Formation of cometary O${}_2$ ice and related ice species on grain surfaces in the midplane of the pre-Solar nebula

Abstract

[Abridged] The detection of abundant O${}_2$ ice at 1-10% with respect to H${}_2$O ice in the comae of comets 1P/Halley and 67P/Churyumov-Gerasimenko motivated attempts to explain the origin of the high O${}_2$ ice abundance. Recent chemical modelling of the outer, colder regions of a protoplanetary disk midplane has shown production of O${}_2$ ice at the same abundance as that measured in the comet. An updated chemical kinetics code is utilised here to evolve chemistry under pre-Solar nebula midplane conditions. Four different chemical starting conditions, and the effects of various chemical parameters are tested. The parameter space investigation revealed a sweet spot for production of O${}_2$ ice at an abundance matching those in 67P and 1P, and O${}_3$ and H${}_2$O${}_2$ ices abundances matching those in 67P. This means that there is a radial region in the pre-Solar nebula from 120-150 AU, within which O${}_2$ could have been produced in-situ via ice chemistry on grain surfaces. However, it is apparent that there is a high degree of sensitivity of the chemistry to the assumed chemical parameters (e.g. binding energy, activation barrier width, and quantum tunnelling barrier). Hence, because the more likely scenario starting with a percentage of elemental oxygen locked in O${}_2$ also reproduces the O${}_2$ ice abundance in 67P at early stages, this supports previous suggestions that the cometary O${}_2$ ice could have a primordial origin.

Accepted for publication in Astronomy and Astrophysics. 22 pages, 15 figures