Phosphine and Ammonia Photochemistry in Jupiter’s Troposphere

Introduction: The last comprehensive photochemical model for Jupiter’s troposphere was presented by Edgington et al. [1,2], based upon the work of Atreya et al. [3] and Kaye and Strobel [4-6]. Since these early studies, numerous laboratory experiments have led to an improvement in our understanding of PH3, NH3-PH3, and NH3-C2H2 photochemistry [7-13]. Furthermore, recent Galileo, Cassini, and Earth-based observations have better defined the abundance of key atmospheric constituents as a function of altitude and latitude in Jupiter’s troposphere. These new results provide an opportunity to test and improve theoretical models of Jovian atmospheric chemistry. We have therefore developed a photochemical model for Jupiter’s troposphere considering the updated experimental and observational constraints. Using the Caltech/JPL KINETICS code [14] for our photochemical models, our basic approach is twofold. The validity of our selected chemical reaction list is first tested by simulating the laboratory experiments of PH3, NH3-PH3, and NH3-C2H2 photolysis with photochemical “box” models. This reaction list is then used to produce a photochemical model for the Jovian troposphere. Simulations of Laboratory Experiments: A box model involving 30 species and 220 reactions is used to simulate the photolysis experiments of Ferris et al. [5,6], in which PH3 or NH3/PH3 mixtures were UVirradiated in photochemical cells. Wherever possible, reaction rate coefficients used in our model are obtained from experimental and literature data or calculated from reverse reaction rates. For important reactions with poorly known rate coefficients, estimated reaction rates are adjusted until good agreement was achieved between our model and experimental results. One outcome is shown in Figure 1, which compares abundances in a photochemical box model to the laboratory results of Ferris et al. [8] for a 50/50 NH3/PH3 mixture at 298 K and 11 torr total pressure. As in the experiments, the dominant products in our box model were H2, diphosphine (P2H4), and red phosphorus (Px). The net conversion of phosphine into red phosphorus in the model follows the pathway PH3 → PH2 → P2H4 → P2H3 → P2H2 → P2H → P2 → Px. This pathway generally occurs in either the absence or presence of NH3. In mixtures containing a greater proportion of ammonia, amino-phosphine (NH2PH2) also becomes an important product and chemical reactions involving the amino radical (NH2) play a larger role. We note that this is the first photochemical model to include P2H as an intermediate species in PH3 photolysis. The equivalent N2Hx species are believed to be important in the combustion chemistry of nitrogen compounds [15].