Triple oxygen isotopes of lunar water unveil indigenous and cometary heritage

The origin of water in the Earth-Moon system is a pivotal question in planetary science, particularly with the need for water resources in the race to establish lunar bases. The candidate origins of lunar water are an indigenous lunar component, solar wind water production, and the delivery of meteoritic and cometary material. Characterizing the oxygen isotopic composition of water provides information on lunar oxygen sources. The scarcity of lunar water required the development of a high-precision analytical technique for small samples. This method employs stepwise heating, fluorination, and oxygen isotopic measurements using a dual inlet isotope ratio mass spectrometer. The three heating steps were selected based on other extraterrestrial material studies to release loosely bound water that may have been terrestrially contaminated (50 °C), loosely bound water (150 °C), and tightly bound water (as OH) (1,000 °C). This method was applied to a suite of 9 Apollo samples (basalts, breccias, and a regolith), along with terrestrial and meteoritic controls. We present here measurements of the triple oxygen isotopic composition of this water. Our data predominantly show high Δ'¹⁷O values (≥ 0‰) for lunar water. These values are consistent with enstatite, ordinary, and CI chondrite-like signatures, although coupling Δ'¹⁷O with δ¹⁸O forms mixing trends that dominantly overlap enstatite signatures. The other end of the mixing line is in the positive Δ'¹⁷O space with cometary δ¹⁸O values, providing constraints for cometary Δ'¹⁷O between 0.75 to 1.75‰.