MOON — China’s Chang’e-6 mission returned lunar samples in 2024 that were analyzed and reported in the journal Science Advances on July 23. The findings indicate that moon rocks from the lunar farside do not support the theory of a cataclysmic Late Heavy Bombardment spike 3.9 billion years ago.

The Chang’e-6 samples are the first analyzed rocks to originate from the moon’s farside. Planetary scientists previously theorized that lunar impacts spiked suddenly around 3.9 billion years ago, a period known as the Late Heavy Bombardment. This concept was first introduced in 1975 by planetary scientist William K. Hartmann, who proposed that the Moon and Earth were subjected to a sudden surge of impacts around that time.

Wanfeng Zhang is an engineer at the Guangzhou Institute of Geochemistry in China. Zhang and colleagues analyzed 28 bits of impact melt, which are rocks that were once liquified by the heat of an impact. The team measured the relative amounts of two isotopes of argon in the rocks to date when they last melted and resolidified. Zhang said the lunar farside bombardment was not dominated by a cataclysmic spike.

The Chang’e-6 samples were collected from a farside impact basin named Apollo. The samples range in age from 4.33 billion to 1.13 billion years old. The Chang’e-6 samples show no sign of a cluster of impacts around 3.9 billion years ago. Zhang said the lunar bombardment history is severely masked.

Prior to the Chang’e-6 findings, lunar samples from NASA’s Apollo missions (1969–1972) were the primary source of data on the Moon’s impact history, with most of these samples coming from the nearside. Many moon rocks from NASA’s Apollo and the Soviet Union’s Luna missions in the 1960s and 1970s appear to be around 3.9 billion years old based on argon measurements. The Apollo and Luna samples all came from a part of the moon’s nearside covered in solidified lava pools called mare. The Chang'e-6 mission marked the first time that samples from the Moon’s far side were returned to Earth for detailed analysis, with the samples collected from the Apollo Basin within the South Pole–Aitken Basin, the largest and oldest impact basin on the Moon.

Mark Harrison is a geochemist at UCLA. He questioned the direct link between the samples and broader bombardment theories. Harrison said the main issue is how to know this in any way relates to the bombardment history.

He noted a shift in scientific consensus regarding lunar impact timelines. Harrison said the mood in the room has changed and the paradigm is shifting, or has shifted. Simone Marchi is a planetary scientist at the Southwest Research Institute in Boulder, Colo. The Chang'e-6 findings challenge the long-standing assumption that the Moon and Earth were subjected to a cataclysmic spike in impacts 3.9 billion years ago, as the new data suggest a smoother, more gradual decline in impact activity following planetary accretion.