New clues from the Moon’s largest crater
A fresh interpretation of the Moon’s oldest giant impact hinges on the South Pole–Aitken (SPA) basin, the largest known impact structure in the solar system. Led by planetary scientist Jeffrey Andrews-Hanna of the University of Arizona, the study argues that the downrange rim of SPA lies near the Moon’s south pole, a region that may be stacked with rocks hurled from deep inside the lunar interior. If correct, Artemis astronauts landing near the south pole could literally be stepping onto a natural archive of the Moon’s deepest secrets.
Rethinking SPA’s shape and direction
SPA stretches more than 1,200 miles (about 2,000 kilometers) long and 1,000 miles wide, carved into the Moon’s far side about 4.3 billion years ago. Its elongated, teardrop-like silhouette has long suggested an oblique, glancing impact. In a broader comparison with other enormous basins across the solar system, the researchers noticed a consistent pattern: giant craters tend to narrow in the downrange direction, creating that avocado-like shape.
Applying this pattern to SPA flips the traditional idea: the impactor would have struck from the north and plowed toward the south, depositing thick ejecta toward the downrange end. That means the south-pole region—where Artemis plans to land—could be where the Moon’s deepest interior rocks accumulate, offering an unprecedented sample of its innermost material.
Supporting evidence beyond the crater’s outline
The team’s case rests on more than geometry. They analyzed topography, crustal thickness, and surface chemistry and found a coherent story: the downrange end would capture more material from the interior. A striking asymmetry in thorium abundance adds weight to the hypothesis. Thorium, associated with late-stage magmatic activity, is more concentrated on the basin’s western flank than its eastern side, hinting at a crustal boundary where the Moon’s primordial magma ocean left its last traces.
These observations dovetail with long-standing puzzles about the Moon’s near side and far side. The near side is thinner, volcanic, and rich in heat-producing elements, while the far side is thicker and more heavily cratered. The study suggests that as the Moon’s magma ocean froze, heavier elements settled to form the mantle while lighter ones formed the crust, but the process left a lopsided distribution of remnants—precisely what SPA’s downrange rocks could reveal.
KREEP, the magma ocean’s last whispers
After the Moon’s global melt, a small fraction of elements—potassium, rare earth elements, and phosphorus, known collectively as KREEP—stayed liquid longer and concentrated at the boundary between mantle and crust. The near side’s abundance of KREEP explains the dark volcanic plains, but why those remnants ended up near the near side remains a question. Andrews-Hanna’s team argues that the crustal thickening on the far side squeezed the remaining magma ocean sideways, pushing KREEP-rich liquids toward the near side. The SPA impact likely punctured a patchy remnant of that layer on the far side, scattering radioactive ejecta across one basin flank while leaving the other relatively clean.
Why Artemis samples could redefine lunar history
Orbital data can map thorium and infer crustal properties, but true understanding requires rocks in hand. If Artemis astronauts collect specimens from SPA’s downrange rim, scientists can test whether these rocks originated deep within the Moon and constrain the late-stage chemistry of the magma ocean. Such samples could illuminate when the Moon’s crust formed, why the near/far-side dichotomy emerged, and how giant impacts sculpt rocky worlds elsewhere in the solar system.
Broader implications for planetary science
Confirming a universal rule for the shape of oblique craters would extend beyond the Moon. The teardrop geometry observed at SPA appears in large basins across the solar system, suggesting common principles governing how massive impacts decorate rocky planets and moons. Ground-truthing these rules on the Moon—by combining orbital data, sample analysis, and field geology—could sharpen interpretations of craters on Mars, Mercury, and beyond.
A mission with the potential to rewrite a chapter of lunar history
In the end, the practical upshot is straightforward: Artemis could land in the perfect place to access the Moon’s oldest, most consequential impact and its richest interior material. The ensuing analyses would neither merely confirm a model nor fill gaps in a textbook; they could rewrite a century-long narrative about the Moon’s origin and early evolution.
Published in Nature, the study invites a new era of lunar exploration where the south-pole region becomes a critical laboratory for understanding how the Moon—and other worlds—formed and evolved after the earliest cataclysms of the solar system.
