Laboratory Studies of Ammonium Salts

Ammonium (NH₄⁺) salts have been detected by the instruments onboard ESA’s Rosetta spacecraft in the dust ejected from comet 67P/Churyumov-Gerasimenko. Estimates of the ammonium abundances in the dust indicate that they make up a significant fraction (as much as 50% by mass) of the cometary dust particles. Measurements of the gases in the coma made by the ROSINA mass spectrometer indicated that the dissociation of these salts contributed to the observed abundances of ammonia (NH₃), hydrogen cyanide (HCN), and cyanide radical CN. Although a contribution from ammonium cyanide (NH₄CN) was unclear, cyanamide (NH₂CN) was detected and considered to be a potential solid tracer of this species.

Besides in the surface ice and the coma gas of comet 67P, ammonium has been detected in other parts of the solar system, such as on the surfaces of Ceres and on Charon. As-yet-unidentified infrared features in the spectra of dense interstellar clouds suggest that NH₄⁺ may be even present in interstellar ice and dust.

Our recent work in the Cosmic Ice Laboratory at NASA Goddard Space Flight Center has focused on the IR spectra, physical properties (refractive indices and densities), band strengths, and optical constants of both HCN and NH₃ in their amorphous and crystalline forms. Here, we report the results of similar measurements for NH₄CN, where we have determined its visible refractive index, density, IR absorption band strengths, optical constants, and sublimation rates between ~120 and 165 K. The significance of ammonium salts in different environments will also be discussed along with plans for future lab measurements.