Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes from 0 to 400 km. Ionization due to both galactic cosmic rays and electron precipitation from the Saturnian magnetosphere is considered. This ionization results in free electrons and the primary ions N₂⁺ and N⁺ which are then rapidly converted into secondary ions such as H₂CN⁺ and NH₄⁺ which in turn form ion clusters such as H₂CN⁺(HCN)_n and NH₄⁺(NH₃)_m. In contrast to the atmospheres of Venus and Earth, we find no species in the Titan atmosphere that lead to the formation of appreciable concentrations of negative ions. Consequently, the predicted conductivity is quite different in that a substantial concentration of electrons exists all the way to the surface of Titan. The ubiquitous aerosols observed in the Titan atmosphere also play an important role in determining the charge distribution in the atmosphere. At altitudes above 100 km and for aerosol concentrations above approximately 10/cc, the recombination of electrons and positive ions is controlled by the recombination on the surface of the aerosols rather than by the gas-kinetic recombination rate. For small aerosol concentrations, the ratio of the number of charges per particle to the radius of the particle is approximately 30, for radii in microns. This value is similar to that obtained by previous investigators for terrestrial noctilucent clouds. Because the aerosol particles are highly charged, coagulation is inhibited, particle sizes are smaller, and their settling rates are reduced. As a consequence, the optical depth of the atmosphere is much higher than it would be if the particles were uncharged.