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 cometary comae 67P/Churyumov-Gerasimenko 1P/Halley, respectively. In both cases, a surprisingly large amount molecular oxygen (O 2 ) was detected well correlated with observed abundances H O. Laboratory experiments simulating chemical processing for various astronomical environments already showed that formation solid state O is linked to water. However, quantitative study upon UV photolysis pure dominated interstellar ice analogues still missing. Aims. goal this work investigate whether irradiation O-rich produced at earliest stages star efficient enough explain abundance . Methods. photochemistry 16 (H 18 O) as mixed O:CO (ratio 100:11, 100:22, 100:44) :O (100:22:2) ices quantified during photolysis. Laser desorption post-ionisation time flight mass spectrometry (LDPI TOF MS) used probe in function fluence. Results. Upon amorphous ice, deposited 20 K, of, respectively, (0.9 ± 0.2)% /H (1.3 0.3)% O). To best our knowledge, first characterisation kinetics process. During ices, relative compared increases level (1.6 0.4)% (for ratio 100:22), while yield remains similar an enriched (2%), up 7% regard O, low fluence, which higher than expected on basis enrichment alone. resulting values derived may account (substantial) part high amounts found 67P 1P.