Chang'e 6 findings challenge Earth water theories

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 scientists to long believe that carbonaceous asteroids from the outer solar system were a major source of the building blocks for life on Earth.

Now, a study on lunar samples from China's Chang'e 6 mission is challenging conventional theories about this cosmic process. By analyzing tiny metal grains in lunar soil, Chinese scientists have reconstructed changes in asteroid impacts between 4.3 billion and 2.8 billion years ago.

The findings were published on April 27 in the Journal of Geophysical Research: Planets.

Frequent geological activity on Earth, such as plate movement and erosion, has erased most traces of ancient asteroid impacts. As a result, meteorites — fragments of asteroids that fall to Earth — mainly preserve records from only the past 2 million years.

In contrast, the moon, which formed around the same time as Earth but has remained largely inactive geologically, serves as a well-preserved "diary of the solar system", retaining impact records spanning nearly 4 billion years.

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

Liu said these metal grains are chemically different from the moon's native rocks and contain distinct trace elements depending on the type of asteroid involved. These differences act like "chemical fingerprints", allowing scientists to identify what kinds of asteroids struck the moon.

Using these "fingerprints", the research team examined 40 fragments of impact debris from the Chang'e 6 lunar samples.

Among them, 13 were found in ancient lunar highland rocks and recorded impacts dating back 4.3 billion years. The other 27 were preserved in relatively more recent volcanic rock debris, recording impacts accumulated about 2.8 billion years ago.

The study found that the 13 older fragments mainly came from ordinary rocky asteroids and iron-rich meteorites originating in the inner solar system. Metals from carbonaceous asteroids accounted for less than 8 percent.

By comparison, among the 27 fragments, the proportion linked to carbonaceous asteroids rose to about 26 percent.

The findings suggest impacts from carbonaceous asteroids became significantly more frequent between 4.3 billion and 2.8 billion years ago.

However, this increase occurred during a relatively late stage, when overall asteroid bombardment had already declined, possibly limiting the total amount of water and other volatile substances delivered to the Earth-moon system.

The team proposed several scenarios for the change in asteroid types, including giant-planet migration that scattered carbonaceous asteroids inward, the Yarkovsky effect gradually 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.

Lin added that with more lunar samples expected from future missions, follow-up studies will focus on different periods in lunar history to further uncover the mysteries of asteroid impacts and the evolution of the solar system.

limenghan@chinadaily.com.cn