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

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 (O-2) 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 O-2 is linked to water. However, a quantitative study of O-2 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 O-2. Methods. The photochemistry of pure (H2O)-O-16 ((H2O)-O-18) as well as mixed H2O:CO2 (ratio of 100:11, 100:22, 100:44) and H2O:CO2:O-2 (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 O-2 and H2O2 is observed at abundances of, respectively, (0.9 +/- 0.2)% (O-2/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 O-2 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 O-2 (2%), the O-2 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 O-2/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.