Scientists have long sought to determine the age of the Moon's South Pole-Aitken (SPA) basin, the largest and oldest known impact crater on the lunar surface. Recently, a research team led by Prof. CHEN Yi from the Institute of Geology and Geophysics at the Chinese Academy of Sciences, has dated the formation of the basin to 4.25 billion years ago by analyzing the first-ever rock samples returned from the SPA basin by China's Chang'e-6 mission. Their findings were published in National Science Review.
The SPA basin, a vast impact structure spanning the Moon's far side, is believed to have formed during a period of intense asteroid bombardment that shaped much of the Solar System within its first few hundred million years. Despite its significance, the exact age of the SPA basin has remained elusive, hindering efforts to understand its role in lunar and planetary history.
Previous estimates of the basin's age varied widely, highlighting the need for direct analysis of SPA-derived materials. The Chang'e-6 mission's return of lunar samples from the SPA basin has given scientists the opportunity to resolve this mystery. The research team focused on impact melt rocks within these samples, which provide crucial clues to the basin's formation.
"The SPA impact event generated a massive sheet of impact melt," explained Prof. CHEN. "To accurately determine the basin's age, we first needed to identify the products of this melt within the Chang'e-6 samples."
The researchers meticulously analyzed approximately 1,600 fragments from two soil samples, identifying 20 norite clasts with textures, mineralogy, and geochemical signatures consistent with an impact origin. Using lead-lead dating of zirconium-bearing minerals within these clasts, they uncovered evidence of two distinct impact events, dated to 4.25 and 3.87 billion years ago. The older norites, dated to 4.25 billion years, exhibited structural and compositional features suggesting they crystallized at varying depths within a common impact melt sheet produced by the SPA-forming event.
"Our geological surveys and comparative lithological analyses strongly indicate that the 4.25-billion-year age corresponds to the formation of the SPA basin," Prof. CHEN stated.
This study provides the direct, sample-based evidence that the Moon's largest impact basin formed approximately 320 million years after the birth of the Solar System. The precise age of 4.25 billion years for the SPA basin serves as a critical anchor point for refining the lunar cratering chronology and reconstructing the timeline of the Moon's early evolution. These findings not only advance our understanding of lunar history but also shed light on the dynamic processes that shaped the early Solar System.
Schematic illustrating that Chang'e-6 norites document the 4.25 Ga lunar South Pole-Aitken impact event (Image by SU Bin)
