  The Alice far-ultraviolet imaging spectrograph onboard Rosetta observed
emissions from atomic and molecular species from within the coma of comet
67P/Churyumov-Gerasimenko during the entire escort phase of the mission from
2014 August to 2016 September. The initial observations showed that emissions
of atomic hydrogen and oxygen close to the surface were produced by energetic
electron impact dissociation of H2O. Following delivery of the lander, Philae,
on 2014 November 12, the trajectory of Rosetta shifted to near-terminator
orbits that allowed for these emissions to be observed against the shadowed
nucleus that, together with the compositional heterogeneity, enabled us to
identify unique spectral signatures of dissociative electron impact excitation
of H2O, CO2, and O2. CO emissions were found to be due to both electron and
photoexcitation processes. Thus we are able, from far-ultraviolet spectroscopy,
to qualitatively study the evolution of the primary molecular constituents of
the gaseous coma from start to finish of the escort phase. Our results show
asymmetric outgassing of H2O and CO2 about perihelion, H2O dominant before and
CO2 dominant after, consistent with the results from both the in situ and other
remote sensing instruments on Rosetta.
