Spatial and Temporal Variations of Atomic Species in the Coma of Comet 67P/Churyumov-Gerasimenko as Observed by Rosetta’s ALICE UV Spectrograph during Great Circle Scans

The Alice far-ultraviolet (FUV) imaging spectrograph on the Rosetta orbiter obtained spatially resolved spectra of 67P/Churyumov-Gerasimenko (67P) from 700-2050 Å with a spectral resolution of 8-12 Å. Observations of 67P were obtained by Alice continually from arrival at the comet in August 2014 through the end of the mission in September 2016. “Great Circle” observations were performed every few weeks from January 2015 through May 2016 to survey the coma away from the nucleus. These sequences consisted of a series of slews along a celestial great circle passing through the nucleus, e.g., covering off-nadir angles from approximately 0-180°, with pauses for observations by Alice and other instruments. Alice’s line of sight during these scans included signal to the edge of the coma, thus sampling very different parts of the coma than most other instruments.We report here on observations acquired during these Great Circle scans that allow us to investigate the spatial distributions of various emissions, as well as seasonal variations in the coma composition. Bright lines consistently included H Ly-b, the OI triplet near 1304 Å, CI near 1657 Å, and the SI triplet near 1820 Å. Spatial distributions of the OI, CI, and SI brightnesses have been determined and are being fitted with Haser models. The process is more complicated than for traditional remote sensing FUV observations due to Rosetta’s location in the coma and because resonant scattering does not always dominate the excitation. Preliminary modeling yields H₂O and CO₂ production rates consistent with contemporaneous measurements obtained by other instruments on Rosetta and production rates that generally peak a few weeks after perihelion. A surprising phenomenon is a slight increase in OI brightness at large off-nadir angles for some Great Circles while the other measured emissions continue to decrease. We are investigating possible explanations.Rosetta is an ESA mission with contributions from its member states and NASA. The Alice team acknowledges continuing support from NASA’s Jet Propulsion Laboratory through contract 1336850 to the Southwest Research Institute.