A study of lunar samples collected by China's Chang'e-6 mission has provided evidence of a shift in the composition of asteroids that bombarded the Earth-moon system between approximately 4.3 billion and 2.8 billion years ago.
Methodology and Sample Analysis
Researchers from the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) analyzed 40 impact clasts isolated from lunar far-side soil. These clasts contain metal particles that record the composition of past asteroid impacts.
The clasts were categorized into two groups based on their origin and age:
- 13 ancient clasts from lunar highland anorthosites, dating to approximately 4.3 billion years.
- 27 younger clasts from basaltic debris, associated with the last 2.8 billion years.
Key Findings
Analysis of the metal particles within the clasts revealed a change in the dominant types of impacting bodies over time.
- In the ancient clasts, metals primarily originated from non-carbonaceous asteroids, such as ordinary chondrites and iron meteorites from the inner solar system. Carbonaceous asteroid metals accounted for less than 8% of the total.
- In the younger clasts, 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 between 4.3 billion and 2.8 billion years ago.
"The moon serves as a pristine archive of the Earth-moon system's impact history." — Lin Yangting, IGGCAS
Implications and Proposed Mechanisms
Carbonaceous asteroids are rich in water and organic matter and are considered a potential source of Earth's early water. The later arrival of this type of asteroid, at a time when the overall impact flux is understood to have declined, suggests that the total volume of water and volatiles delivered to the Earth-moon system may have been less than some previous estimates.
The study attributes the shift in asteroid types to three possible mechanisms:
- Giant Planet Migration: The movement of giant planets in the early solar system scattering carbonaceous asteroids inward.
- Yarkovsky Effect: A thermal force causing gradual orbital drift of small bodies.
- Collisional Breakup: The fragmentation of large carbonaceous bodies, creating debris fields.
Future Research
Lin Yangting added that future sampling of lunar regions with different ages could refine the understanding of evolutionary patterns in asteroid types.
Publication
The study was published in the Journal of Geophysical Research: Planets.