Dynamic molecular oxygen production in cometary comae

Yunxi Yao,Konstantinos P Giapis

doi:10.1038/ncomms15298

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.

Highlights

Abundant molecular oxygen was discovered in the coma of comet 67P/Churyumov– Gerasimenko
We first demonstrate the production of molecular oxygen and hydroperoxyl radicals, detected as anions, from H2O þ ions bombarding Si, and Fe targets
We have uncovered high-energy reaction channels for dynamic production of negative ions from collisions of energetic water ions with oxidized surfaces. The latter surfaces include: SiOx, FeOy, Pt(O), NiOz, Pd(O), Au(O) and TiOw. Such interactions are applicable to plasmas and astrophysical environments whenever H2O þ ions are encountered with kinetic energies between 50 and 300 eV

Summary

Introduction

Abundant molecular oxygen was discovered in the coma of comet 67P/Churyumov– Gerasimenko. 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. Collisions of energetic water ions with oxidized minerals on the nucleus surface are probable, with several possible outcomes: (1) collision-induced dissociation (CID) of H2O þ produces atomic O, atomic H and OH radicals and ions; (2) physical sputtering ejects mineral constituents, including metal and oxygen atoms; and (3) collisional excitation of H2O þ drives an intramolecular water-splitting reaction[18], producing molecular H2 directly.

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Results

Conclusion