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CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula

Abstract

The abundances of oxygen isotopes in the most refractory mineral phases (calcium-aluminium-rich inclusions, CAIs) in meteorites1 have hitherto defied explanation. Most processes fractionate isotopes by nuclear mass; that is, 18O is twice as fractionated as 17O, relative to 16O. In CAIs 17O and 18O are nearly equally fractionated, implying a fundamentally different mechanism. The CAI data were originally interpreted as evidence for supernova input of pure 16O into the solar nebula1, but the lack of a similar isotope trend in other elements argues against this explanation2. A symmetry-dependent fractionation mechanism3,4 may have occurred in the inner solar nebula5, but experimental evidence is lacking. Isotope-selective photodissociation of CO in the innermost solar nebula6 might explain the CAI data, but the high temperatures in this region would have rapidly erased the signature7. Here we report time-dependent calculations of CO photodissociation in the cooler surface region of a turbulent nebula. If the surface were irradiated by a far-ultraviolet flux 103 times that of the local interstellar medium (for example, owing to an O or B star within 1 pc of the protosun), then substantial fractionation of the oxygen isotopes was possible on a timescale of 105 years. We predict that similarly irradiated protoplanetary disks will have H2O enriched in 17O and 18O by several tens of per cent relative to CO.

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Figure 1: Model results for the time evolution of molecular abundances and isotope ratios at the nebula midplane at a heliocentric distance of 30  au.
Figure 2: The time evolution of the non-mass-dependent oxygen isotope component of total nebular H 2 O for a range of FUV flux enhancement factors.
Figure 3: Three-isotope plot (δ 17 O SMOW versus δ 18 O SMOW ) of total nebular H 2 O at the midplane, with time labelled along the trajectory.
Figure 4: Three-isotope plot of total nebular H2O in the model, including an estimate of wavelength-dependent absorption by H2.

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Acknowledgements

J.R.L. thanks K. McKeegan, J. Cuzzi and A. Boss for discussions, and P. Plavchan for assistance with IDL. J.R.L. acknowledges funding from the NASA Origins Program and from the UCLA Center for Astrobiology. E.D.Y. acknowledges support from the UCLA Center for Astrobiology.Author Contributions J.R.L. conceived and carried out the calculations presented here, using a modified version of a photochemical code provided by J. Kasting. The paper was written by J.R.L. and E.D.Y.

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Correspondence to J. R. Lyons.

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Supplementary Methods

A detailed description of the mathematics used in this work, including the following: the 1-D continuity equation for chemical volume fractions; the photodissociation loss rates for CO isotopologues, including the shielding functions; the power law expressions for the CO shielding functions including the correction terms to account for H2 absorption; the calculation of oxygen isotope δ-values. (DOC 85 kb)

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Lyons, J., Young, E. CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula. Nature 435, 317–320 (2005). https://doi.org/10.1038/nature03557

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