BEIJING -- Using lunar samples collected by the Chang'e 6 mission, Chinese scientists have discovered a significant shift in the types of asteroids bombarding the Earth-moon system between 4.3 billion and 2.8 billion years ago, transitioning from the dominance of non-carbonaceous ones to the arrival of carbonaceous ones.

The results of the study, carried out by researchers from the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), were recently published in the Journal of Geophysical Research: Planets.

For scientists, the moon records the history of asteroid impacts in the solar system over the past 4 billion years. The research team isolated 40 impact clasts containing tiny metal particles from the lunar far-side soil, which act as "time capsules" preserving a record of ancient collisions.

Based on the mineral composition associated with these metals, the team categorized them into two distinct groups. Those preserved in basaltic debris represent asteroid fragments accumulated over the last 2.8 billion years following basaltic eruptions, while those originating from older lunar highland anorthosites splashed from other regions record impact events dating back as far as 4.3 billion years.

The analysis revealed a stark contrast in impactor types over the ages. The 13 ancient clasts showed metal particles corresponding primarily to ordinary chondrites and iron meteorites from the inner solar system, with carbonaceous asteroid metals accounting for less than 8 percent.

However, in the 27 younger clasts, the proportion of metals from carbonaceous asteroids rose to approximately 26 percent. This indicates that the contribution of carbonaceous asteroids increased significantly between 4.3 billion and 2.8 billion years ago.

This finding has major implications for human understanding of how water arrived in the inner solar system. While carbonaceous asteroids are rich in water and organic matter, and are believed to be a key source of Earth's early water, the study suggests their impact history is characterized by a "lag."

Because these water-rich asteroids appeared later -- when the solar system's impact flux had already decayed significantly -- the total volume of water and volatiles they could deliver to the Earth-moon system was likely limited compared to previous assumptions.

The team attributes this shift to three potential mechanisms: the migration of giant planets that scatter carbonaceous asteroids inward, the Yarkovsky effect driving gradual orbital drift, or the collisional breakup of large carbonaceous bodies, which generates vast debris fields.

"The moon serves as a pristine archive of the Earth-moon system's impact history," said Lin Yangting, a researcher from the IGGCAS, adding that future sampling of lunar regions with different ages will allow scientists to refine the evolutionary patterns of asteroid types, which will consequently deepen human understanding of the inner solar system's impact history and provide key parameters for the evolutionary dynamics of celestial orbits.