Introduction: A New Look at an Ancient Moon
As NASA’s Artemis program prepares to touch down near the Moon’s south pole, a bold question guides researchers: could the South Pole-Aitken (SPA) basin hold the deepest clues about the Moon’s origin? A team led by Jeffrey Andrews-Hanna at the University of Arizona argues that SPA sits on the “downrange” rim of the Moon’s largest impact crater, implying that this region may be covered with rocks blasted from deep beneath the lunar surface. If true, Artemis astronauts could collect samples that crystallize how the Moon formed and evolved in its earliest chapters.
What is SPA, and why does its shape matter?
SPA is the Moon’s largest impact structure, a vast, oblong basin more than 1,200 miles by 1,000 miles. Formed roughly 4.3 billion years ago on the far side of the Moon, its elongated shape has long hinted at a glancing, oblique collision rather than a head-on strike. A new comparison with other giant basins across the solar system reveals a consistent pattern: these basins tend to narrow in the downrange direction, creating a teardrop-like silhouette. Applying this insight to SPA suggests the impactor came from the north and moved southward, a reversal of prior assumptions about impact direction.
Why the downrange rim matters for samples
In large, oblique impacts, the downrange end of a crater tends to collect thick ejecta—material hurled from deep within the Moon. If SPA’s downrange rim indeed marks the deepest interior rocks, then the southern lunar landscape around Artemis landing sites could be piled with rocks sourced from the Moon’s interior layers. This sets up a rare opportunity: astronauts could retrieve real samples that carry the last whispers of the Moon’s primordial magma ocean and the early crust formation.
Evidence beyond the shape: geology and chemistry
The study goes beyond shape by examining topography, crustal thickness, and surface chemistry. A striking asymmetry in thorium abundance surrounds SPA: higher on the western flank and lower on the eastern side. Thorium is a tracer linked to late-stage magma-ocean materials and KREEP elements (potassium, rare earth elements, and phosphorus) that remained dissolved as the Moon cooled. This pattern hints at a boundary between the Moon’s remaining magma ocean and the more conventional crust elsewhere. The researchers propose that a lopsided cooling history caused a migration of KREEP-rich liquids toward the near side, while the far side retained thicker crust. The SPA impact may have punctured through a patchy remnant of that late-stage layer, scattering thorium-rich ejecta predominantly to one side of the basin, aligning with the observed chemistry.
Why Artemis samples could solve long-standing mysteries
Remote sensing can map thorium and other crustal properties from orbit, but ground-truth samples are essential to confirm hypotheses. If Artemis collects rocks from SPA’s downrange rim, scientists can test whether these materials truly come from deep within the Moon and whether they preserve the late-stage chemistry of the lunar magma ocean. Such findings could constrain debates over when the Moon’s crust formed, why its near side hosts vast volcanic plains, and why the far side remains thicker and more cratered. More broadly, confirming the universal rule that teardrop-shaped basins record oblique impacts could illuminate how giant collisions shape rocky worlds beyond Earth.
Broader implications for planetary science
Artemis could do more than add new pages to lunar lore. By corroborating the proposed impact direction and interior sampling, scientists would sharpen models of early crust formation, magma-ocean refreezing, and the distribution of heat-producing elements. The revelation would ripple outward to how we interpret cratered landscapes on Mars, Mercury, and other planetary bodies. The Moon would remain a Rosetta Stone for understanding planet formation and the violent processes that forge rocky planets.
A promising future for lunar exploration
In practical terms, SPA’s south-pole region becomes a scientifically fertile ground for Artemis. The mission could deliver the deepest interior rocks yet studied in situ, enabling a new era of lunar geology. As Andrews-Hanna notes, SPA is the largest and oldest basin on the Moon, with its downrange rim concealing the deepest interior rocks—exactly the kind of material that could unlock the Moon’s earliest story.
Conclusion
The idea that a glancing impact helped sculpt the Moon’s face challenges long-held assumptions and opens actionable avenues for sample-based science. If Artemis confirms the SPA-downrange thesis, we may finally read the missing pages of our natural satellite’s origin story, stitched together from the rocks that formed in the furnace of the early solar system.
