A primordial origin for molecular oxygen in comets: a chemical kinetics study of the formation and survival of O₂ ice from clouds to discs

Molecular oxygen has been confirmed as the fourth most abundant molecule in cometary material (O2/H2O ∼ 4 per cent) and is thought to have a primordial nature, i.e. coming from the interstellar cloud from which our Solar system was formed. However, interstellar O2 gas is notoriously difficult to detect and has only been observed in one potential precursor of a solar-like system. Here, the chemical and physical origin of O2 in comets is investigated using sophisticated astrochemical models. Three origins are considered: (i) in dark clouds; (ii) during forming protostellar discs; and (iii) during luminosity outbursts in discs. The dark cloud models show that reproduction of the observed abundance of O2 and related species in comet 67P/C-G requires a low H/O ratio facilitated by a high total density (≥105 cm−3), and a moderate cosmic ray ionization rate (≤10−16 s−1) while a temperature of 20 K, slightly higher than the typical temperatures found in dark clouds, also enhances the production of O2. Disc models show that O2 can only be formed in the gas phase in intermediate disc layers, and cannot explain the strong correlation between O2 and H2O in comet 67P/C-G together with the weak correlation between other volatiles and H2O. However, primordial O2 ice can survive transport into the comet-forming regions of discs. Taken together, these models favour a dark cloud (or ‘primordial’) origin for O2 in comets, albeit for dark clouds which are warmer and denser than those usually considered as Solar system progenitors.