Quantification of O₂ formation during UV photolysis of water ice: H₂O and H₂O:CO₂ ices

¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: bulak@strw.leidenuniv.nl
² Research Laboratory for Astrochemistry, Ural Federal University, Kuibysheva St. 48, 620026 Ekaterinburg, Russia
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 26 July 2021
Accepted: 30 August 2021

Abstract

Context. The Rosetta and Giotto missions investigated the composition of the cometary comae of 67P/Churyumov-Gerasimenko and 1P/Halley, respectively. In both cases, a surprisingly large amount of molecular oxygen (O₂) was detected and was well correlated with the observed abundances of H₂O. Laboratory experiments simulating chemical processing for various astronomical environments already showed that formation of solid state O₂ is linked to water. However, a quantitative study of O₂ formation upon UV photolysis of pure H₂O and H₂O dominated interstellar ice analogues is still missing.

Aims. The goal of this work is to investigate whether the UV irradiation of H₂O-rich ice produced at the earliest stages of star formation is efficient enough to explain the observed abundance of cometary O₂.

Methods. The photochemistry of pure H₂¹⁶O (H₂¹⁸O) as well as mixed H₂O:CO₂ (ratio of 100:11, 100:22, 100:44) and H₂O:CO₂:O₂ (100:22:2) ices was quantified during UV photolysis. Laser desorption post-ionisation time of flight mass spectrometry (LDPI TOF MS) was used to probe molecular abundances in the ice as a function of UV fluence.

Results. Upon UV photolysis of pure amorphous H₂O ice, deposited at 20 K, formation of O₂ and H₂O₂ is observed at abundances of, respectively, (0.9 ± 0.2)% (O₂/H₂O) and (1.3 ± 0.3)% (H₂O₂/H₂O). To the best of our knowledge, this is the first quantitative characterisation of the kinetics of this process. During the UV photolysis of mixed H₂O:CO₂ ices, the formation of the relative amount of O₂ compared to H₂O increases to a level of (1.6 ± 0.4)% (for H₂O:CO₂ ratio of 100:22), while the (H₂O₂/H₂O) yield remains similar to experiments with pure water. In an ice enriched with O₂ (2%), the O₂ level increases up to 7% with regard to H₂O, at low UV fluence, which is higher than expected on the basis of the enrichment alone. The resulting O₂/H₂O values derived for the H₂O and H₂O:CO₂ ices may account for a (substantial) part of the high oxygen amounts found in the comae of 67P and 1P.