Time-dependent oxygen isotopic effects of CO self shielding across the solar protoplanetary disk

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 Δ17O) H2O ice on a timescale of 105 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 H2O, 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-Δ17O H2O that passed from surface regions through the outer disk and into the rocky planet-forming region on a timescale of 105 to 106 yr.