Voyager 2 imaging eclipse observations of the Jovian high altitude haze

Abstract

Images of Jupiter's northern hemisphere dawn and dusk limbs taken by the Voyager 2 wide angle camera were analyzed to explore the nature of the high stratospheric haze layer. At most latitudes the scattered intensities at 0.43- and 0.60-μm wavlengths are smaller than those for a conservatively scattering molecular atmosphere due to extinction by aerosols in the upper 100-mbar region. Considerable latitudinal structure is evident, with intensities in the latitude range 40°N to 50°N being higher, at 0.60 μm only, than those for a conservatively scattering molecular atmosphere. Asymmetry in the dawn and dusk limbs is also observed at high latitude. These variations are evidence for a dynamically active stratosphere and may be the most visible indicators of processes such as diabatic circulation, latitudinally dependent eddy mixing, or wave dissipation and breaking. Two kinds of aerosols models were compared with the observations. In the first, free parameters of the model were the particle radius, constant with altitude, the pressure level of the top of the haze, the haze optical depth at 0.43 μm, and the haze scale height. Although the solutions are multiple, the top of the haze must be in the range of a few millibars to a few tenths of a millibar, the optical depth above the 100-mbar level can be either small ( τ ∼ 0.1) or large (many times that value) at low latitudes but is always large at high latitudes, and the most frequent particle radii found were in the range between 0.01 and 0.05 μm. In the second type of model, the haze radius and number density were prescribed by a microphysical model which has particle radius increasing with depth. The free parameters were the total column density and the pressure of the haze top. Sensitivity studies were performed to assess the influence of particle size in the top layers and the refractive index at 0.60-μm wavelength. Again the solutions are not unique, but the most economical solution can explain the variations at latitudes less than 50°N by a haze with optical depth (above the 100-mbar level) near 0.1 at 0.43 μm, with a minimum particle radius, r min, of 0.05 μm and with variations in haze top pressure, from 7 mbar at 5.9°N to 0.5 mbar at 39.7°N, producing most of the variation in limb intensity. Simplistic calculations of particle transport suggest that, at low latitudes, hydrazine mixed upward from the tens of millibars region where it is produced can account for the inferred particle number densities. At high latitudes the same may be true, pending the results of a realistic microphysical model.