Unveiling the Moon’s Largest Crater
The South Pole-Aitken (SPA) basin is not just the Moon’s largest impact feature by size; its shape and composition may hold the key to rewriting lunar history. A recent study led by Jeffrey Andrews-Hanna at the University of Arizona argues that the SPA region sits on the downrange rim of the basin. In plain terms, this means the area near the Moon’s south pole could be piled high with rocks ejected from deep beneath the lunar surface when the SPA formed, roughly 4.3 billion years ago.
SPA stretches more than 1,200 miles in length and about 1,000 miles across, a vast basin carved into the Moon’s far side after a colossal debris-slinging event. The team’s analysis suggests the impactor struck from the north and plowed southward, flipping previous assumptions about the direction of the ancient collision. This has profound implications for what scientists expect to find at Artemis landing sites.
A New Shape, A New Story
Giant craters across the solar system often display a teardrop or avocado silhouette due to oblique impacts. By comparing SPA’s outline with other basins, the researchers found a consistent pattern: craters tend to narrow in the downrange direction. Applying this to SPA implies the downrange end, at the southern rim, would be rich in deep-origin ejecta. If true, the south pole could be a natural archive of the Moon’s deepest interior rocks.
Beyond Shape: Crust, Chemistry, and a Glowing Clue
The study does not rely on topography alone. Its conclusions are reinforced by analyses of crustal thickness and surface chemistry. A striking asymmetry in thorium abundance around SPA—more on the western flank, less on the eastern side—serves as a geological clue. Thorium is a trace marker linked to the Moon’s primordial magma ocean and its long-lived heat-producing elements. This uneven distribution suggests the crustal boundary between near-side volcanism and far-side crust thickening may have frozen heretofore hidden dynamics of the magma ocean.
According to the authors, the Moon’s near side shows thicker, more KREEP-rich (potassium, rare earth elements, phosphorus) material that fueled volcanic plains, while the far side hosts a thicker crust with fewer of these late-stage leftovers. The proposed explanation is a crustal asymmetry that squeezed the remaining magma ocean toward the near side as the Moon cooled. The SPA impact would then have punched into a patchier remnant of that last-dregs layer, scattering thorium-rich ejecta predominantly on one side of the basin.
Artemis Samples: A Test for Deep Interior Origins
A key implication of this work is the potential value of Artemis samples from the SPA downrange rim. If astronauts return rocks that truly originate from deep within the Moon, scientists can test whether these materials preserve the late-stage chemistry of the lunar magma ocean. Such data would constrain when the Moon’s crust solidified and why the near/far-side dichotomy developed so early in planetary history. It could also sharpen our understanding of how giant, oblique impacts sculpt rocky worlds beyond Earth.
Broader Impacts: Rules for Cratered Worlds
The study’s suggested teardrop geometry may reflect universal rules for large, oblique impacts. Confirming these patterns on the Moon—and then applying them to Mars, Mercury, and other cratered bodies—could transform crater analysis across the solar system. Orbiters map thorium and gravity, but only in-situ rock analysis can validate the story the data tell. Artemis has the potential to turn a paradigm into a tangible, testable record.
The Road Ahead
As science moves from remote sensing to tangible samples, the Moon’s south-pole region becomes a focal point for discovering the earliest chapters of our solar system’s history. If the SPA downrange rocks confirm the new direction of the impact, Artemis may not merely plant a flag; it may unlock the missing pages of a story written in the Moon’s deepest interior.
