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Project Progress

Oxygen isotopes in the solar nebula

In work supported both by Astrobiology (Young —PI; Lyons-Co-I) and by the NASA Origins program (PI-Lyons), J. Lyons has completed astrochemical modeling calculations relevant to the outer solar nebula. The goal of the modeling is to understand the role of isotope-selective photodissociation of CO in producing the well-known oxygen isotope anomaly in meteorites, and its impact on water in the nebula. Lyons converted a one-dimensional atmospheric chemistry code into a solar nebula chemistry code, and computed the expected isotopic composition of water at the nebula midplane. The required ultraviolet radiation to get sufficient CO dissociation at 20 — 30 AU from the protosun is consistent with birth of the solar system in a cluster of about 200 stars. This is an important step forward in understanding the implications of Clayton’s discovery over 30 years ago. This work was recently published in Nature.

E. Young has completed a fully-functional reaction network code that includes for the first time the mass-dependent oxygen isotope fractionation as well as photochemistry and parameterized radiative transfer. The code integrates the rates of 7063 relevant reactions among 546 chemical and isotopologue gas and solid species in the context of a circumstellar disk environment. It permits comparisons between the competing effects of mass-dependent fractionation and photolysis as a function of temperature, total gas pressures, cosmic ray photon flux, cosmic ray proton flux, stellar ultraviolet and x-ray fluxes, and 1-D advective transport. Results are just now being collated. Findings to date suggest regions of our own protoplanetary disk where the photodissociation of CO could have produced a reservoir of water ice with high ¹⁸O/¹⁶O ~ ¹⁷O/¹⁶O ratios as indicated by the meteorite record. It also shows where such a reservoir could not have been made.

These results represent substantial steps towards integrating the fields of astrochemistry and cosmochemistry with the goal of placing the history of the early solar stystem on a firmer footing.