A primordial origin for molecular oxygen in comets: A chemical kinetics study of the formation and survival of O${}_2$ ice from clouds to disks

Abstract

Molecular oxygen has been confirmed as the fourth most abundant molecule in cometary material O${}_2$/H${}_2$O $\sim 4$ %) and is thought to have a primordial nature, i.e., coming from the interstellar cloud from which our solar system was formed. However, interstellar O${}_2$ 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 O${}_2$ in comets is investigated using sophisticated astrochemical models. Three origins are considered: i) in dark clouds, ii) during forming protostellar disks, and iii) during luminosity outbursts in disks. The dark cloud models show that reproduction of the observed abundance of O${}_2$ and related species in comet 67P/C-G requires a low H/O ratio facilitated by a high total density ($\ge 10^5$ cm${}^{-3}$), and a moderate cosmic ray ionisation rate ($\le 10^{-16}$ s${}^{-1}$) while a temperature of 20 K, slightly higher than the typical temperatures found in dark clouds, also enhances the production of O${}_2$. Disk models show that O${}_2$ can only be formed in the gas phase in intermediate disk layers, and cannot explain the strong correlation between O${}_2$ and H${}_2$O in comet 67P/C-G together with the weak correlation between other volatiles and H${}_2$O. However, primordial O${}_2$ ice can survive transport into the comet-forming regions of disks. Taken together, these models favour a dark cloud (or "primordial") origin for O${}_2$ in comets, albeit for dark clouds which are warmer and denser than those usually considered as solar system progenitors.

Accepted for publication in MNRAS. 20 pages, 13 figures, 2 tables

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• Insight into the origin of cometary ices from Rosetta/ROSINA mass spectrometer data