Age samples reinforce rising tide casting doubt on singular 3.9 billion-year-old lunar cataclysm
Rock fragments from the far side of the Moon add weight to scientific reflection on the adolescence of the solar system.
The Moon’s craters and basins bear witness to its history of being battered by incoming space rocks. Planetary scientists have long thought that these impacts suddenly increased around 3.9 billion years ago, a period called the Late Heavy Bombardment.
But samples returned to Earth by the Chinese spacecraft Chang’e-6 in 2024 do not support this ideathe scientists report on July 23 in Scientific advances. The rocks analyzed are the first to come from the far side of the Moon, outside the range of a particularly influential nearby impact.
“The bombardment of the far side of the Moon was not dominated by a cataclysmic peak,” says Wanfeng Zhang, an engineer at the Guangzhou Institute of Geochemistry in China. Instead, the Moon may have experienced a longer period of less frequent impacts.
Zhang and his colleagues analyzed 28 pieces of impact melt, rocks once liquefied in the heat of a strike. The team measured the relative amounts of two argon isotopes in the rocks until they last melted and resolidified, an indicator of when a crater formed.
This technique provided the original evidence of late heavy bombardment. Many moon rocks from NASA’s Apollo and the Soviet Union’s Luna missions in the 1960s and 1970s appear to be about 3.9 billion years old, based on the same argon measurements.
This idea of bombing influenced theories about evolution of the solar system And the origins of life on Earth. But more recently, planetologists began to wonder whether there was a single, short-lived cataclysm 3.9 billion years ago, a series of smaller peaks, or something more gradual.
Part of the problem with the Apollo and Luna samples is that they all come from a part of the near side of the Moon covered in huge pools of solidified lava called a pool. The mare makes up the face of the iconic “man in the moon” (or rabbit, depending on how you look). Previous work suggested that the impact that created one of these basins, Mare Imbrium, spread debris so far across the face of the Moon that many of the existing samples came from this single event, which occurred about 3.92 billion years ago.
“As such, the history of the lunar bombardments is severely obscured,” says Zhang. “This problem can be overcome by examining impact-melted rocks from lunar meteorites or samples [returned] from the far side of the Moon.
The Chang’e-6 samples come from a hidden impact basin also called Apollo. They are between 4.33 billion and 1.13 billion years old and show no signs of clustering around 3.9 billion years ago, Zhang and colleagues report. This suggests a long period of regular impacts, rather than a brief cataclysm.
It’s difficult to draw conclusions about impact cadence with data from just one region, says planetary scientist Simone Marchi of the Southwest Research Institute in Boulder, Colorado. And it’s true that past data tends to be “plagued” by the influence of Mare Imbrium, and more samples from more parts of the Moon would be useful, Marchi says.
UCLA geochemist Mark Harrison, an expert in argon dating, thinks the measured rock ages appear accurate. Or at least as precise as argon dating can be; Harrison’s earlier work called into question whether argon can date reliably a single ancient impact.
But he also questions whether these rocks actually came from impacts or are simply volcanic rocks that have cooled. The rocks studied do not exhibit the morphological characteristics that one would expect from impact melting, he says.
“The main question is: How do we know this has anything to do with the history of the bombings? » he said.
Still, if the discovery is confirmed, it will add to a growing number of researchers who doubt the late heavy bombardment, Harrison said. “The mood in the room has changed. The paradigm is changing, or has changed.”
