Ion composition at comet 67P near perihelion: Rosetta observations and model-based interpretation

2017
We present the ion composition in the comaof comet67P with newly detected ion species over the 28–37 u mass range, probed by Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Double Focusing Mass Spectrometer (DFMS). In summer 2015, the nucleus reached its highest outgassingrate and ion-neutral reactions started to take place at low cometocentric distances. Minor neutrals can efficiently capture protons from the ion population, making the protonated version of these neutrals a major ion species. So far, only NH+₄ has been reported at comet67P. However, there are additional neutral species with proton affinitieshigher than that of water (besides NH₃) that have been detected in the comaof comet67P: CH₃OH, HCN, H₂CO and H₂S. Their protonated versions have all been detected. Statistics showing the number of detections with respect to the number of scans are presented. The effect of the negative spacecraft potential probed by the Rosetta Plasma Consortium/ LAngmuir Probeon ion detection is assessed. An ionospheric model has been developed to assess the different ion density profiles and compare them to the ROSINA/DFMS measurements. It is also used to interpret the ROSINA/DFMS observations when different ion species have similar masses, and their respective densities are not high enough to disentangle them using the ROSINA/DFMS high-resolution mode. The different ion species that have been reported in the comaof 67P are summarized and compared with the ions detected at comet1P/Halley during the Giotto mission.
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