Differences in isotopic abundances between meteorites and rocks on Earth leave unclear the true composition of the gas out of which the Solar System formed1,2,3,4. 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 meteorites5. 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 underabundant, 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.
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This file contains all Supplementary Information as follows: Supplementary Figure 1 and legend, Supplementary Figure 2 and legend, Supplementary Methods, and Supplementary Table 1. (DOC 223 kb)
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Nature Astronomy (2017)
Nature Geoscience (2011)
Space Science Reviews (2007)