A new study reveals that a massive asteroid struck the moon from the north about 4.3 billion years ago, forming the vast South Pole-Aitken basin and exposing deep interior materials. This glancing blow explains the basin's uneven terrain and the concentration of key elements on the moon's near side. The findings could aid NASA's Artemis missions in uncovering lunar history.
Around 4.3 billion years ago, during the solar system's early days, a colossal asteroid impacted the moon's far side from the north, carving out the South Pole-Aitken (SPA) basin. This immense crater spans roughly 1,200 miles north to south and 1,000 miles east to west, with an elongated oval shape indicating a glancing strike rather than a direct hit.
Led by Jeffrey Andrews-Hanna, a planetary scientist at the University of Arizona, the research published on October 8 in Nature analyzed the basin's topography, crustal thickness, and surface chemistry. The team found that the SPA tapers toward the south, suggesting the impactor traveled northward to southward. The southern rim, or down-range end, is buried under thick layers of ejecta from the moon's deep interior, while the northern up-range end has less debris.
"This means that the Artemis missions will be landing on the down-range rim of the basin -- the best place to study the largest and oldest impact basin on the moon, where most of the ejecta, material from deep within the moon's interior, should be piled up," Andrews-Hanna said.
The impact also sheds light on the moon's early evolution from a global magma ocean. As it cooled, denser minerals formed the mantle, while lighter ones created the crust. KREEP elements—potassium, rare earth elements, and phosphorus—concentrated in the last molten remnants, akin to syrup in a freezing soda. The far side's thicker crust squeezed this material toward the near side, fueling volcanism that formed the familiar 'man in the moon' features.
An asymmetry in the ejecta blanket supports this: the western side is rich in radioactive thorium, but the eastern flank is not, indicating the impact pierced a boundary between KREEP-enriched and regular crust. "Our theory is that as the crust thickened on the far side, the magma ocean below was squeezed out to the sides, like toothpaste being squeezed out of a tube, until most of it ended up on the near side," Andrews-Hanna explained.
Artemis astronauts may soon collect samples from this region, offering detailed insights into the moon's composition beyond remote sensing data.