Triple Oxygen Isotopes of Lunar Water Reveal Indigenous and Cometary Heritage

The origin of water on the Moon has long been a pivotal question in planetary science, especially with the increasing need for water resources in the race to establish lunar bases. Potential sources of lunar water include indigenous lunar components, water production by solar wind, and the delivery of water through meteoritic and cometary impacts. However, due to the scarcity of lunar water, traditional analytical techniques have struggled to provide precise measurements of its isotopic composition, limiting our understanding of its origins. To address this, researchers developed a high-precision analytical technique capable of measuring the triple oxygen isotopes (δ¹⁷O and δ¹⁸O) in ultra-small samples, allowing for a more detailed investigation into the sources of lunar water.

Research Process

1. Sample Selection and Preparation

The study analyzed nine Apollo mission samples, including basalts, breccias, and regolith, representing diverse geographical locations on the lunar surface. These samples were chosen to cover a wide range of exposure ages and mineral compositions. Terrestrial and meteoritic samples were also analyzed as controls to ensure the accuracy of the measurements.

2. Water Extraction and Isotopic Analysis

The researchers employed a stepwise heating technique to extract water from the lunar samples. The process involved three heating steps:

• 50°C Heating: Released physisorbed molecular water (H₂O), which could potentially be contaminated by Earth’s atmosphere.
• 150°C Heating: Released any remaining physisorbed water, further eliminating terrestrial contamination.
• 1000°C Heating: Released tightly bound water, primarily from glasses and structural OH, which is most likely to reflect the isotopic composition of primordial lunar water.

The extracted water was then converted to molecular oxygen (O₂) through fluorination, and its oxygen isotopic composition was measured using a dual-inlet isotope ratio mass spectrometer.

3. Data Analysis

The researchers used a three-isotope approach to analyze the sources of lunar water. The isotopic composition of oxygen (δ¹⁷O and δ¹⁸O) was used to identify mass-dependent or mass-independent fractionation processes, which can reveal the origins of the water. By comparing the isotopic signatures of lunar water with potential sources such as meteorites and comets, the researchers were able to infer the contributions of different reservoirs to the lunar water inventory.

Key Findings

1. Isotopic Characteristics of Lunar Water

The results showed that lunar water predominantly exhibits high δ¹⁷O values (≥0‰), consistent with signatures from enstatite chondrites, ordinary chondrites, and CI-type carbonaceous chondrites. The mixing trends between δ¹⁷O and δ¹⁸O largely overlap with enstatite chondrite signatures, while the other end of the mixing line aligns with cometary δ¹⁸O values, suggesting a secondary contribution from comets.

2. Water Release at Different Heating Steps

The amount of water released and its isotopic composition varied significantly across the three heating steps. For example, sample 10060 released the most water at 50°C, while samples 10057 and 79035 released the most water at 1000°C. This indicates that the primary water-bearing phases differ among samples and are closely related to their mineralogical composition.

3. Constraints on Cometary Contributions

The study also provided estimates for cometary δ¹⁷O, ranging from 0.75‰ to 1.75‰. This offers valuable constraints on the potential contribution of cometary water to the Moon.

Conclusions

The findings suggest that the primary source of lunar water is likely indigenous, with an isotopic signature inherited from the early Earth. Additionally, comets may have contributed to the lunar water inventory, particularly in certain regions of the Moon. This research not only enhances our understanding of the origin of lunar water but also provides critical insights for future lunar exploration and resource utilization.

Research Highlights

1. High-Precision Analytical Technique: The study developed a method for measuring triple oxygen isotopes in ultra-small samples, offering a new tool for investigating the origins of lunar water.
2. Multi-Source Mixing Model: Through triple oxygen isotope analysis, the researchers proposed a multi-source mixing model for lunar water, revealing the combined contributions of indigenous lunar water and cometary water.
3. Quantitative Estimates of Cometary Contributions: The study provided the first estimates of cometary δ¹⁷O, laying the groundwork for quantitative assessments of cometary contributions to lunar water.

Significance of the Research

This research not only provides new insights into the origin of lunar water but also offers valuable information for future lunar exploration and resource utilization. By understanding the sources and distribution of lunar water, researchers can better plan future missions to ensure the sustainable use of water resources on the Moon. Additionally, the high-precision analytical techniques developed in this study can be applied to other areas of planetary science, opening new avenues for research.

Additional Insights

The study also highlighted that the isotopic composition of lunar water may be influenced by various processes, including meteorite impacts, solar wind, and cosmic rays. These processes not only alter the isotopic signatures of lunar water but also affect its distribution across the lunar surface. Future research could further explore these processes to gain a more comprehensive understanding of the origin and evolution of lunar water.