Formation of cometary O$_{2}$ ice and related ice species on grain surfaces in the midplane of the pre-Solar nebula

TL;DR

This study uses chemical modeling to demonstrate that O$_{2}$ ice observed in comets could have formed in the pre-Solar nebula's midplane, highlighting a specific radial zone and the sensitivity of the process to chemical parameters.

Contribution

It provides a detailed chemical kinetics model showing in-situ formation of cometary O$_{2}$ ice in the pre-Solar nebula, supporting a primordial origin hypothesis.

Findings

O$_{2}$ ice can form in the pre-Solar nebula at 120-150 AU.

The model reproduces observed O$_{2}$ and related ice abundances in comets.

Chemical parameters significantly influence ice formation outcomes.

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.