Dynamic molecular oxygen production in cometary comae

Abstract

Abundant molecular oxygen was discovered in the coma of comet 67P/Churyumov–Gerasimenko. Its origin was ascribed to primordial gaseous O2 incorporated into the nucleus during the comet's formation. This thesis was put forward after discounting several O2 production mechanisms in comets, including photolysis and radiolysis of water, solar wind–surface interactions and gas-phase collisions. Here we report an original Eley–Rideal reaction mechanism, which permits direct O2 formation in single collisions of energetic water ions with oxidized cometary surface analogues. The reaction proceeds by H2O+ abstracting a surface O-atom, then forming an excited precursor state, which dissociates to produce O2−. Subsequent photo-detachment leads to molecular O2, whose presence in the coma may thus be linked directly to water molecules and their interaction with the solar wind. This abiotic O2 production mechanism is consistent with reported trends in the 67P coma and raises awareness of the role of energetic negative ions in comets.

Origin of Molecular Oxygen in Comets: Current Knowledge and Perspectives

“…While endogenic radiolysis is unlikely to have produced all the observed O 2 today, it may have significantly contributed to it (Bouquet et al 2018). In situ O 2 formation is possible via water-group ions accelerated by the solar wind and energetic electrons hitting various oxidized surfaces and triggering Eley-Rideal (ER) reactions (Yao and Giapis 2017). Thus, O 2 may be formed on the surface of the nucleus, spacecraft, or the ROSINA/DFMS instrument itself (hyperthermal surface ionization).…”

“…alternative hypothesis concerning a class of rarelyconsidered Eley-Rideal mechanisms (ER) was recently proposed byYao and Giapis (2017) to explain the presence of O 2 in the coma of 67P/C-G. In these ER reactions, energetic water-derived ions (e.g.…”

Reply to “On the origin of molecular oxygen in cometary comae”

“…Heritier, Altwegg, Berthelier (HAB) et al discount the contribution of our proposed Eley-Rideal (ER) reaction mechanism 1 to the observed O 2 abundance 2 in the 67P/G–C coma by positing that: (1) the flux of energetic water-group ions (H 2 O + , H 3 O + , and OH + ) hitting the nucleus is not sufficient to produce the observed O 2 signal, and (2) there are no instrumental effects in response to energetic O 2 − ions and energetic O 2 neutrals entering the DFMS.…”

“…The ion flux deficiency was conceded in our paper 1 . However, HAB et al offer a new argument that shifts the debate.…”

Effect of the Surface Roughness of Icy Grains on Molecular Oxygen Chemistry in Molecular Clouds

“…For a thorough review of these different mechanisms, the reader is redirected to Luspay-Kuti et al (2018). To summarize briefly, though several mechanisms have been advanced to support the in situ production of O 2 (Dulieu et al 2017;Yao & Giapis 2017), the most likely origin for molecular oxygen in comets is a primordial one, i.e., it was incorporated in icy grains at the MC stage, which were themselves incorporated in comets. This scenario is supported by the observed profiles of O 2 and the other main cometary molecules (Bieler et al 2015;Heritier et al 2018) and by astrochemical models of various stages of star formation that predict a possible conservation of molecular oxygen formed during the MC stage throughout the remainder of the stellar system evolution up to the accretion of cometary bodies (e.g., Mousis et al 2016;Taquet et al 2016).…”