Aitken region up close during the next era of human exploration.
