METHANE-NITROGEN BINARY NUCLEATION: A NEW MICROPHYSICAL MECHANISM FOR CLOUD FORMATION IN TITAN'S ATMOSPHERE

It is known that clouds are present in the troposphere of Titan; however, their formation mechanism, particle size, and chemical composition remain poorly understood. In this study, a two-component (CH4 and N-2) bin-microphysics model is developed and applied to simulate cloud formation in the troposphere of Titan. A new process, binary nucleation of particles from CH4 and N-2 gases, is considered. The model is validated and calibrated by recent laboratory experiments that synthesize particle formation in Titan-like environments. Our model simulations show that cloud layers can be formed at about 20 km with a particle size ranging from one to several hundred mu m and number concentration 10(-2) to over 100 cm(-3) depending on the strength of the vertical updraft. The particles are formed by binary nucleation and grow via the condensation of both CH4 and N-2 gases, with their N-2 mole fraction varying from <10% in the nucleation stage to >30% in the condensation growth stage. The locally occurring CH4-N-2 binary nucleation mechanism is strong and could potentially be more important than the falling condensation nuclei mechanism assumed in many current models.