Giant-planet chemistry: Ammonium hydrosulfide (NH4SH), its IR spectra and thermal and radiolytic stabilities

Here we present our recent studies of proton-irradiated and unirradiated ammonium hydrosulfide, NH₄SH, a compound predicted to be an important tropospheric cloud component of Jupiter and other giant planets. We irradiated both crystalline and amorphous NH₄SH at 10-160 K and used IR spectroscopy to observe and identify reaction products in the ice, specifically NH₃ and long-chained sulfur-containing ions. Crystalline NH₄SH was amorphized during irradiation at all temperatures studied with the rate being the fastest at the lowest temperatures. Irradiation of amorphous NH₄SH at ∼10-75 K showed that 60-80% of the NH₄⁺ remained when equilibrium was reached, and that NH₄SH destruction rates were relatively constant within this temperature range. Irradiations at higher temperatures produced different dose dependence and were accompanied by pressure outbursts that, in some cases, fractured the ice. The thermal stability of irradiated NH₄SH was found to be greater than that of unirradiated NH₄SH, suggesting that an irradiated giant-planet cloud precipitate can exist at temperatures and altitudes not previously considered.