Using just 3.5 grams of lunar soil collected from the far side of the Moon, Chinese researchers have precisely dated the formation of the Apollo basin — one of the far side’s largest craters — to 4.16 billion years ago. The findings push back the start of the Moon’s major impact era by at least 100 million years.

The three rock fragments used in the research were retrieved by Chang’e-6, which returned to Earth on June 25 last year, marking China’s sixth robotic lunar exploration mission and its second successful lunar sample-return effort. Like its predecessors, the mission is named after the Chinese moon goddess Chang’e, who flew from the Earth to the Moon after swallowing a pill of immortality.

Scientists have long debated whether the Moon’s Late Heavy Bombardment period encompassed a sudden spike in impacts or a gradual decline over time. Most lunar samples — mainly taken from near-side landing sites — were dated to between 4 and 3.8 billion years, appearing to support the idea of a brief, intense surge in collisions.

The latest research, which pushes that timeline back by approximately 100 million years, was published Aug. 20 in Nature Astronomy by a team led by Xu Yigang, a researcher at the Chinese Academy of Sciences and the Guangzhou Institute of Geochemistry.

“The controversy stems from the lack of precise age data for key impact basins on the Moon,” Xu told state news agency Xinhua.

The Apollo basin, where the Chang’e-6 sampling site is located, lies within the South Pole–Aitken basin, the largest known impact crater in the solar system, taking up a quarter of the Moon’s surface, and is the largest secondary crater in the region.

The rock fragments retrieved by Chang’e-6 are impact-melted rocks produced during the formation of the Apollo basin. These samples serve as ideal “rock clocks” for recording ancient lunar impact events.

Through high-precision chronological analysis of the lunar soil samples, combined with remote sensing imagery and geochemical data, researchers demonstrated that the 4.16-billion-year-old rock fragments mark the formation of the Apollo basin.

“The impact flux analysis shows that during the Moon’s Late Heavy Bombardment, the rate of impacts gradually declined, rather than surging between 4 billion and 3.8 billion years ago,” Xu emphasized. He added that ongoing research on Chang’e-6 samples will continue to deepen humanity’s understanding of the Earth-Moon system’s evolution.

Multiple studies stemming from the lunar samples retrieved by the Chang’e-6 mission have catapulted to the international stage. In July, four studies by the Chinese Academy of Sciences analyzing the same batch of lunar samples were published as cover articles in Nature. These studies revealed the magmatic activity, ancient magnetic field, mantle water content, and mantle evolution of the lunar far side, shedding light on its evolutionary history.

Chang’e-6 is the latest lunar mission within China’s Chang’e Program, also known as the Chinese Lunar Exploration Project (CLEP), which began in 2004. Currently, China has two more lunar missions planned: Chang’e-7, in 2026, and Chang’e-8, in 2029, with the respective goals of looking for evidence of water on the Moon’s South Pole and laying the groundwork for a lunar research base.

China hopes to formally establish a research base on the Moon by 2035.

Editor: Marianne Gunnarsson.

(Header image: A researcher shows a lunar sample retrieved by Chang’e-6 at the lunar sample laboratory in the National Astronomical Observatories of the Chinese Academy of Sciences (CAS) in Beijing, Sept. 24, 2024. Xinhua)