Chang'e-6 lunar soil reveals changing history of asteroid impacts

A research team from the Institute of Geology and Geophysics, Chinese Academy of Sciences has uncovered new insights into the history of asteroid impacts on the Earth-moon system by analyzing lunar soil samples returned by China's Chang'e-6 mission.

The study, led by researcher Lin Yangting's team, suggests that carbonaceous asteroids – long considered potential carriers of water and organic materials to the Earth-moon system and closely linked to the emergence of Earth's habitable environment – began impacting the Earth-moon system later than previously thought.

Unlike Earth, where meteorite records only reflect impact events from the past 2 million years and earlier records are extremely scarce, the moon preserves a more complete archive of cosmic collisions. Scientists can identify asteroid types through iron-nickel metal particles embedded in lunar soil.

From the Chang'e-6 lunar samples, researchers identified 40 impact-related fragments containing metallic particles. These fragments were divided into two groups. One originated from lunar basalt formed about 2.8 billion years ago, recording relatively recent impact events. The other came from much older lunar highland material, tracing back to about 4.3 billion years ago.

Analysis shows a clear change in the composition of impacting asteroids over time. In the older samples, metallic particles linked to carbonaceous asteroids were extremely rare. However, in younger samples, their proportion increased significantly.

This indicates that between 4.3 billion and 2.8 billion years ago, the dominant impactors in the Earth-moon system shifted from mainly non-carbonaceous asteroids to a higher fraction of carbonaceous asteroids.

The new findings suggest that increased bombardment of carbonaceous asteroids occurred during a later period, when the overall asteroid impact rate had already declined. This implies the total amount of water and organic materials delivered to Earth by carbonaceous asteroids may have been more limited than previously assumed.

Scientists propose several possible causes for this shift, including orbital migration of the giant planets, gradual drift of asteroid orbits or the breakup of large carbonaceous asteroids.