Based on research on lunar samples retrieved by Chang'e-6 mission, a team of Chinese scientists confirmed that the oldest and largest impact crater on the moon, the South Pole-Aitken (SPA) basin, was formed 4.25 billion years ago, providing critical insights into the understanding of the early evolution of the moon and the solar system.

According to the study paper published Friday on National Science Review, the team, led by Chen Yi from the Institute of Geology and Geophysics under the Chinese Academy of Sciences, has analyzed the samples returned by the Chang'e-6 mission, and precisely dated SPA basin's formation to 4.25 billion years ago.

The SPA basin, a colossal scar stretching across the moon's far side, was likely formed during a barrage of asteroids pummeling much of the solar system within the first few hundred million years of its history. However, it has been difficult to date the basin.

As indirect estimates of the time of the SPA impact range widely from 4.26 to 4.35 billion years ago, planetary scientists have long waited to get their hands on direct evidence--rock samples from the SPA basin itself--to unravel the mysteries of the moon and the solar system.

China's Chang'e-6 mission returned the first samples from the SPA basin, finally providing the long-anticipated opportunity for direct dating of the SAP basin.

However, the Chang'e-6 lander landed on the mare basalt area of the Apollo Basin within the SPA Basin. This area has experienced multiple impacts and a basaltic eruption after the SPA impact. This has led to the fact that Chang'e-6 lunar samples contain fragments of materials from different periods, posing a challenge to the accurate dating of the SPA basin.

"The SPA impact event produced a massive impact melt sheet," explained Chen. "To precisely determine its formation age, we first need to identify the products of this impact melt sheet in the Chang'e-6 lunar samples."

The researchers examined approximately 1,600 fragments from 5 grams of samples, identifying 20 representative norite clasts with textures, mineralogy and geochemistry indicative of an impact origin.

Through precise lead-lead dating of zirconium-bearing minerals within these clasts, the team uncovered evidence of two distinct impact events at 4.25 and 3.87 billion years ago. The older norites, dating back to 4.25 billion years, exhibited structural and compositional characteristics that suggest they crystallized at different levels within a common impact melt sheet generated by the SPA impact.

The extensive geological surveys and comparative lithological analyses of the SPA basin strongly suggest that the older impact age of 4.25 billion years most likely represents the timing of the SPA impact, Chen added.

According to the study, this finding provides the first direct, sample-based evidence that the moon's largest impact basin formed approximately 320 million years after the beginning of solar system. The definitive age of 4.25 billion years for the SPA basin can serve as a crucial anchor point for refining the lunar cratering chronology and establishing a more complete temporal sequence of the moon's early evolution.

The Chang'e-6 probe was launched from China on May 3, 2024. On June 25, 2024, its returner landed in north China, bringing back 1,935.3 grams of samples from the far side of the moon.