A group of Chinese researchers revealed that a giant impact that formed the South Pole-Aitken (SPA) Basin on the moon's far side resulted in volatile loss from the lunar mantle.

The study was published in the Proceedings of the National Academy of Sciences on Tuesday, providing crucial evidence for understanding how large-scale collisions shaped the moon's internal evolution and contributed to the differences between its near and far sides.

Since the formation of the moon, asteroid impacts have been the dominant geological processes triggered by events outside the moon, producing impact craters and basins across the lunar surface and profoundly modifying its topography and geochemical characteristics. However, the extent to which large-scale impact events have affected the moon's deep interior remains unclear.

A research team led by Tian Hengci from the Institute of Geology and Geophysics, Chinese Academy of Sciences, examined the isotopic compositions of potassium (K) in Chang'e-6 lunar basalts collected from the SPA basin.

The isotopic systems of moderately volatile elements, such as K, are prone to volatilization and fractionation under the high-temperature conditions generated by impacts. Their compositions can record information such as temperature, pressure, and material sources during impact events, making them key evidence for determining impact scales, thermal histories, and modifications to the lunar crust and mantle.

The team found that the Chang'e-6 lunar basalts exhibit significantly heavier K isotopic compositions than all previously reported lunar basalts from Apollo missions and lunar meteorites. Then they evaluated potential mechanisms that could modify K-isotope composition, including long-term cosmic-ray irradiation, magmatic differentiation and impactor input. However, the results showed that these factors exert only minor effects.

Further research showed that the impact events altered the K-isotope composition of the lunar mantle, leading to an increase in isotope values. During the high-temperature and high-pressure processes generated by impacts, lighter K isotopes were preferentially lost compared to heavier ones, leading to elevated isotopic ratios in the residual materials.

The study also implied that such volatile depletion may have suppressed magma generation and volcanic activity on the lunar far side, potentially contributing to the long-recognized asymmetry in volcanic activity between the near side and far side of the moon.