Chinese scientists, using lunar soil samples brought back by Chang'e-6, have determined that the moon's oldest and largest impact crater, the South Pole-Aitken (SPA) Basin, was formed 4.25 billion years ago. Photo: CCTV

Chinese scientists, using lunar soil samples brought back by Chang'e-6, have determined that the Moon's oldest and largest impact crater, the South Pole-Aitken (SPA) Basin, was formed 4.25 billion years ago. This provides a key reference point for the history of early large impacts in the solar system, China Central Television (CCTV) reported on Friday.
This significant scientific discovery, which is crucial for understanding the early evolution of the moon and the solar system, was published in the academic journal National Science Review.

As Earth's only natural satellite, the moon is covered with impact craters of various sizes, marking the history of collisions in the solar system. Among them, the giant SPA Basin, about 2,500 km in diameter, is the moon's oldest impact scar, recording the massive "wounds" from its early years.

The formation of the SPA Basin is a key milestone in the moon's history and a starting reference for impact events. It also serves as a crucial benchmark for calibrating the impact history of the solar system, helping establish a unified age scale for craters on planets like Mars and Mercury—essentially acting as a "cosmic clock." Accurately determining its formation age has long been a top scientific goal in deep space exploration and one of the biggest unsolved mysteries in lunar science.

Currently, there are two main methods for estimating the formation age of the SPA Basin. One method is based on impact crater statistics, which suggest an age ranging from 4.26 billion to more than 4.33 billion years. The other method involves studies of lunar meteorite samples and Apollo samples from the moon's near side, which indicate that a global thermal event occurred around 4.35-4.33 billion years ago and is speculated to be linked to the SPA impact event, per CCTV.

However, direct sample evidence from the SPA Basin on the moon's far side has never been obtained

Previously, China's Chang'e-6 mission successfully returned lunar soil samples from the interior of the SPA Basin on the moon's far side, presenting a rare opportunity to precisely determine the formation age of the basin.

The Chang'e-6 lander, touched down in the Apollo Basin, a region of lunar mare basalt within the SPA Basin. Over 2.8 billion years after the large SPA impact, this area experienced multiple impacts and volcanic basalt eruptions, leading to lunar soil samples containing fragments from different periods.

Remote sensing spectral data and impact sputtering simulation results show that the non-mare material in both the SPA and Apollo basins is predominantly Norite. The Chang'e-6 lunar soil samples may contain sputtered material from the Apollo Basin, SPA Basin, and other nearby craters. These non-mare materials might hold clues to the moon's early impact history. Decoding the impact signatures of the SPA Basin is the key challenge for determining its formation age.

Researcher Chen Yi from the Institute of Geology and Geophysics at the Chinese Academy of Sciences led a team that selected over 1,600 fragments larger than 200 microns from 5 grams of lunar soil. Through detailed petrological classification, they identified 20 representative Norite fragments for further petrological, geochemical, and chronological studies, said the report.

The study found that although these Norite fragments were compositionally and mineralogically consistent with Apollo samples from the moon's near side, there were notable differences in their major and trace element compositions. Extremely low volatile element content, higher Ni/Co ratios, and the presence of residual molten minerals indicate that these fragments are impact melt rocks, a new type of lunar Norite, which the team named South Pole-Aitken Norites (SPANs).

The team further conducted lead isotopic dating on tiny (1-5 microns) zircon minerals within the fragments and identified two impact events, at 4.25 billion and 3.87 billion years ago. The 4.25 billion-year-old Norite showed distinct mineral grain sizes and structures, with clear evolution trends in major and trace elements from coarse to fine grains, suggesting they originated from different layers of the same impact magma pool.

To trace the source region of this ancient impact-derived Norite, the team conducted large-scale lithological mapping and sourcing using 447 GB of remote sensing data. They discovered that the 4.25 billion-year-old Norite came from the compositionally anomalous region in the inner ring of the SPA Basin. Combining all evidence, the team concluded that the SPA Basin formed 4.25 billion years ago.

This finding confirms that a large impact event occurred around 320 million years after the formation of the solar system, creating the moon's largest impact feature, the SPA Basin. It provides an initial anchor point for lunar impact dating based on samples from the far side of the moon. 

The model age of the SPA Basin, based on the impact curve corrected using near-side lunar samples, is consistent with isotopic dating results, indicating that the impact flux on both the near and far sides of the moon was roughly the same during the moon's early history.

Additionally, this work confirms that the global thermal event from 4.35-4.33 billion years ago is unrelated to the SPA impact, providing a reference point and scientific basis for reshaping the moon's early evolutionary sequence, CCTV reported.

Global Times