VAPOR-LIQUID-EQUILIBRIUM THERMODYNAMICS OF N2 + CH4 - MODEL AND TITAN APPLICATIONS

Calculations of the vapor-liquid equilibrium thermodynamics of the N2 + CH4 system show that the tropospheric clouds of Titan are not pure CH4, but solutions of CH4 containing substantial quantities of N2. The conditions for saturation, latent heat of condensation, and droplet composition all depend on this equilibrium. We present a thermodynamic model for vapor-liquid equilibrium in the N2 + CH4 system which, by its structure, places strong constraints on the consistency of experimental equilibrium data, and confidently embodies temperature effects by also including enthalpy (heat of mixing) data. Selected equilibrium and enthalpy data are used in a maximum likehood determination of model parameters. The model can be readily evaluated to compute the saturation criteria, composition of condensate, and latent heat in Titan's atmosphere for a given pressure-temperature (p-T) profile. For a nominal p-T profile, the partial pressure of CH4 required for formation of CH4 + N2 condensate is ∼20% lower than that required to saturate pure CH4, and ∼25% higher than that which would be computed by Raoult's law. N2 constitutes 16-30% of the cloud condensate, and higher altitude clouds are generally more N2-rich. The N2 content of condensate is ∼ 1 2 of that computed from Raoult's law and about 30% greater than that computed from Henry's law. Heats of condensation are ∼10% lower than for pure CH4. Above 14 km altitude, the liquid solution becomes metastable with respect to a solid solution containing less N2: freezing of liquid droplets will be accompanied by the exsolution of about 30% of the dissolved N2, probably leading to an underdense, porous texture. The refractive index, single-scattering albedo, and density of CH4 + N2 cloud droplets of the appropriate composition and phase should be used in modeling and spacecraft planning studies for Titan. Cassini investigations with sufficient altitude resolution (primarily Huygens probe experiments) can potentially detect vertical motion of particles by determining whether condensate and gas are in local thermodynamic equilibrium. © 1992.