Visitors view Chang'e-6 mission lunar samples, collected from the far side of the moon, on display at the 15th China International Aviation and Aerospace Exhibition in Zhuhai, south China's Guangdong Province, Nov. 13, 2024. China's Chang'e-6 mission lunar samples, collected from the far side of the moon, are showcased at the 15th China International Aviation and Aerospace Exhibition. (Photo: Xinhua)

Basalt samples returned by the Chang'e-6 mission have revealed volcanic events on the lunar far side at 2.8 billion years ago (Ga) and 4.2 Ga, according to a latest research conducted by Professor Li Qiuli's lab at the Institute of Geology and Geophysics of the Chinese Academy of Sciences.

"Unraveling the volcanic history of the lunar far side is crucial for understanding the hemispheric dichotomy of the Moon," Li told the Global Times.

The asymmetry between the Moon's near side and far side - encompassing differences in basalt distribution, topography, crustal thickness, and thorium (Th) concentration - has long been a mystery. However, China's Chang'e-6 mission - the first to return samples from the lunar far side - has created a unique opportunity to explore volcanic activity in this hemisphere, with the 1,935.3-gram lunar soil samples it retrieved.

Led by Li, postdoctoral researcher Zhang Qian conducted systematic radioisotope dating on 108 basalt fragments from the sample. Of these, 107 fragments revealed a consistent formation age of 2807 ± 3 million years ago (Ma), indicating the eruption age of local basalts at the Chang'e-6 landing site. Notably, this 2.8 Ga volcanic episode has not been observed in near side samples.

The remaining fragment, of high-aluminum basalt and dating to 4203 ± 4 Ma, is thought to have originated from a cryptomare region south of the landing site. It is the oldest lunar basalt sample yet returned whose age has been precisely determined.

These data indicate that volcanic activity on the lunar far side persisted for at least 1.4 billion years, from 4.2 Ga to 2.8 Ga. Initial lead isotope analysis suggests that these basalts derive from distinct mantle sources: The 4.2 Ga basalt came from a KREEP-rich source, i.e., one with abundant potassium (K), rare earth elements (REE), and phosphorous (P), while the 2.8 Ga basalt came from a KREEP-poor source.

The close alignment between the 2.8 Ga basalt age and crater-counting estimates suggests that the cratering chronology model, established based on near side observations, is also applicable to the lunar far side. The radioisotope ages of the Chang'e-6 basalts provide an essential calibration point for refining lunar crater-counting chronology, thus enhancing its accuracy, according to Li.

Li's team's work was published in Nature on Friday. The same day, another study on the Chang'e-6 samples conducted by a team led by Professor Xu Yigang from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences, which found evidences that indicate existence of young magmatism on the lunar far side, was published in Science.

The samples returned by Chang'e-6, which was the far side South Pole-Aitken (SPA) Basin of the Moon, provide a best opportunity to investigate the lunar global dichotomy, Xu said.

Crustal thickness has been suggested as a key factor in accounting for asymmetry in the abundance of volcanism between the lunar near side and far side. However, this model has been questioned since the SPA Basin on the far side, which has an anomalously thin crust, appears deep and significantly underfilled by volcanism.

In the latest study, Xu's team also suggested that the composition of the mantle source is another important factor controlling the generation of lunar volcanic activity. "Although the SPA Basin has a thin crust, the depleted and refractory mantle source beneath the SPA basin hinders partial melting to a large degree," Xu said.

This work also provides an additional calibration point at 2.83 Ga for the lunar crater chronology and implies a constant impact flux after 2.83 Ga. This newly calibrated chronology model improves the age estimation tool based on crater statistics for both the Moon and other terrestrial bodies, and also has additional implications for the evolution of lunar impactors, potentially related with early planet migration in the early Solar System, according to Xu.