New research forces scientists to reconsider what they know about the Moon’s South Pole-Aitken basin (SPA), the location where NASA’s Artemis mission is planning to set down astronauts on the lunar surface in the coming years.
When Artemis lands, scientists may find themselves immersed in clues to the moon’s origins, says a new study published in Nature, led by Jeffery Andrews-Hanna of the University of Arizona. Landing on the heavily cratered far side of the moon would provide researchers with insight into the forces that produced its dichotomy with the relatively smooth near side, where the Apollo program landed decades ago.
The South Pole-Aitken Basin, Artemis Landing Spot
When an asteroid passing through the early solar system impacted the far side of our moon, it left behind the South Pole-Aitken (SPA) basin. The 1,200 by 1,000-mile crater was formed in an oblong shape, stretching longer from north to south than east to west, suggesting a sideswipe instead of a head-on collision.
In Andrew-Hanna’s new study, they discovered that a tear-drop-shaped narrowing, as an asteroid traveled downrange, was common in impact features across the solar system. When they applied this finding to SPA, it conflicted with long-held assumptions about the crater’s formation.
Until now, it had been believed that SPA was formed by an asteroid coming from the south. However, the narrowing evident at the site, when viewed through the lens of the new research, indicates that the asteroid was actually traveling from the north. The reason for this is that the direction of travel would deposit the material excavated by the glancing impact at the downrange end of the basin. This changes scientists’ current understanding of the proposed Artemis landing area and what they may expect to find there.
“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.
Supporting a Revised Impact
To bolster their findings, the team conducted further topographical analysis targeted at the crust thickness and surface composition, work that has implications for our understanding of the Moon’s interior structure and evolution.
Our traditional understanding has been that energy from the Moon’s formation melted the surface into a global magma ocean. When the surface finally cooled and solidified, researchers expected heavy minerals to have sunk into the mantle, leaving the lighter minerals on in the crust. However, there were some odd exceptions to this rule.
A handful of elements remained in the final molten soup that represented the last vestige of the magma ocean. These elements included potassium, rare earth elements, and phosphorus, which are collectively referred to by the acronym KREEP. Andrews-Hanna’s team discovered that KREEP elements were far more common on the smoother, near side of the Moon.
“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.”
The KREEP Layer
Millions of years of cooling eventually turned the magma ocean into solid rock, stratified into a crust and mantle. KREEP-rich material acted as a buffer, a small amount of still liquid magma hidden between the crust and mantle.
“All of the KREEP-rich material and heat-producing elements somehow became concentrated on the moon’s near side, causing it to heat up and leading to intense volcanism that formed the dark volcanic plains that make for the familiar sight of the ‘face’ of the Moon from Earth”, according to Andrews-Hanna.
The reasons for the preponderance of KREEP elements on the near side of the Moon have puzzled scientists. One of the driving factors, according to Andrew-Hanna, may be another element of the Moon’s asymmetry: its thinner near-side crust. The imbalance is believed to have contributed to how the Moon evolved.
“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,” he said.
What Artemis May Find
Supporting the team’s theory is the concentration of radioactive Thorium present in the ejecta blanket’s western side, which is absent in the eastern portion. Such a finding indicates that impact produced a rip in the Moon’s surface at the boundary between the final pools of molten KREEP and the already solidified crust.
“The last dregs of the lunar magma ocean ended up on the near side, where we see the highest concentrations of radioactive elements,” Andrews-Hanna said. “But at some earlier time, a thin and patchy layer of magma ocean would have existed below parts of the far side, explaining the radioactive ejecta on one side of the SPA impact basin.
Sample returns from the Artemis mission will aid researchers like Andrews-Hanna to continue to pursue the mysteries of lunar evolution. Some of the most essential materials to their work, such as Thorium, are simple to spot on the Lunar surface, yet getting a more thorough analysis will require first-hand access to samples.
“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,” Andrews-Hanna said. “Our study shows that these samples may reveal even more about the early evolution of the moon than had been thought.”
The paper, “Southward Impact Excavated Magma Ocean at the Lunar South Pole–Aitken Basin,” appeared in Nature on October 08, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.