Dynamic molecular oxygen production in cometary comae

Abstract

Abundant molecular oxygen was discovered in the coma of comet 67P/Churyumov–Gerasimenko. Its origin was ascribed to primordial gaseous O₂ incorporated into the nucleus during the comet's formation. This thesis was put forward after discounting several O₂ 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 O₂ 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 O₂−. Subsequent photo-detachment leads to molecular O₂, whose presence in the coma may thus be linked directly to water molecules and their interaction with the solar wind. This abiotic O₂ production mechanism is consistent with reported trends in the 67P coma and raises awareness of the role of energetic negative ions in comets.

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