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Determination of the initial hydrogen isotopic composition of the solar system


The initial isotopic composition of water in the Solar System is of paramount importance to understanding the origin of water on planetary bodies but remains unknown, despite numerous studies1,2,3,4,5. Here we use the isotopic composition of hydrogen in calcium–aluminium-rich inclusions (CAIs) from primitive meteorites, the oldest Solar System rocks, to establish the hydrogen isotopic composition of water at the onset of Solar System formation. We report the hydrogen isotopic composition of nominally anhydrous minerals from CAI fragments trapped in a once-melted host CAI. Primary minerals have extremely low D/H ratios, with δD values down to −850‰, recording the trapping of nebular hydrogen. Minerals rich in oxidised iron formed before the capture of the fragments record the existence of a nebular gas reservoir with an oxygen fugacity substantially above the solar value and a D/H ratio within 20% of that of the Earth’s oceans. Hydrogen isotopes also correlate with oxygen and nitrogen isotopes, indicating that planetary reservoirs of volatile elements formed within the first 2 × 105 years of the Solar System, during the main CAI formation epoch. We propose that the isotopic composition of inner Solar System water was established during the collapse of the protosolar cloud core owing to a massive admixture of interstellar water.

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Fig. 1: Petrographic description of CAI E101.1.
Fig. 2: Distribution of H isotopic compositions.
Fig. 3: Comparison between H isotopes and isotopes of other volatile elements in CAIs and planetary components.
Fig. 4: Schematic representation of H isotope distribution in the innermost young Solar System during CAI formation.

Data availability

All data used in the manuscript are presented in the Supplementary Data and are available on request from the corresponding author.


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Discussions with L. Remusat and F. Robert and help from the NanoSIMS staff R. Duhamel and A. Gonzalez-Cano are appreciated. The FIB section preparation was partly supported by the French Renatech network. D. Troadec is warmly thanked. This work was supported by ATM grants from the Museum National d’Histoire Naturelle and by the French National Program of Planetology PNP INSU/CNRS.

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Authors and Affiliations



J.A. and A.A.-T. designed and supervised the study. D.L. characterized the sample, performed the NanoSIMS analyses and reduced the data. H.B. and H.K. characterized the standards by ERDA. F.B. performed scanning electron microscope imaging of the FIB sections. J.A., A.A.-T. and D.L. interpreted the data. J.A. and A.A.-T. wrote the manuscript with input from all other co-authors.

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Correspondence to J. Aléon.

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Nature Astronomy thanks Romain Tartese and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Discussion, Figs. 1–10 and references.

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Aléon, J., Lévy, D., Aléon-Toppani, A. et al. Determination of the initial hydrogen isotopic composition of the solar system. Nat Astron 6, 458–463 (2022).

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