The Oxygen Isotope Identity of the Earth and Moon with Implications for the Formation of the Moon and Source of Volatiles

Academic Background

The isotopic similarity between Earth and Moon rocks has long been a significant puzzle in geochemistry and cosmochemistry, contradicting prevailing models of Moon formation, particularly the “Giant Impact Theory.” According to this theory, the Moon formed approximately 4.5 billion years ago from a collision between Earth and a Mars-sized body named Theia. However, the oxygen isotope similarity between Earth and Moon rocks suggests that Theia and proto-Earth may have had very similar oxygen isotope compositions, or that intense mixing occurred after the collision. Additionally, this finding provides new insights into the origin of water on Earth and the Moon, suggesting that water may not have been delivered by the late veneer.

To further explore this issue, researchers conducted precise measurements of oxygen isotopes in lunar and terrestrial rocks and proposed new interpretations based on existing Moon formation models. This study not only helps to understand the process of Moon formation but also provides new clues about the origin of volatiles on Earth and the Moon.

Source of the Paper

This research was conducted by Meike Fischer, Stefan T. M. Peters, Daniel Herwartz, Paul Hartogh, Tommaso Di Rocco, and Andreas Pack, affiliated with the Geoscience Center at the University of Göttingen, the Max Planck Institute for Solar System Research, Thermo Fisher Scientific (Bremen), the Leibniz Institute for the Analysis of Biodiversity Change, the Institute of Mineralogy and Petrology at the University of Cologne, and the Institute of Geology, Mineralogy, and Geophysics at Ruhr University Bochum. The paper was published on December 16, 2024, in the Proceedings of the National Academy of Sciences (PNAS), titled “The Oxygen Isotope Identity of the Earth and Moon with Implications for the Formation of the Moon and Source of Volatiles.”

Research Process

1. Sample Collection and Processing

The research team obtained 14 lunar rock samples from NASA’s Apollo program, including low- and high-Ti mare basalts, volcanic glasses, and highland rocks. These samples were classified as “pristine,” meaning they were not contaminated by impacts. To ensure data accuracy, the researchers also analyzed terrestrial San Carlos olivine and UWG2 garnet as control samples.

2. Oxygen Isotope Analysis

All samples were analyzed using laser fluorination. The research team designed a new laser fluorination line to improve the precision and automation of the analysis. Specific steps included: - Sample Pretreatment: Samples were heated overnight under vacuum and pre-melted to reduce surface contamination. - Fluorination Reaction: BrF5 was used as the fluorinating agent, and oxygen was released by laser heating the samples. - Gas Purification: Impurities were removed using cold traps and molecular sieve columns to ensure the purity of the oxygen. - Mass Spectrometry Analysis: A Thermo MAT253plus mass spectrometer was used for dual-inlet mode measurements to record oxygen isotope ratios (δ17O and δ18O).

3. Data Analysis

The researchers conducted detailed analyses of the oxygen isotope data from lunar and terrestrial samples and compared them with published data. By calculating the differences in δ17O, they assessed the oxygen isotope identity between Earth and the Moon. Additionally, the study explored the volatile content in lunar rocks and its implications for Moon formation models.

Key Findings

1. Oxygen Isotope Identity Between Earth and the Moon

The results showed that the oxygen isotope compositions of Earth and the Moon are identical at the sub-ppm (parts per million) level. Specifically, the average δ17O value of lunar rocks was -51.4 ± 1.4 ppm, nearly identical to the δ17O value of Earth’s mantle (-51.6 ± 1.0 ppm). This finding indicates no significant difference in oxygen isotope composition between Earth and the Moon, further supporting the idea that Theia and proto-Earth had similar oxygen isotope compositions.

2. Volatile Content in Lunar Rocks

The study also found that the volatile content in lunar rocks is comparable to that of Earth’s mantle, contradicting the traditional “dry Moon” model. In particular, the water content in lunar volcanic glasses was as high as 150 ppm, suggesting that the Moon’s interior may contain water levels similar to Earth’s. This finding challenges the assumption that volatiles were completely lost during Moon formation.

3. Implications for Moon Formation Models

Based on the oxygen isotope identity, the researchers proposed several possible Moon formation models: - Similar Oxygen Isotope Compositions of Theia and Proto-Earth: If Theia and proto-Earth had similar oxygen isotope compositions, the oxygen isotope identity between Earth and the Moon can be explained. - Intense Post-Impact Mixing: If Theia and proto-Earth had different oxygen isotope compositions, intense mixing after the collision could have led to the oxygen isotope identity between Earth and the Moon. - Loss of Theia’s Silicate Mantle: Another possibility is that Theia lost its silicate mantle before the collision, resulting in the Moon being primarily composed of Earth’s mantle material.

Conclusions and Significance

Through high-precision oxygen isotope measurements, this study revealed the high degree of oxygen isotope identity between Earth and the Moon, providing new constraints for Moon formation models. The results suggest that Theia and proto-Earth may have had very similar oxygen isotope compositions, or that intense mixing occurred after the collision. Additionally, the study indicates that water on Earth and the Moon may not have been delivered by the late veneer but rather originated from an early mixed reservoir.

This finding is significant for understanding the process of Moon formation and provides new insights into the origin of volatiles on Earth and the Moon. The study also highlights the importance of samples obtained through space missions, particularly compared to meteorite samples, which may have interacted with terrestrial water.

Research Highlights

1. High-Precision Oxygen Isotope Measurements: The research team achieved sub-ppm level oxygen isotope measurements using an improved laser fluorination method, providing high-precision data to support the study of Earth-Moon oxygen isotope identity.
2. Challenging the “Dry Moon” Model: The study found that the volatile content in lunar rocks is comparable to that of Earth’s mantle, challenging the traditional “dry Moon” model.
3. New Moon Formation Models: The research proposed several possible Moon formation models, including similar oxygen isotope compositions of Theia and proto-Earth, intense post-impact mixing, and the loss of Theia’s silicate mantle, providing new directions for future research.

Additional Valuable Information

The study also noted that future lunar sample return missions should focus on obtaining mantle rocks from the Moon to further validate Moon formation models. Additionally, the automated laser fluorination line developed by the research team provides a new technological tool for high-precision oxygen isotope analysis, which can be widely applied in other planetary science and geochemistry studies.