Dynamic molecular oxygen production in cometary comae

Abundant molecular oxygen was discovered in the coma of comet 67P/Churyumov-Gerasimenko. Its origin was ascribed to primordial gaseous O-2 incorporated into the nucleus during the comet's formation. This thesis was put forward after discounting several O-2 production mechanisms in comets, including photolysis and radiolysis of water, solar wind-surface interactions and gas-phase collisions. Here we report an original Eley-Rideal reaction mechanism, which permits direct O-2 formation in single collisions of energetic water ions with oxidized cometary surface analogues. The reaction proceeds by H2O+ abstracting a surface O-atom, then forming an excited precursor state, which dissociates to produce O-2(-). Subsequent photo-detachment leads to molecular O-2, whose presence in the coma may thus be linked directly to water molecules and their interaction with the solar wind. This abiotic O-2 production mechanism is consistent with reported trends in the 67P coma and raises awareness of the role of energetic negative ions in comets.