When asteroids struck the Earth-moon system, they devastated ancient landscapes, triggered global catastrophic events and delivered water and organic compounds essential for life, leading to a long-held belief that carbonaceous asteroids — from the outer solar system and rich in water and organic compounds — were a major source of the building blocks of life.

Now, new evidence from China's Chang'e 6 mission, which returned the first-ever samples from the moon's far side, is challenging conventional theories about this cosmic process. By analyzing tiny metal grains from lunar soil, Chinese scientists have reconstructed changes in asteroid impacts between 4.3 billion and 2.8 billion years ago. The findings were published Apr 27 in the Journal of Geophysical Research: Planets.

Frequent geological activity on Earth, such as plate tectonics and erosion, has erased most ancient impact records, meaning meteorites — fragments of asteroids that have fallen to Earth — reflect only impacts that occurred within the last 2 million years. In contrast, the moon, which formed at the same time as Earth but has remained geologically quiet, serves as a well-preserved "diary of the solar system", preserving a long record of impact history spanning nearly 4 billion years.

"When an asteroid strikes the moon, the impactor vaporizes and shatters under extreme heat, leaving behind tiny fragments — among them iron-nickel metal grains," said Liu Xiaoying, first author of the study and a postdoctoral researcher at the Institute of Geology and Geophysics under the Chinese Academy of Sciences.

Liu added that these metal grains are chemically distinct from the moon's own rocks and exhibit varying trace-element compositions across different asteroid types. They act as distinctive "chemical fingerprints" that allow scientists to identify the type of asteroid that struck the moon.

Using these "fingerprints," the team examined 40 impact-debris fragments from the Chang'e 6 samples. Of these, 13 were preserved in ancient lunar highland anorthosite, recording impacts dating back 4.3 billion years. The other 27 were preserved in younger basaltic debris, recording impacts accumulated since about 2.8 billion years ago.

The analysis showed that the 13 ancient fragments mainly came from ordinary chondrites and iron meteorites — dry, rocky impactors from the inner solar system — with metals from carbonaceous asteroids accounting for less than 8 percent. In contrast, among the 27 younger fragments, the proportion of carbonaceous metals rose to about 26 percent.

This indicates that carbonaceous asteroid impacts became significantly more frequent between 4.3 billion and 2.8 billion years ago, a relatively late stage when overall asteroid bombardment was already waning, likely limiting the total amount of water and other volatile materials delivered to the Earth-moon system.

The team proposed several possible explanations for the change in asteroid types. These include giant-planet migration that scattered carbonaceous asteroids inward, the Yarkovsky effect slowly shifting them into new orbits, or the breakup of one or more large carbonaceous bodies into numerous fragments.

"The study may challenge existing theories about the origin of Earth's water," said Lin Yangting, the study's corresponding author and a researcher at the institute.

He added that with more lunar samples expected from future missions, follow-up research will focus on different time windows to further unveil the mysteries of the solar system's impact history.