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On March 21, 2025, Chinese scientists made a major breakthrough through the study of lunar soil samples collected by Chang'e-6 on the far side of the moon: they determined that the oldest and largest impact relic on the moon, the SPA basin, was formed 4.25 billion years ago, providing an initial anchor point for the history of large-scale impacts in the early solar system. It has great scientific significance for understanding the early evolution of the moon and even the solar system. The results were published in the academic journal National Science Review.

As the only natural satellite of the Earth, the moon is covered with meteorite impact craters of various sizes on its surface, which are the historical marks of the impact of the solar system. Among them, the giant impact crater SPA basin with a diameter of about 2,500 kilometers is the oldest impact scar on the moon, recording the huge "trauma" suffered by the moon in its infancy. The formation time of the SPA basin is not only a key node in the evolutionary history of the moon and the starting anchor point of the impact history, but also can be used as a golden reference for calibrating the impact history of the solar system, establishing a unified age scale for impact craters on planets such as Mars and Mercury, and is a precious "cosmic clock" left to mankind by the solar system. Therefore, accurately defining the formation time of the SPA basin has long been one of the primary scientific goals in the field of international deep space exploration, but it is also one of the biggest unsolved cases in the field of lunar scientific research.

Chen Yi, a researcher at the Institute of Geology and Geophysics, Chinese Academy of Sciences, led a research team to sort out more than 1,600 rock fragments >200 microns from 5 grams of lunar soil. Based on detailed petrological classification, 20 representative sulongite rock fragments were selected from them to carry out petrological, geochemical and chronological research.

The study found that although these sulonite fragments are consistent with the sulonite in the Apollo samples on the front of the moon in terms of whole rock composition and mineral composition, there are obvious differences in the major and trace element composition of the minerals. The extremely low volatile element content, high Ni/Co ratio and molten residual minerals all indicate that these sulonite fragments are impact lava, a new type of lunar sulonite, which the team named South Pole-Aitken Sulonite (SPANs). The team further conducted lead isotope dating analysis on tiny (1-5 microns) zircon-containing minerals in the fragments and identified two impact events of 4.25 billion years and 3.87 billion years. Among them, the 4.25 billion-year-old sulonite presents different mineral grain sizes and structures, and the major and trace elements of the minerals show a clear evolution trend from coarse grains to fine grains, indicating that they are products of separation and crystallization from different layers of the same impact magma pool.

In order to trace the source area of ​​this ancient impact-induced sulongite, the team carried out large-scale lithology mapping and tracing in the SPA basin and surrounding areas based on 447GB of remote sensing data, and found that the 4.25 billion-year-old sulongite came from the compositional anomaly area in the inner ring of the SPA basin. Combining the above evidence, the team finally determined that the SPA basin was formed 4.25 billion years ago.

This result proves that about 320 million years after the formation of the solar system, a large impact event formed the largest impact relic on the moon, the SPA basin, providing an initial anchor point from the far side of the moon for the statistical dating of lunar impact craters. The model age of the SPA basin calculated based on the impact curve calibrated by the lunar front sample is basically consistent with the isotope dating results, indicating that the impact flux on the front and back of the early moon is roughly the same. In addition, this work confirms that the full-moon thermal event of 4.35-4.33 billion years is not related to the SPA large impact, providing a reference base point and scientific basis for reshaping the early primary evolutionary sequence of the moon.