image: Lead-lead isotopic ages of zirconium-bearing minerals in Chang’e-6 norites reveal two distinct impact events at 4.25 and 3.87 billion years ago.
Credit: ©Science China Press
Scientists have long sought to determine the age of the South Pole–Aitken (SPA) basin, the Moon’s largest and oldest impact crater. Now, a research team led by Prof. CHEN YI from the Institute of Geology and Geophysics of the Chinese Academy of Sciences has analyzed the first-ever rock samples from the elusive SPA basin, returned by the Chang'e-6 mission, and precisely dated its formation to 4.25 billion years ago. This groundbreaking discovery provides critical insights into the early history of the Moon and the Solar System.
The SPA basin, a colossal scar stretching across the Moon's far side, likely formed during a barrage of asteroids pummeling much of the Solar System within the first few hundred million years of its history. However, as important as the massive SPA basin is, its age—and therefore also its greater role and importance—has been difficult to pin down.
As indirect estimates of the age of the SPA impact range widely, planetary scientists have long waited to get their hands on rocks from the SPA basin itself to unravel the mysteries of this tumultuous early epoch in the history of the Moon and the Solar System.
The Chang’e-6 mission recently returned the first rock samples from the SPA basin, thus finally providing the long-anticipated opportunity for direct dating. Prof. CHEN's team focused on analyzing impact melt rocks within the returned lunar soils to pinpoint the basin's formation age.
“The South Pole-Aitken impact event produced a massive impact melt sheet,” explained Prof. CHEN. “To precisely determine its formation age, we first need to identify the products of this impact melt sheet within the Chang'e-6 lunar soils.”
The researchers meticulously examined approximately 1,600 fragments from two Chang’e-6 soil 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 suggesting they crystallized at different levels within a common impact melt sheet generated by the SPA impact.
“Our 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,” Prof. CHEN stated.
This work provides the first direct, sample-based evidence that the Moon’s largest impact basin formed approximately 320 million years after the Solar System's birth. 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.
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See the article:
South Pole-Aitken massive impact 4.25 billion years age revealed by Chang’e-6 samples
https://doi.org/10.1093/nsr/nwaf103
Journal
National Science Review