Striped areas represent relative O₂/H₂O abundances detected in cometary comae of 67P and 1P from Rubin et al. (2019). The data points represent the laboratory experiments presented here. Left panel: molecular abundance of O₂ normalised to the amount of H₂O at each UV dose for different initial compositions of the ice. Right panel: molecular abundance of O₂ enhanced by the decomposition of H₂O₂ (see text fordetails).