About 4.3 billion years ago, when our solar system was still in its infancy, a giant asteroid crashed into the far side of the Moon, exploding a huge crater called the South Pole-Aitken Basin. This impactor is the largest crater on the Moon, stretching more than 1,200 miles from north to south and 1,000 miles from east to west. The oblong shape of the bowl is the result of a glancing blow rather than a frontal impact. Contrary to conventional wisdom that the basin was formed by an asteroid coming from a southern direction, the new study shows that the shape of the basin tapers towards the south, indicating an impact coming from the north.
The South Pole-Aitken impact basin on the far side of the Moon was formed by a southward-directed impact. Image credit: Jeff Andrews-Hanna/University of Arizona/NASA/NAOJ.
“The bottom end of the pool should be covered by a thick layer of material extracted from the lunar interior by the impact, and the top end should not be covered,” said Dr. Jeffrey Andrews-Hanna, a planetary scientist at the University of Arizona.
“This means that the Artemis missions will land on the lower edge of the basin, the best place to study the largest and oldest impact basin on the Moon, where most of the ejecta, material from the deep interior of the Moon, should accumulate.”
It was long believed that the early Moon melted due to the energy released during its formation, resulting in an ocean of magma that covered the entire Moon.
As this ocean of magma crystallized, heavy minerals sank to form the lunar mantle, and light minerals floated to the surface to form the crust.
However, some elements were excluded from the solid mantle and crust and instead became concentrated in the final fluids of the magma ocean.
These “residual” elements included potassium, rare earth elements and phosphorus, which are collectively called KREEP.
According to Dr Andrews-Hanna and his colleagues, these elements were found to be particularly abundant on the 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 hardens, the high fructose corn syrup resists freezing until the very end and instead concentrates in the last bits of liquid,” Dr. Andrews-Hanna said.
“We think something similar happened on the Moon with KREEP.”
“As it cooled over many millions of years, the magma ocean gradually solidified into crust and mantle.”
“And eventually you get to the point where there's just a tiny bit of liquid between the mantle and the crust, and that's the KREEP-rich material.”
“All the KREEP-rich material and heat-producing elements somehow concentrated on the near side of the Moon, causing it to heat up and lead to intense volcanism that formed the dark volcanic plains that make the face of the Moon familiar from Earth.”
“However, the reason why the KREEP-rich material ended up on the near side, and how this material evolved over time, remains a mystery.”
“The Moon’s crust on the far side is much thicker than on the near, Earth-facing side, and this asymmetry still puzzles scientists.”
“This asymmetry influenced every aspect of the Moon's evolution, including the most recent stages of the magma ocean.”
“Our theory is that as the crust on the far side thickened, the ocean of magma below was squeezed outward, like toothpaste being squeezed out of a tube, until most of it was on the near side.”
A new study of the South Pole-Aitken Basin has revealed a striking and unexpected asymmetry around the basin that supports exactly this scenario: a blanket of emissions on its western side is rich in radioactive thorium, but not on its eastern flank.
This suggests that the crack created by the impact created a window in the Moon's skin right at the boundary separating the crust underlain by the last remnants of the KREEP-enriched magma ocean from the “regular” crust.
“Our study shows that the distribution and composition of these materials matches the predictions we get from modeling the final stages of magma ocean evolution,” Dr Andrews-Hanna said.
“The last remnants of the lunar magma ocean are on the near side, where we see the highest concentrations of radioactive elements.”
“But once before, a thin and patchy layer of magma ocean would have existed beneath parts of the far side, which explains the radioactive emissions on one side of the South Pole-Aitken basin.”
study appears in the magazine Nature.
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JC Andrews-Hanna etc.. 2025. A southward impact led to the formation of an ocean of magma in the region of the Moon's South Pole – the Aitken Basin. Nature 646, 297-302; doi:10.1038/s41586-025-09582-y
