O 2 Activity in Comet 67P/churyumov-Gerasimenko from Observations of Electron Dissociative Excitation by the Rosetta-Alice Far-ultraviolet Spectrograph

We have shown that far-ultraviolet emissions of atomic hydrogen, oxygen, and carbon in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko result primarily from electron impact dissociative excitation of H2O, CO2, and O2. Our initial detection of gas outbursts was based on detection of enhanced atomic oxygen emissions relative to those expected from H2O or CO2 and were attributed to electron impact on O2. This spectral signature of O2 was also observed in long-term limb observations. Molecular oxygen was first reported to be a significant constituent of the coma from Rosetta/ROSINA mass spectrometer measurements. Of the remote sensing instruments on Rosetta, only the Alice far-ultraviolet spectrograph is capable of measuring spatial and temporal variations of O2, both from atomic emissions as well as from stellar absorption measurements. Here we report on the detection and use of the far-ultraviolet emissions to estimate the abundance of O2 relative to H2O along lines-of-sight above the limb, and its variation over the period February 2015 to January 2016, corresponding to heliocentric distances within ~2.0 AU.