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

The abundances of oxygen isotopes in the most refractory mineral phases (calcium-aluminium-rich inclusions, CAIs) in meteorites 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 nebula, but the lack of a similar isotope trend in other elements argues against this explanation. A symmetry-dependent fractionation mechanism may have occurred in the inner solar nebula, but experimental evidence is lacking. Isotope-selective photodissociation of CO in the inner-most solar nebula might explain the CAI data, but the high temperatures in this region would have rapidly erased the signature. 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 ∼10 ³ 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.