A colossal asteroid may have warped the moon from the inside out

An unusual ratio of potassium isotopes, found in samples of basaltic rock brought back to Earth from the moon's South Pole–Aitken Basin by China's Chang'e 6 sample-return mission, has provided further evidence for how the impact that formed this gigantic basin is responsible for the asymmetry between the moon's near and far sides.

The moon's near side is familiar to us through the pattern of the 'Man in the Moon' that is made from the dark shapes of the lunar maria, which are vast volcanic plains. In contrast, the far side, visible only to spacecraft that go around the back side of the moon, has barely any dark maria.

The huge 1,600-mile (2,500-kilometer) wide expanse of the South Pole–Aitken Basin extends considerably onto the far side of the moon. It's one of the largest impact features in the entire solar system and is between 4.2 and 4.3 billion years old – much older than the lunar maria, most of which are estimated to be around 3.6 billion years old.

Chang'e 6 touched down inside the 334-mile (537 km) crater Apollo, which sits inside the South Pole–Aitken Basin, on June 1, 2024, and returned to Earth precious samples from its landing site 25 days later. Ever since, Chinese scientists have been carefully analyzing the samples to try to learn why the far side is so much different from the near side.

Now, a team led by Heng-Ci Tian of the Institute of Geology and Geophysics at the Chinese Academy of Sciences in Beijing has analyzed samples of lunar basalt brought back by Chang'e 6. The scientists found that the ratio of the heavier potassium isotope, potassium-41, relative to potassium-39 is greater in the samples from the South Pole–Aitken Basin than in samples from the near side collected by the Apollo missions and lunar meteorites.

Tian's team explored several possible explanations for this baffling isotopic composition. They considered whether the long-term irradiation of the lunar surface by cosmic rays could have resulted in the unusual isotopic ratio. They looked at whether the various melting, cooling, and eruptive processes of magma could have changed the composition of the basalts. And they explored whether the isotope ratio is a consequence of meteoritic contamination. Ultimately, they concluded that all of these processes would have only a minor effect, if any.

That left one other option: that the potassium isotope ratio is a relic of the giant impact that formed the South Pole–Aitken Basin. The intense temperature and pressure of the impact heated the moon's crust and mantle so much that many of the volatile elements present (volatiles are elements with low boiling points), including potassium, evaporated and escaped into space. Previous results support this – Chang'e 6 has already discovered that the mantle on the far side contains less water than the near side. Since the lighter potassium-39 isotope would be more susceptible to evaporating than the heavier isotope, the impact resulted in this greater ratio of potassium-41 to potassium-39.

Isotopic curiosities aside, the findings show how deeply the impact affected the moon's interior, and how isotopic ratios can provide windows into the conditions of such impacts, and how those impacts altered the moon's crust and mantle. The reduction in volatiles would limit volcanism by suppressing the formation of magma, providing a strong explanation for why the lunar far side contains so few maria.

The latest findings from the Chang'e 6 mission were presented on 12 January in Proceedings of the National Academy of Sciences.