Production of O₂ through dismutation of H₂O₂ during water ice desorption: a key to understanding comet O₂ abundances

¹ LERMA, Université de Cergy Pontoise, Sorbonne Universités, UPMC Univ. Paris 6, PSL Research University, Observatoire de Paris, UMR 8112 CNRS, 5 mail Gay Lussac, 95000 Cergy Pontoise, France
e-mail: Francois.Dulieu@obspm.fr
² LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris-Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France

Received: 7 March 2016
Accepted: 11 August 2016

Abstract

Context. Detection of molecular oxygen and prediction of its abundance have long been a challenge for astronomers. The low abundances observed in few interstellar sources are well above the predictions of current astrochemical models. During the Rosetta mission, an unexpectedly high abundance of O₂ was discovered in the comet 67P/Churyumov-Gerasimenko’s coma. A strong correlation between O₂ and H₂O productions is observed, whereas no such correlation is observed between O₂ and either of CO or N₂.

Aims. We suggest that the O₂ molecule may be formed during the evaporation of water ice. We propose a possible reaction: the dismutation of H₂O₂ (2 H₂O₂−→ 2 H₂O + O₂), a molecule which should be co-produced during the water ice mantle growth on dust grains. We aim to test this hypothesis under realistic experimental conditions.

Methods. We performed two sets of experiments. They consist of producing a mixture of D₂O and D₂O₂ via the reaction of O₂ and D on a surface held at 10 K. The first set is made on a silicate substrate, and explores the limit of thin films, in order to prevent any complication due to trapping during the desorption. The second set is performed on a pre-deposited H₂O ice substrate and mimics the desorption of mixed ice.

Results. In thin films, O₂ is produced by the dismutation of H₂O₂, even at temperatures as low as 155 K. Mixed with water, H₂O₂ desorbs after the water ice sublimation and even more desorption of O₂ is observed.

Conclusions. H₂O₂, synthesised during the growth of interstellar ices (or by later processing), desorbs at the latest stage of the water sublimation and undergoes the dismutation reaction. Therefore an O₂ release in the gas phase should occur at the end of the evaporation of ice mantles. Temperature gradients along the geometry of clouds, or interior of comets, should blend the different stages of the sublimation. Averaged along the whole process, a mean value of the O₂/H₂O ratio of a few percent in the gas phase seems plausible.