Spanning more than 1,200 miles north to south and 1,000 miles east to west, the oblong basin formed from a glancing blow rather than a head-on strike. Comparing SPA with other giant craters, the team shows these basins narrow in the down-range direction. For SPA, that tapering points south, indicating the projectile came from the north and piled most ejecta on the down-range rim.
"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," said Andrews-Hanna. Additional evidence from topography, crustal thickness and surface composition supports the southward-directed excavation.
The work connects SPA's asymmetry to the Moon's early magma ocean. As that global melt cooled, leftover liquids concentrated potassium, rare earth elements and phosphorus known as KREEP. "If you've ever left a can of soda in the freezer, you may have noticed that as the water becomes solid, the high fructose corn syrup resists freezing until the very end and instead becomes concentrated in the last bits of liquid," he said. "We think something similar happened on the moon with KREEP."
Because the lunar far-side crust thickened earlier, magma was squeezed toward the near side, helping explain why KREEP and heat-producing elements accumulated there and fueled extensive volcanism. Around SPA, the team finds a striking imbalance: thorium-rich ejecta blankets the western flank but not the east, consistent with the impact slicing into a boundary where patchy, KREEP-enriched magma ocean residue once lingered beneath parts of the far side.
Many questions about the Moon's infancy remain, but Artemis samples from the SPA rim could be decisive. "Those samples will be analyzed by scientists around the world, including here at the University of Arizona, where we have state-of-the-art facilities that are specially designed for those types of analyses," he said. "With Artemis, we'll have samples to study here on Earth, and we will know exactly what they are," he added, noting the potential to illuminate the Moon's earliest evolution.
Research Report:Southward impact excavated magma ocean at the lunar South Pole-Aitken basin
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