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

Abstract

NH _3 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 (NH _3 ) and ozone (O _3 ) in a H _2 O + NH _3 + O _3 mixture may contribute to the low abundance of NH _3 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 ${\mathrm{NH}}_{4}^{+}$ and ${\mathrm{NO}}_{3}^{-}$ at low temperatures, both of which are observable with infrared spectroscopy. Warming our H _2 O + NH _3 + O _3 mixtures through sublimation, we find a number of higher-temperature phases, such as ammonia hemihydrate, nitric acid, and ammonium nitrate (NH _4 NO _3 ). The most stable of these is NH _4 NO _3 , 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 μ m, 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.

Keywords

• Astrochemistry
• Laboratory astrophysics
• Natural satellites (Solar system)
• Surface ices