Models for Polar Haze Formation in Jupiter's Stratosphere

Abstract

We present coupled chemical–microphysical models for the formation, growth, and physical properties of the jovian polar haze based on a gas-phase photochemical model for the auroral regions developed by A. S. Wong et al. (2000, Astrophys. J. 534, L215–217). In this model, auroral particle precipitation provides an important energy source for enhanced decomposition of methane and production of benzene and polycyclic aromatic hydrocarbons (PAHs). We find that at high altitude, A 4 (pyrene, a hydrocarbon consisting of four fused aromatic rings) should homogeneously nucleate to form tiny primary particles. At lower altitudes, A 3 (phenanthrene) and A 2 (naphthalene) heterogeneously nucleate on the A 4 nuclei. These particles subsequently grow by additional condensation of A 2 on the nucleated particles and by coagulation and eventually sediment out to the troposphere. We run different cases of the aerosol microphysical model for different assumptions regarding the fractal dimension of aggregate particles formed by the coagulation process. If coagulation is assumed to produce spherical particles (of dimensionality 3), then their mean radius at altitudes below the 20-mbar pressure level is computed to be approximately 0.1 μm. If coagulation produces fractal aggregates of dimension 2.1, then their equivalent mean radius below the 20-mbar level is much larger, of order 0.7 μm. Aggregates with fractal dimensions between 2.1 and 3 form with equivalent mean radii between 0.1 and 0.7 μm. In every case, mean particle radius is found to decrease with increasing altitude, as expected for a system approximately in sedimentation–coagulation equilibrium. The predicted range of altitudes where aerosol formation occurs and the mean size to which particles grow are found to be generally consistent with observations. However, our calculations cannot presently account for the large amount of total aerosol loading inferred by M. G. Tomasko et al. (1986, Icarus 65, 218–243). We suggest that the primarily neutral chemical pathway to heavy hydrocarbon and PAH formation proposed by Wong et al. (2000) may proceed too slowly to produce a sufficient amount of condensible material. Inclusion of ion and ion–neutral reactions in the chemical scheme could potentially lead to the prediction of higher PAH production rates, higher nucleation rates, and greater aerosol loading, producing better agreement with the observations.