SPA samples help analyze moon scar age

Chinese scientists have determined that the South Pole-Aitken basin, the largest and oldest impact scar on the moon, was formed 4.25 billion years ago, after analyzing lunar samples returned by the Chang'e 6 mission.

According to a news release published on Friday by the Chinese Academy of Sciences, the new finding, achieved by researchers at its Institute of Geology and Geophysics in Beijing, is a major scientific breakthrough that enables scientists to establish a key reference point for understanding massive collisions during the early stages of solar system.

It has tremendous scientific value in the research of early histories of the solar system and the moon, the academy said.

It noted that knowing the formation time of the 2,500-kilometer-wide SPA basin, which is located on the moon's far side, allows researchers to use the site as a cosmic timekeeper to calibrate impact chronologies across planetary bodies like Mars and Mercury.

Previous estimates of its age ranged from 4.26 to over 4.33 billion years using crater-counting methods, while samples collected from the Apollo missions suggested that a major global thermal event took place on the silver sphere between 4.35 and 4.33 billion years ago.

However, scientists were unable to validate their hypotheses and accurately determine the basin's age due to the lack of SPA samples.

The situation changed last year thanks to the Chang'e 6 mission, which made history by retrieving the first-ever samples from the basin.

A group of researchers led by Professor Chen Yi from the Institute of Geology and Geophysics isolated 20 representative norite fragments from five grams of lunar soil collected by the Chinese probe for detailed analysis.

Through lead isotope dating of microscopic zircon-bearing minerals, the team detected two distinct impact events at 3.87 billion and 4.25 billion years ago.

The older 4.25-billion-year date, supported by mineral crystallization patterns and remote sensing data matching SPA's inner ring composition, resolves long-standing debates about the basin's formation timeline.

This finding aligns the lunar impact chronology between the moon's near and far sides, while disproving connections between the SPA impact and the previously observed 4.35-to-4.33-billion-year thermal event.

According to the academy, the study provides critical evidence that this monumental collision that created the SPA basin occurred approximately 320 million years after the solar system's formation, offering new insights into early planetary bombardment processes.

The Chang'e 6 robotic mission, representing the world's first attempt to bring samples from the far side of the moon, was launched on May 3 last year from the Wenchang Space Launch Center in Hainan province.

After a series of complex maneuvers, the lander of the Chang'e 6 probe touched down at the SPA basin on June 2 and then began to collect surface and underground samples.

The landing marked the second time a spacecraft ever arrived on the lunar far side.

The vast region had never been reached by any spacecraft until January 2019, when the Chang'e 4 probe landed in the South Pole-Aitken Basin. Chang'e 4 surveyed areas surrounding its landing site, but did not collect samples.

The lander worked for 49 hours on the far side, using a mechanical arm and a drill to collect surface and underground materials.

The Chang'e 6 mission successfully concluded on June 25 as a total of 1,935.3 grams of samples from the far side was retrieved.