Abstract
ABSTRACT A full-mission analysis has been conducted of Cl-bearing species in the coma of comet 67P/Churyumov–Gerasimenko as detected by the Double Focusing Mass Spectrometer (DFMS) of Rosetta's ROSINA instrument. The isotope ratio of the two stable chlorine isotopes 37Cl/35Cl is found to be 0.336 ± 0.017, to be compared with the standard mean ocean chloride value of 0.320. The isotope ratio does not change appreciably throughout the mission. The Cl-bearing species fingerprint in DFMS indicates that there is at least one additional chlorine-bearing species in the coma next to HCl, CH3Cl, and NH4Cl. The identity of this volatile or semivolatile species is unknown at this time.
Highlights
ED The Double Focusing Mass Spectrometer (DFMS), part of Rosetta’s ROSINA instrument (Balsiger et al 2007), has been remarkably successful in investigating the atmo-N sphere of comet 67P/Churyumov-Gerasimenko in exquisite detail
N ting the micro-channel plate (MCP) at the acceleration energy used in DFMS, A relative to the reference species and energy
For the analysis presented here, it is required that at least part of the comet is in the 20◦ × 20◦ field of view (FOV) of DFMS, that is θ < FOV + arctan dmin, (4)
Summary
ED The Double Focusing Mass Spectrometer (DFMS), part of Rosetta’s ROSINA instrument (Balsiger et al 2007), has been remarkably successful in investigating the atmo-. R ion source can produce multiple types of ions for each of the Oparent neutrals in the coma gas In some situations it has Dhooghe et al (2017) presented the first in-situ coma observations for the halogen-containing species HF, HCl and HBr. The Cl/O elemental abundance ratio was found to vary as a function of distance from the comet and a followup article by De Keyser et al (2017) explained these observations in terms of a distributed source model. The Cl+/HCl+ ratio measured in DFMS plays a key role in establishing which neutral chlorine-bearing species are present in the coma. The overall and position-dependent gain factors have been determined from on-board instrument calibration measurements throughout the mission (Schroeder et al 2019, supplementary material). N ting the MCP at the acceleration energy used in DFMS, A relative to the reference species and energy
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