Halogen-containing species at Comet 67P/Churyumov-Gerasimenko: Full mission results

Dhooghe et al. (2017) studied halogen-bearing compounds in the coma of 67P/C-G with the Double Focusing Mass Spectrometer (DFMS) of Rosetta’s ROSINA instrument during a few time periods from first encounter up to perihelion (August 2014-August 2015). The main halogen-bearing compounds identified in the comet atmosphere were the hydrogen halides HF (hydrogen fluoride), HCl (hydrogen chloride) and HBr (hydrogen bromide). The halogen to oxygen ratios were found to vary between ~10^-4 (Cl/O and F/O) to ~10^-6 (Br/O), which shows these compounds have a very low abundance. In a follow-up article, De Keyser et al. (2017) observed an increase in the halogen-to-oxygen ratio as a function of distance, which suggests a distributed source for HF and HCl, probably through progressive release of these compounds from grains. Fayolle et al. 2017 and recent work by Altwegg et al. 2020 show that also CH₃Cl and NH₄Cl, respectively are present in the coma.

 

To further our knowledge on halogen containing species, we have applied recent improvements in DFMS data analysis techniques (De Keyser et al. 2019) to obtain a high quality time series for the complete mission duration. These data analysis techniques improve the retrieval of the abundances for overlapping mass peaks (¹⁸OH⁺ for F⁺, H₂¹⁸O⁺ for HF⁺, H³⁴S⁺ for ³⁵Cl⁺, and ³⁶Ar⁺ and H₂³⁴S⁺ for H³⁵Cl⁺). The contribution of CS₂⁺⁺ to the signal of H³⁷Cl⁺ has been determined from data for CS₂⁺.

 

Based on the full mission data, and focusing on chlorine, we determine the ³⁷Cl/³⁵Cl isotopic ratio. An interesting finding is that the ³⁵Cl⁺/H³⁵Cl⁺ and ³⁷Cl⁺/H³⁷Cl⁺ ratios in the DFMS mass spectrometer do not match the NIST ones for the H³⁵Cl and H³⁷Cl parents. This indicates that at least one additional chlorine source must be present. The variability of halogen-containing species as a function of time is discussed, as well as the possible role of distributed sources.

 

Altwegg, K. et al. (2020): Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae. Nature Astronomy, in press

Dhooghe, F. et al (2017): Halogens as tracers of protosolar nebula material in comet 67P/Churyumov-Gerasimenko, MNRAS, 472, Issue 2, 1336, doi 10.1093/mnras/stx1911.

De Keyser, J. et al (2017): Evidence for distributed gas sources of hydrogen halides in the coma of comet 67P/Churyumov–Gerasimenko, MNRAS, 469, Issue Suppl_2, S695, doi 10.1093/mnras/stx2725.

De Keyser, J. et al. (2019): Position-dependent microchannel plate gain correction in Rosetta's ROSINA/DFMS mass spectrometer. IJMS, 446, 116232, doi 10.1016/j.ijms.2019.116232.

Fayolle et al. (2017): Protostellar and cometary detections of organohalogens. Nature Astronomy 1, 703, doi.org/10.1038/s41550-017-0237-7