# A gas-phase primordial origin of O2 in comet 67P/Churyumov-Gerasimenko

## TL;DR

This paper proposes a primordial gas-phase origin for molecular oxygen in comet 67P, supported by astrochemical modeling of ice evolution from dark clouds to the protosolar nebula, explaining observed O2 levels.

## Contribution

It introduces a new gas-phase formation pathway for O2 in comets, contrasting with surface chemistry hypotheses, supported by detailed astrochemical simulations.

## Key findings

- O2 abundance can be produced in primordial gas-phase conditions.
- Models match observed O2:H2O ratios in the comet.
- N2 is over-produced, suggesting a unique thermal history.

## Abstract

Recent observations made by the Rosetta/ROSINA instrument have detected molecular oxygen in the coma of comet 67P/Churyumov-Gerasimenko with abundances at the 1-10% level relative to H2O. Previous studies have indicated that the likely origin of the O2 may be surface chemistry of primordial (dark cloud) origin, requiring somewhat warmer, denser and extreme H-atom poor conditions than are usually assumed. In this study we propose a primordial gas-phase origin for the O2 which is subsequently frozen and effectively hidden until the ice mantles are sublimated in the comet's coma. Our study presents results from a three-phase astrochemical model that simulates the chemical evolution of ices in the primordial dark cloud phase, its gravitational collapse, and evolution in the early protosolar nebula. We find that the O2 abundance can be produced and is fairly robust to the choice of the free parameters. Good matches for the O2:H2O ratio and, to a lesser extent, the N2:CO and CO:H2O ratios are obtained, but the models significantly over-produce N2. We speculate that the low value of N2:O2 that is observed is a consequence of the specific thermal history of the comet.