A shift in the types of asteroids that bombarded the Earth-moon system has been identified, pinpointing a dramatic change between 4.3 billion and 2.8 billion years ago.
A new study published in the Journal of Geophysical Research: Planets has analyzed lunar samples collected by China's Chang'e-6 mission, revealing a significant change in the composition of impacting asteroids over time. The research, conducted by the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), suggests that water-rich asteroids arrived much later than previously assumed.
Methodology and Sample Analysis
Researchers isolated 40 impact clasts from lunar far-side soil. These clasts, containing metal particles, were categorized into two groups based on their geological origin and age:
- 13 ancient clasts from lunar highland anorthosites, dating to approximately 4.3 billion years ago.
- 27 younger clasts from basaltic debris, dated to the last 2.8 billion years.
Key Findings
An analysis of the metal particles within the clasts revealed a clear evolution in the source of impacting asteroids.
- In the ancient clasts (4.3 billion years old), metals primarily originated from non-carbonaceous (inner solar system) sources, such as ordinary chondrites and iron meteorites. Carbonaceous metals accounted for less than 8% of the total.
- In the younger clasts (from the last 2.8 billion years), the proportion of metals from carbonaceous asteroids rose to approximately 26%.
This data indicates a significant increase in the contribution of carbonaceous asteroid impacts to the Earth-moon system occurring between 4.3 and 2.8 billion years ago.
Implications for Earth's Water
Carbonaceous asteroids are known to be rich in water and organic matter and are considered a potential source of Earth's early water. The study suggests that the later arrival of these water-rich asteroids implies a limited delivery of water and volatiles to the Earth-moon system compared to some prior assumptions, as the overall impact flux had already declined by that period.
Proposed Mechanisms
The research team attributed the observed shift in asteroid types to three possible mechanisms:
- The migration of giant planets, which scattered carbonaceous asteroids inward.
- The Yarkovsky effect, a force that can cause gradual orbital drift of small bodies.
- The collisional breakup of large carbonaceous bodies, creating debris fields.
Statement
Lin Yangting, a researcher at IGGCAS, stated that the moon serves as a pristine archive of the Earth-moon system's impact history. He added that future sampling of lunar regions of different ages could refine the evolutionary patterns of asteroid types and improve understanding of the inner solar system's impact history.