Revisiting the Moon’s oldest scar
A new Nature study led by planetary scientist Jeffrey Andrews-Hanna from the University of Arizona revisits a landmark event in our solar system’s history: the giant impact that created the Moon’s largest basin, the South Pole–Aitken (SPA) basin. This colossal feature stretches about 1,931 kilometres from north to south and 1,600 kilometres east to west, making it the biggest and oldest known impact site in the solar system. The research team argues that the impact origin wasn’t from the south as previously thought but from the Moon’s northern region, reshaping how scientists understand the Moon’s early geology.
How the northern crash shaped lunar geology
The team’s analysis of the SPA basin’s teardrop shape—the narrowing toward the south—points to a northern approach by the asteroid. This orientation implies that the impact did more than excavate a massive hole; it actively sculpted the Moon’s crust in an uneven, long-lasting way. As the blast carved through the crust, molten material from deeper layers surged and redistributed, helping to create the Moon’s characteristic hemispheric differences that we observe today.
KREEP: the radioactive signature of a formative collision
One of the most intriguing outcomes of the SPA impact is the exposure and concentration of KREEP materials—potassium, rare earth elements, and phosphorus. These heat-producing, radioactive elements were once suspended in the Moon’s molten early layers. The northern strike exposed these KREEP-rich zones, allowing them to concentrate on one side of the basin. The aftermath created a thermal boost that persisted for millions of years, contributing to localized volcanic activity and helping to forge the dark lava plains that we can spot on the Moon with a telescope.
Why the Moon’s uneven crust matters for future missions
NASA’s Artemis program aims to return humans to the lunar south pole, near the SPA basin rim. The new interpretation of a northern origin for the SPA impact suggests that the area around the south pole could still host a rich archive of debris and rocks from the ancient collision. These materials could hold vital clues about the Moon’s early heat engine and how radioactive elements moved through the Moon after the impact. By studying samples from this region, scientists hope to confirm models of interior differentiation and material migration that shaped both the Moon and Earth early in the solar system’s history.
Implications for lunar science and exploration
The research provides fresh context for why the Moon’s far and near sides look so different: a giant nearly two-billion-year-old crash may have set in motion the divergent crustal architecture and volcanic activity that created the familiar lunar landscapes. If the northern origin of the SPA basin is confirmed, it could redefine timelines for lunar cooling, crust formation, and mantle mixing. The study thus helps guide the next steps in lunar exploration, inviting missions to sample and analyze rocks that record the Moon’s fiery past and its long-lasting radioactive wink from that ancient dawn.
Looking ahead
As NASA and international space agencies plan to extend human and robotic presence on the Moon, the SPA basin remains a treasure trove of scientific questions. The possibility that a northern asteroid strike seeded the Moon’s radioactive reservoirs invites a more nuanced view of lunar history and informs mission planning, target selection, and scientific priorities for returning to Earth’s natural satellite.
