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 H(2)O(+) 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.