Abstract
Optically thin surfaces of the solar circumstellar disk were likely sites for generating 16O isotope variability in the early Solar System. Astrochemical reaction network calculations predict that a robust feature of these photoactive horizons of the disk was conversion of CO gas to 16O-poor (high Δ 17O) H 2O ice on a timescale of 10 5 yr. Within several AU of the central star ultraviolet fluxes were too great for the oxygen isotopic effects of CO photodissociation to be sequestered in H 2O, meaning that the CO self shielding oxygen isotopic effect was an outer disk phenomenon. Calculations depicting transport in the circumstellar disk suggest that CO photodissociation at disk surfaces triggered a wave of high-Δ 17O H 2O that passed from surface regions through the outer disk and into the rocky planet-forming region on a timescale of 10 5 to 10 6 yr.
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