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

Abstract

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.