Photolysis and Sublimation Chemistry of Ammonium Cyanide with Relevance to Cometary Environments

Abstract

Bulk nitrogen is depleted in solar system objects, including the terrestrial planets, chondritic meteorites, and comets. Recently, it was suggested that cometary nitrogen may be locked up in ammonium salts—ionic solids comprising ammonia (NH3) and an acid such as hydrogen cyanide (HCN)—based on Rosetta/ROSINA data from comet 67P/Churyumov-Gerasimanko. Among the proposed salts to have been found in 67P were ammonium cyanide ( NH 4 CN ) and ammonium cyanate ( NH 4 OCN ), which were inferred from identifying HCN, cyanamide ( NH 2 CN ), and isocyanic acid (HNCO) in the mass spectrometry data collected in the coma of 67P. However, the relationship between these salts and the proposed tracers had not previously been studied experimentally. We investigated the ultraviolet photolysis and sublimation chemistry of NH 4 CN and NH 3 + HCN + H 2 O samples formed at 125 K in the laboratory and monitored the ice samples and resulting gas mixtures using infrared and submillimeter spectroscopies, respectively. We find NH 2 CN forms from NH 4 CN but only in the presence of water. When starting from salts, both NH 2 CN and HNCO seem to form only in the gas phase as the salts dissociate upon sublimation and not in the photolyzed ice.