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

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 H 2 O. Previous studies have indicated that the likely origin of the O 2 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 O 2 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 O 2 abundance can be produced and is fairly robust to the choice of the free parameters. Good matches for the O 2 :H 2 O ratio and, to a lesser extent, the N 2 :CO and CO:H 2 O ratios are obtained, but the models significantly over-produce N 2 . We speculate that the low value of N 2 :O 2 that is observed is a consequence of the specific thermal history of the comet.

Molecule-dependent oxygen isotopic ratios in the coma of comet 67P/Churyumov–Gerasimenko

“…Interaction between energetic water ions and the surface of 67P is very unlikely to yield highly fractionated O2. This leaves a primordial origin of O2 as described in Taquet et al (2016) for grain-surface chemistry or gas phase chemistry as described in Rawlings et al (2019).…”

Prestellar grain-surface origins of deuterated methanol in comet 67P/Churyumov–Gerasimenko

“…Some works argue that the only way that O 2 can be produced at the abundance level ∼4 per cent relative to water in 67P/C-G and show such a strong correlation with water is for it to be formed in a prestellar core that has a slightly elevated temperature of ∼15-25 K (in contrast to the typical core temperature of 10 K) through a combination of gas-phase and grain-surface processes (Taquet et al 2016(Taquet et al , 2018Eistrup & Walsh 2019). However, other models claim this to not necessarily be the case (Garrod 2019;Rawlings, Wilson & Williams 2019). Most of the alternative mechanisms summarized in Luspay-Kuti et al ( 2018) have been ruled out (e.g.…”

Water ice deposition and growth in molecular clouds

“…In our model we follow the procedures described in the STARCHEM model (Rawlings, Keto & Caselli 2021) and pay careful attention to (a) the composition of individual ice layers, and (b) the geometrical growth of grains due to ice deposition. For (a) we note that an important consequence of the ice layering/surface layer desorption scenario is that the ices act as a 'stack' that records and retains the prior chemical evolution of a cloud; a fact that has been used, for example, to explain the abundance anomalies in cometary ices (Rawlings, Wilson & Williams 2019). Most desorption processes only occur in the upper layer(s) of the ices, so we model the layering of the ices and follow the chemical composition of each individual layer.…”