  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.
