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The riddle of the halogens

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In the zoo of neutral gases discovered by Rosetta at comet 67P/Churyumov-Gerasimenko, the group of halogen-containing species represented by the saltwater fish grows. The ROSINA-DFMS instrument discovered ammonium chloride (NH4Cl) in a dust grain from the comet and a careful analysis of data from the complete mission revealed that chlorine exists in at least one other form different from HCl or NH4Cl.
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The ROSINA instrument

ROSINA’s DFMS sensor onboard Rosetta measured the gas around comet Chury. To do that, DFMS needed to charge (ionize) the cometary gas, sort the formed ions according to their mass and detect the number of ions at each mass.

This means that DFMS did not directly identify the neutrals around the comet; we in fact need to derive this from the masses and numbers of the measured ions. This identification is quite complex as often one type of detected ion in DFMS can be a result of the ionization of different neutral species. For example, O+ can be formed by ionization of H2O, CO or O2. We therefore need to understand how each neutral behaves in the instrument.

Which sea fish in the comet zoo?

In previous comet Chury research, we discovered species containing the halogens fluorine (F), chlorine (Cl) and bromine (Br) on Chury, mainly in the form of the hydrogen halides HF, HCl and HBr. We have labeled these species as “sea fish” because HCl in water is linked to sea salt. We also found evidence that these hydrogen halides are not readily released from the comet – they are semi-volatile – and mainly come from the dust grains around the comet.

However, one question remained concerning the chlorine-bearing species: we know the relative proportion of Cl+ to HCl+ ions for HCl, but we found a lot more Cl+ relative to HCl+ than is expected for neutral HCl alone. For sure, there must be an additional source of Cl in the cometary gas, but which one(s)?

The discovery of ammonium salts

Luckily, close to the end of the Rosetta mission, a dust grain entered the instrument, which allowed us to get a glimpse at the gas released directly from this grain. Not only did we find that the amount of HCl+ and Cl+ from dust was a lot higher than what comes directly from the comet, but also NH3+ was measured with DFMS in a considerably larger amount (see Figure 1). This indicates the presence of ammonium salts, which was confirmed by measuring several different ammonium salts (NH4X) in the instrument copy on Earth. The Cl+/HCl+ ratio for ammonium chloride (NH4Cl) is closer to the ratio obtained in the cometary gas.

An update on Cl+/HCl+

Since our initial work on the halogen-containing species, further improvements were made to the data analysis to correctly take into account instrumental effects. We applied the updated techniques to data from the complete mission to update our estimates of the Cl+/HCl+ ratio. The Cl+/HCl+ ratio for the complete mission was even higher than during the beginning of the mission. All these findings point to at least one additional source of Cl.

The SeVoCo project

At present, except for the newly discovered ammonium salts, not much is known about the chemical or physical form in which semi-volatile species are present on the comet. Thanks to the BRAIN-be 2.0 SeVoCo project we can continue our investigation of these largely unknown species using the same approach as for our saltwater fish.

products of ammonium salts in the gas from a fresh dust grain
Figure 1 Rosetta and the ‘fish’ in Chury’s atmosphere: products of ammonium salts in the gas from a fresh dust grain (Credits: Altwegg et al/Nature Astronomy)


Want to know more?

  • Altwegg, K., Balsiger, H., Hänni, N., Rubin, M., Schuhmann, M., Schroeder, I., Sémon, T., Wampfler, S., Berthelier, J.-J., Briois, C., Combi, M., Gombosi, T.I., Cottin, H., De Keyser, J., Dhooghe, F., Fiethe, B., and Fuselier, S.A. (2020). Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae. Nature Astronomy, 4(5), 533-540.

  • Dhooghe, F., De Keyser, J., Altwegg, K., Briois, C., Balsiger, H., Berthelier, J.-J., Calmonte, U., Cessateur, G., Combi, M.R., Equeter, E., Fiethe, B., Fray, N., Fuselier, S., Gasc, S., Gibbons, A., Gombosi, T., Gunell, H., Hässig, M., Hilchenbach, M., Le Roy, L., Maggiolo, R., Mall, U., Marty, B., Neefs, E., Rème, H., Rubin, M., Sémon, T., Tzou, C.-Y., and Wurz, P. (2017). Halogens as tracers of protosolar nebula material in comet 67P/Churyumov–Gerasimenko. Monthly Notices of the Royal Astronomical Society, 472(2), 1336-1345.

  • De Keyser, J., Altwegg, K., Gibbons, A., Dhooghe, F., Balsiger, H., Berthelier, J.-J., Fuselier, S.A., Gombosi, T.I., Neefs, E., and Rubin, M. (2019). Position-dependent microchannel plate gain correction in Rosetta's ROSINA/DFMS mass spectrometer. International Journal of Mass Spectrometry, 446, A116232. Open Access Logo

  • De Keyser, J., Gibbons, A., Dhooghe, F., Altwegg, K., Balsiger, H., Berthelier, J.-J., Fuselier, S.A., Gombosi, T.I., Neefs, E., and Rubin, M. (2019). Calibration of parent and fragment ion detection rates in Rosettas ROSINA/DFMS mass spectrometer. International Journal of Mass Spectrometry, 446, A116233.

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Comet 67P/Churyumov-Gerasimenko on Jan. 31, 2015. Image Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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