Revisiting the Moon’s Biggest Crater
The Moon’s South Pole-Aitken basin (SPA), the largest known impact feature in our solar system, has long been a subject of fascination for planetary scientists. Recent research digging into the basin’s shape and composition suggests a surprising twist: the basin’s oblong form and the distribution of interior materials point to a northward impact, not a southern one. This finding offers fresh clues about the Moon’s early interior and how its crust and mantle evolved as the magma ocean cooled.
New Clues from an Ancient Smash
Roughly 4.3 billion years ago, a colossal asteroid struck the Moon’s far side, creating a basin that stretches more than 1,200 miles north to south and about 1,000 miles east to west. The elongated shape, researchers say, reflects a glancing blow. The latest analysis shows the down-range end of the basin—traditionally thought to be the direction of the incoming impact—should be buried under a thick layer of material excavated from deep within the Moon. Conversely, the up-range end would lack this heavy ejecta buildup.
Why the Direction Matters
“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,” explained Dr. Jeffrey Andrews-Hanna of the University of Arizona. “This means that the 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, where most of the ejecta, material from deep within the Moon’s interior, should be piled up.”
KREEP and the Moon’s Early Heat
A long-standing idea in lunar science is that a magma ocean once covered the Moon as it cooled after formation. During crystallization, heavier minerals sank while lighter ones rose, forming the crust and mantle. Yet some elements—potassium, rare earth elements, and phosphorus—remained sequestered in the last pockets of liquid, a mix scientists call KREEP. The new study finds KREEP material is unusually abundant on the Moon’s near side, a pattern that helps explain why the near side became so volcanically active and darker in appearance.
As the magma ocean cooled over millions of years, a tiny amount of liquid lingered between the mantle and crust. This residual, KREEP-rich melt would heat up the surrounding rocks, fueling volcanism that created the familiar dark plains visible from Earth. But the mystery has always been: why is KREEP concentrated on the near side, and how did that distribution evolve?
Two Views Unified by a Crustal Asymmetry
The Moon’s crust is notably thicker on the far side than on the near side, a disparity that has puzzled scientists. The new interpretation posits that as the crust thickened on the far side, the underlying magma ocean was squeezed outward, like toothpaste from a tube, concentrating most of the residual magma on the near side. This asymmetry would later influence the Moon’s thermal and volcanic history and the way the early crust interacted with the interior.
Evidence Inside the SPA Basin
The study highlights a striking asymmetry around SPA: the western ejecta blanket is rich in radioactive thorium, while the eastern flank is comparatively deficient. This distribution aligns with a boundary where crust overlies the last remnants of the KREEP-enriched magma ocean. “Our study shows that the distribution and composition of these materials match the predictions that we get by modeling the latest stages of the evolution of the magma ocean,” said Dr. Andrews-Hanna. The inference is that the far side once hosted a patchy layer of magma ocean beneath parts of the crust, leaving a detectable signature in the basin’s ejecta.
Implications for the Moon and Artemis
Beyond satisfying scientific curiosity, the findings have practical implications for future lunar exploration. With Artemis targets pointing toward the SPA region, understanding the ejecta distribution and crustal history helps scientists interpret samples and plan precise landing sites. The basin’s down-range rim stands out as a laboratory where researchers can sample material from deep within the Moon’s interior and test theories about the Moon’s early magma ocean and thermal evolution.
In Print and in Space
The study appears in Nature (2025) by J.C. Andrews-Hanna and colleagues, outlining a cohesive narrative that links basin morphology to the Moon’s ancient interior dynamics. As researchers decode the Moon’s earliest chapters, missions like Artemis may soon follow suit, bringing back samples and on-site observations that could confirm or refine this bold new picture of lunar history.
