Introduction: A Giant Crater That Holds Inside Secrets
The Moon’s far side hosts the Solar System’s largest known impact basin: the South Pole-Aitken (SPA) basin. Spanning roughly 1,930 kilometers north-south and about 1,600 kilometers east-west, this ancient scar on the Moon’s crust formed around 4.3 billion years ago when a colossal asteroid struck the young Moon. Recent research from the University of Arizona is breathing new life into this geologic mystery, with implications for our understanding of the Moon’s formation, interior structure, and the future of human exploration.
New Insight: The Basin Isn’t Oriented the Way We Thought
Traditionally, scientists assumed basins like SPA were shaped by impacts that carved from the south toward the north. A meticulous re-analysis by Jeffrey Andrews-Hanna and colleagues, however, shows the basin actually narrows toward the south. In other words, the impact came from the north, not the south. This subtle adjustment is more than a cartographic curiosity; it reshapes expectations about how ejecta—the material blasted from the Moon’s interior—was distributed during the impact.
Impact basins typically deliver a heavy ejecta blanket toward the downrange side. If the SPA basin narrows toward the south, the deeper interior materials would have been excavated and deposited predominantly on the northern portion of the basin. That reversal has direct implications for where Artemis can sample Moon interior rocks at the surface.
What This Means for Artemis: A Natural Core Sample
Artemis missions plan to land near the southern rim of the SPA basin. With the new orientation model, scientists are confident the landing site aligns with a zone that exposes material from the Moon’s deep interior. Rather than drilling, astronauts could collect near-surface samples that serve as a geological core, revealing layers from the crust and mantle mingled during the colossal impact.
The significance lies in the potential to peer into the Moon’s early evolution. The impact and the ensuing circulation of molten rock left a stratified crust, a record of the Moon’s prehistoric magma ocean. The SPA rocks could carry clues about the depth distribution of minerals and how heat and volcanism shaped the near- and far-side crusts.
KREEP, Crustal Asymmetry, and What the Far Side Tells Us
One long-standing puzzle is KREEP—potassium, rare earth elements, and phosphorus—residuals from the Moon’s cooling magma ocean. These elements heated the crust, driving volcanic activity that formed the Moon’s dark basaltic plains (the so-called “faces” we see from Earth). Yet KREEP is conspicuously concentrated on the near side, while the far side remains heavily cratered and volcanically quieter.
The new interpretation of SPA’s formation points to a thick far-side crust that squeezed residual magma toward the thinner near side. The western flank of the SPA shows higher thorium concentrations, a hallmark of KREEP-rich material, while the eastern side does not. This asymmetry supports a model in which the Moon’s crust became uneven during its early evolution, with the SPA basin acting as a window into a transitional zone between near- and far-side crusts.
Why the Discovery Matters for Lunar Science
This research offers a cohesive narrative for how the Moon evolved into its current dual personality: a volcanically active near side with glowing basalts and a far side that bears a long history of impacts. If the samples Artemis returns confirm that the SPA’s southern rim touches deeper crustal material, scientists will gain unprecedented access to the Moon’s early magma ocean history without deep drilling.
In short, the basin acts as a natural laboratory. If we can decode the distribution of heat-producing elements and the timing of crystallization in the early Moon, we improve our broader understanding of planetary formation, differentiation, and the collisional processes that shape rocky worlds.
Looking Ahead: Samples, The Moon, and Our Place in the Solar System
As the Artemis program advances, the SPA findings offer a guiding principle for where to collect samples that best illuminate the Moon’s deep interior. The rocks, if they verify the proposed crustal asymmetry and interior composition, could refine models of how the Moon separated into crust and mantle and how solar system bodies cool and differentiate after giant impacts.
The study, published in Nature, underscores how incremental shifts in data interpretation—like the true downrange direction of a giant impact—can unlock big questions about planetary origins and future human exploration.
Bottom Line: A Giant Crater with a Compact Implication
The South Pole-Aitken basin isn’t just a spectacular lunar feature. It is a strategic portal into the Moon’s earliest chapters. With Artemis aiming at the exact zones that harbor deep crustal and mantle material, scientists anticipate a treasure trove of knowledge that could redefine our understanding of the Moon and the history of the inner Solar System.
