Importance of phase changes in Titanʹs lower atmosphere. Tools for the study of nucleation

The uncertainty about possible supersaturation of methane, condensation of volatile species and the existence of clouds in Titanʹs lower atmosphere affects our understanding of photochemistry, the nature of the surface and the atmospheric thermal structure. Indeed, photochemistry depends on the depth of penetration of energetic photons, affected by methane abundance. Radar and infrared observations of bright surface regions have been explained by rain washing of highlands. As for the thermal profile, it is sensitive to CH4N2 gas opacity, cloud opacity and could be influenced by latent heat exchange. A rudimentary model with no methane supersaturation and gas transport by eddy diffusion indicates a methane latent heat release of 0.2 W m−2 between 20 and 30 km altitude for a surface mole fraction of 4.4% and an eddy diffusion coefficient of 0.2 m2s−1. Description of nucleation seems to be one of the first improvements which should be included in a model of phase changes. The suspicion of difficult methane nucleation comes from analysis of Voyager IRIS spectra. Moreover, species are expected to condense to the solid phase, which excludes very efficient nucleation and condensation processes associated with the presence of a liquid phase, such as deliquescence. The classical theory of heterogeneous nucleation, despite its deficiencies, is employed in atmospheric models, owing to its general nature and relative simplicity. Yet, it requires physical quantities for which experimental values do not exist. It is shown how surface free enthalpies of solids and contact angles may be linked to other material properties which are within reach of laboratory experiments, mainly ultraviolet absorption spectra of solid phases. It is found that a value of 10−9–10−7 s−1 for the “critical nucleation rate” (per nucleus) is adapted to the case of Titan, though we question the ability of the critical rate concept to make predictions for the condensation altitudes. A possible consequence of difficult methane nucleation is periodic evolution of the lower atmosphere, on a timescale of the order of 102 years.