The discovery in primitive components of meteorites1,2 of large oxygen isotopic variations that could not be attributed to mass-dependent fractionation effects has raised a fundamental question: what is the composition of the protosolar gas from which the host grains formed? This composition is probably preserved in the outer layers of the Sun, but the resolution of astronomical spectroscopic measurements is still too poor to be useful for comparison with planetary material3,4. Here we report a precise determination of the oxygen isotopic composition of the solar wind from particles implanted in the outer hundreds of nanometres of metallic grains in the lunar regolith. These layers of the grains are enriched in 16O by >20 ± 4‰ relative to the Earth, Mars and bulk meteorites, which implies the existence in the solar accretion disk of reactions—as yet unknown—that were able to change the 17O/16O and 18O/16O ratios in a way that was not dependent strictly on the mass of the isotope. Photochemical self-shielding of the CO gas irradiated by ultraviolet light5,6,7 may be one of these key processes, because it depends on the abundance of the isotopes, rather than their masses.
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Samples were provided by NASA. We thank J. Aléon, G. Libourel, B. Marty, A. Pack, F. Robert and Z. Sharp for discussions, B. Marty for encouragement, and K. Terada for help in sample preparation. This study was supported by the Mitsubishi Foundation, JSPS, MEXT, Région Lorraine, CNES-CSEEM and by INSU-PNP through a ‘Poste Rouge’ fellowship (K.H.).
The authors declare that they have no competing financial interests.
Depth profiles of Oxygen isotopic composition in metallic lunar grains. (XLS 59 kb)
Example of oxygen isotope depth profiles, plotted for grain 04-21 from lunar soil 79035, in which any negative Δ17O signatures were not observed. (DOC 60 kb)
Example of oxygen isotope depth profiles, plotted for grain 04-32 from lunar soil 79035, in which any negative Δ17O signatures were not observed. (DOC 59 kb)
Oxygen isotopic composition at surfaces of seven metallic grains from lunar soil 79035, which did not show significant negative Δ17O (that is, solar) signatures. (DOC 81 kb)
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Hashizume, K., Chaussidon, M. A non-terrestrial 16O-rich isotopic composition for the protosolar nebula. Nature 434, 619–622 (2005). https://doi.org/10.1038/nature03432
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