H₂O and O₂ absorption in the coma of comet 67P/Churyumov–Gerasimenko measured by the Alice far-ultraviolet spectrograph on Rosetta

Abstract : We have detected H₂O and O₂ absorption against the far-UV continuum of stars located on lines of sight near the nucleus of Comet 67P/Churyumov–Gerasimenko using the Alice imaging spectrograph on Rosetta. These stellar appulses occurred at impact parameters of ρ = 4–20 km, and heliocentric distances ranging from Rh = − 1.8 to 2.3 au (negative values indicate pre-perihelion observations). The measured H₂O column densities agree well with nearly contemporaneous values measured by VIRTIS-H. The clear detection of O₂ independently confirms the initial detection by the ROSINA mass spectrometer; however, the relative abundance of O₂/H₂O derived from the stellar spectra (11–68 per cent, with a median value of 25 per cent) is considerably larger than published values found by ROSINA. The cause of this difference is unclear, but potentially related to ROSINA measuring number density at the spacecraft position while Alice measures column density along a line of sight that passes near the nucleus.