Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae

Kathrin Altwegg (1) , Hans Balsiger (1) , Jean-Jacques Berthelier (2) , Christelle Briois (3) , Mike Combi (4) , Johan de Keyser (5) , Nora Hänni (1) , Frederik Dhooghe (5) , Björn Fiethe (6) , Steven A. Fuselier (7) , Martin Rubin (1) , Tamas I. Gombosi (4) , Hervé Cottin (8) , Markus Schuhmann (1) , Isaac Schroeder (1) , Thierry Sémon (1) , Susanne Wampfler (9)

1 Physikalisches Institut [Bern]
2 PLANETO - LATMOS
3 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
4 CLaSP - Department of Climate and Space Sciences and Engineering
5 BIRA-IASB - Belgian Institute for Space Aeronomy / Institut d'Aéronomie Spatiale de Belgique
6 IDA - Institute of Computer and Network Engineering [Braunschweig]
7 SwRI - Southwest Research Institute [San Antonio]
8 LISA (UMR_7583) - Laboratoire Interuniversitaire des Systèmes Atmosphériques
9 CSH - Center for Space and Habitability

Cometary comae are generally depleted in nitrogen. The main carriers for volatile nitrogen in comets are NH3 and HCN. It is known that ammonia readily combines with many acids, such as HCN, HNCO and HCOOH, encountered in the interstellar medium as well as in cometary ice to form ammonium salts (NH₄⁺X^-) at low temperatures. Ammonium salts, which can have a substantial role in prebiotic chemistry, are hard to detect in space as they are unstable in the gas phase and their infrared signature is often hidden by thermal radiation or by, for example, OH in minerals. Here we report the presence of all possible sublimation products of five different ammonium salts in the comet 67P/Churyumov-Gerasimenko measured by the ROSINA instrument onboard Rosetta. The relatively high sublimation temperatures of the salts leads to an apparent lack of volatile nitrogen in the coma. This then also explains the observed trend of higher NH3/H2O ratios with decreasing perihelion distances in comets.

Domains: Sciences of the Universe [physics]

Kathrin Altwegg, Hans Balsiger, Jean-Jacques Berthelier, Christelle Briois, Mike Combi, et al.. Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae. Nature Astronomy, 2020, 4, pp.533-540. ⟨10.1038/s41550-019-0991-9⟩. ⟨insu-02411099⟩