Thermal Oxidation Reaction between NH₃ and O₃: Low-temperature Formation of an NH₄⁺-bearing Salt

NH₃ has long been predicted to be an important component of outer solar system bodies, yet detection of this compound suggests a low abundance or absence on many objects where it would be expected. Here, we demonstrate that a thermally driven oxidation reaction between ammonia (NH₃) and ozone (O₃) in a H₂O + NH₃ + O₃ mixture may contribute to the low abundance of NH₃ on some of these objects, as this reaction efficiently occurs at temperatures as low as 70 K. We determined the overall activation energy for this reaction to be 17 ± 2 kJ mol⁻¹, which is consistent with other chemical systems that react at cryogenic temperatures. The loss of these two compounds coincides with the formation of NH 4 + and NO 3 − at low temperatures, both of which are observable with infrared spectroscopy. Warming our H₂O + NH₃ + O₃ mixtures through sublimation, we find a number of higher-temperature phases, such as ammonia hemihydrate, nitric acid, and ammonium nitrate (NH₄NO₃). The most stable of these is NH₄NO₃, which remains on the substrate until temperatures near 270 K. The salt product within this sample contains near-infrared spectral features between 2.0 and 2.22 μm, which is a spectral region of interest for several outer solar system objects, including the Uranian satellites Miranda, Ariel and Umbriel, and Pluto's satellite Charon.