The photochemistry of the Jovian stratosphere is currently hard to constrain due to uncertainties in the strength of atmospheric mixing (parameterized through the eddy diffusion coefficient), uncertainties in the roles of auroral chemistry and atmospheric transport in affecting species abundances at mid- and equatorial latitudes, and uncertainties in certain key reaction rate coefficients under relevant conditions. Because the pressure level of the methane homopause is poorly defined at this time (various observations are contradictory; see Moses et al. 2005, J. Geophys. Res. 110, E08001). We currently consider three default models. All three models predict roughly the same abundances of observable hydrocarbons in the Jovian lower stratosphere; the main difference between the models is the eddy diffusion coefficient profile in the upper stratosphere and the pressure level at which methane becomes diffusively separated (i.e., the homopause location). Figure 1 shows the methane mixing-ratio profiles and the eddy diffusion coefficient profiles for these different models.
With the updated set of chemical reactions, rate coefficients, ultraviolet absorption coefficients, and photolysis branching ratios that we use, the models do an excellent job of reproducing the ISO observations of C2H6, C2H2, C4H2 (upper limit), and C6H6, a reasonable job for CH3C2H (models slightly underpredict abundance), and a poor job for C2H4 (models overpredict abundance). Figure 2 shows the results from one of these models compared with various observations. The C2H4 abundance is highly linked to that of C2H2, and with the current reaction set, we cannot simultaneously reproduce both the observed C2H2 and C2H4 abundances. Note, however, that the predicted C2H4 abundance is highly sensitive to several poorly constrained chemical parameters. Other outstanding questions with regard to stratospheric chemistry include the reasons for the differences in the higher C2H2 and C2H4 abundances at Saturn as compared with Jupiter, the details of the production and loss of benzene (C6H6) in these atmospheres, and the roles of auroral chemistry and atmospheric transport in affecting global photochemistry on Jupiter. Figure 3 shows the primary chemical pathways for producing and destroying hydrocarbons in Jupiter's stratosphere and illustrates the complexity of this process.
Methane is photolyzed in the upper stratosphere of Jupiter near the 103 mbar level. Ammonia is photolyzed in the troposphere near the ammonia cloud tops (~100200 mbar). Because the altitude regions for methane and ammonia photolysis are physically separated from each other in Jupiter's atmosphere, the photochemical production of organo-nitrogen compounds is inhibited. In response to the reported detection of HCN in Jupiter's troposphere by Tokunaga et al. (1981, Icarus 48, p. 283), Kaye and Strobel (1983, Icarus 54, p. 417) proposed that the NH2 radicals produced from NH3 photolysis might react with unsaturated hydrocarbons such as C2H2 and C2H3 to eventually form HCN and a variety of other interesting organo-nitrogen molecules in the upper troposphere of Jupiter. Experimental confirmation of this idea has been provided by Ferris and Ishikawa (1988, J. Am. Chem. Soc. 110, p. 4306). However, both the laboratory experiments and the photochemical models supporting this idea assume that C2H2 is as abundant or more abundant in the troposphere as it is in the middle stratosphere. Our photochemical models and various C2H2 observations indicate that the mole fraction of C2H2 falls off significantly with increasing pressure in the lower stratosphere and upper troposphere. Unless the troposphere has independent mechanisms for producing C2H2, our recent more realistic coupled stratospheric-tropospheric models (see LaMothe and Moses 2000, Bull. Amer. Astron. Soc. 32, p. 1104) indicate that HCN formation in Jupiter's upper troposphere will not be significant. These findings are consistent with the non-detection of HCN at infrared wavelengths by Bézard et al. (1995, Icarus 118, p. 384), whose new observations and reanalysis of previous observations shed doubt on the original reported detection by Tokunaga et al. (1981).
We are now expanding our 1-D modeling to include advection and horizontal diffusion. Our 2-D photochemistry/transport models can be compared with the observed evolution of the Shoemaker-Levy 9 debris and with the hydrocarbon distributions observed with Cassini CIRS.
The abundance of H2O in the deep atmosphere of Jupiter is currently uncertain but has major implications with regard to solar-system origin and evolution. Working from ideas developed in Bezard et al. (2002, Icarus 159, 95–111) and Visscher and Fegley (2005, ApJ 623, 1221–1227), we will use an updated kinetics and diffusion model to determine whether the deep H2O abundance can be constrained from the observed CO abundance.