First in-situ detection of the cometary ammonium ion NH⁺₄ (protonated ammonia NH₃) in the coma of 67P/C-G near perihelion

In this paper, we report the first in situ detection of the ammonium ion NH⁺₄ at 67P/Churyumov–Gerasimenko (67P/C-G) in a cometary coma, using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Double Focusing Mass Spectrometer (DFMS). Unlike neutral and ion spectrometers onboard previous cometary missions, the ROSINA/DFMS spectrometer, when operated in ion mode, offers the capability to distinguish NH⁺₄ from H₂O⁺ in a cometary coma. We present here the ion data analysis of mass-to-charge ratios 18 and 19 at high spectral resolution and compare the results with an ionospheric model to put these results into context. The model confirms that the ammonium ion NH⁺₄ is one of the most abundant ion species, as predicted, in the coma near perihelion.