Isotopic enhancements of ¹⁷O and ¹⁸O from solar wind particles in the lunar regolith

Differences in isotopic abundances between meteorites and rocks on Earth leave unclear the true composition of the gas out of which the Solar System formed^1-4. The Sun should have preserved in its outer layers the original composition, and recent work has indicated that the solar wind is enriched in ¹⁶O, relative to Earth, Mars and bulk meteorites ⁵. This suggests that self-shielding of CO due to photo-dissociation, which is a well understood process in molecular clouds, also led to evolution in the isotopic abundances in the early Solar System. Here we report measurements of oxygen isotopic abundances in lunar grains that were recently exposed to the solar wind. We find that ¹⁶O is under-abundant, opposite to an earlier finding⁵ based on studies of ancient metal grains. Our result, however, is more difficult to understand within the context of current models, because there is no clear way to make ¹⁶O more abundant in Solar System rocks than in the Sun.