O2 Activity in Comet 67P/Churyumov-Gerasimenko from Observations of
Electron Dissociative Excitation by the Rosetta-Alice Far-ultraviolet
Spectrograph
Abstract
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.