H${}_2$O and O${}_2$ Absorption in the Coma of Comet 67P/Churyumov-Gerasimenko Measured by the Alice Far-Ultraviolet Spectrograph on Rosetta

Abstract

We have detected H${}_2$O and O${}_2$ 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 $\rho =4$-20 km, and heliocentric distances ranging from $R_h=-1.8$ to 2.3 AU (negative values indicate pre-perihelion observations). The measured H${}_2$O column densities agree well with nearly contemporaneous values measured by VIRTIS-H. The clear detection of O${}_2$ independently confirms the initial detection by the ROSINA mass spectrometer; however, the relative abundance of O${}_2$/H${}_2$O derived from the stellar spectra (11%-68%, with a median value of 25%) 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.

arXiv comment21 pages, accepted by MNRAS

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• Atomic carbon, nitrogen, and oxygen forbidden emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)
• FUV Spectral Signatures of Molecules and the Evolution of the Gaseous Coma of Comet 67P/Churyumov-Gerasimenko
• Ultraviolet Observations of Coronal Mass Ejection Impact on Comet 67P/Churyumov-Gerasimenko by Rosetta Alice
• Evolution of H${}_2$O Production in Comet C/2012 S1 (ISON) as Inferred from Forbidden Oxygen and OH Emission
• O2 signature in thin and thick O2-H2O ices
• Knowledge Gaps in the Cometary Spectra of Oxygen-Bearing Molecular Cations
• Stellar Occultation by Comet 67P/Churyumov-Gerasimenko Observed with Rosetta's Alice Far-Ultraviolet Spectrograph
• A photochemical model of ultraviolet atomic line emissions in the inner coma of comet 67P/Churyumov-Gerasimenko
• Upper Limits for Emissions in the Coma of Comet 67P/Churyumov-Gerasimenko Near Perihelion as Measured by Rosetta's Alice Far-Ultraviolet Spectrograph
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