The ROSINA Perspective on the CN/HCN Ratio at Comet 67P/Churyumov-Gerasimenko

<p>The origin of cyano (CN) radicals in comets presents a long-standing riddle to the science community. Remote observations, e.g. reviewed by Fray et al. [1], show that for some comets the scale lengths, production rates, and spatial distributions of hydrogen cyanide (HCN) and CN using a Haser-based model are not consistent. Consequently, a process additional to photolysis of HCN seems to be required to explain the observed CN densities. Possible scenarios include (1) degradation of CN-producing refractories (e.g. HCN-polymers, tholins, or ammonium salts [2-3]) and (2) photolysis of other gaseous CN-bearing parent species (e.g. HC<sub>3</sub>N or C<sub>2</sub>N<sub>2</sub>).</p><p>The CN/HCN ratio observed in the inner coma of comet 67P/Churyumov-Gerasimenko with the Double Focusing Mass Spectrometer DFMS, part of the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) sensor package [4] onboard ESA’s Rosetta spacecraft, is not compatible with fragmentation of HCN under electron impact ionization. Even though from fragmentation a constant CN/HCN ratio of about 0.15 [5-7] is expected, the observed values range from almost 0.4 at the beginning of the mission (August 2014) to about 0.15 shortly after perihelion passage (August 2015). Towards the end of the mission (September 2016), CN/HCN ratios increase again. This presentation will discuss the data from ROSINA/DFMS in detail and present laboratory-based indications that direct production of CN from sublimating ammonium cyanide (NH<sub>4</sub>CN) occurs, leading to increased CN/HCN ratios. Could this be the process generating a surplus of CN radicals with respect to photolysis of HCN in certain comets?</p><p> </p><p> </p><p>[1] N. Fray et al. The origin oft he CN radical in comets: A review from observations and models Planetary and Space Science 53 (2005) 1243-1262.</p><p>[2] N. Hänni et al. Ammonium Salts as a Source of Small Molecules Observed with High-Resolution Electron-Impact Ionization Mass Spectrometry. J. Phys. Chem. A 123 (2019) 27, 5805-5814.</p><p>[3] K. Altwegg et al. Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae. Nat. Astron. (2019) in print.</p><p>[4] H. Balsiger et al. Rosina - Rosetta Orbiter Spectrometer for Ion and Neutral Analysis. Space Science Reviews 128 (2007) 745-801.</p><p>[5] S.E. Steins in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, National Institute of Standards and Technology, (2018).</p><p>[6] P. Kusch et al. The Dissociation of HCN, C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>N<sub>2</sub>, and C<sub>2</sub>H<sub>4</sub> by Electron Impact. Phys. Rev. 52 (1937) 843-854.</p><p>[7] D. P. Stevenson. Ionization and Dissociation by Electron Impact: Cyanogen, Hydrogen Cyanide, and Cyanogen Chloride and the Dissociation Energy of Cyanogen. J. Chem. Phys. 18 (1950) 1347-1351.</p>