IMPORTANCE OF PHASE-CHANGES IN TITANS LOWER ATMOSPHERE - TOOLS FOR THE STUDY OF NUCLEATION

The uncertainty about possible supersaturation of methane, condensatio n of volatile species and the existence of clouds in Titan's lower atm osphere affects our understanding of photochemistry, the nature the su rface and the atmospheric thermal structure. Indeed, photochemistry de pends on the depth of penetration of energetic photons, affected by me thane abundance. Radar and infrared observations of bright surface reg ions have been explained by rain washing of highlands. As for the ther mal profile, it is sensitive to CH4-N-2 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 indica tes a methane latent heat bf 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 m(2) s(-1). Description of nucleation one of the first improve ments which should included in a model of phase changes. The suspicion difficult methane nucleation comes from analysis of Voyager IRIS spec tra. Moreover, species are expected condense to the solid phase, which excludes very efficient nucleation and condensation processes associa ted with the presence of a liquid phase, such as deliquescence. The cl assical theory of heterogeneous despite its deficiencies, is employed in atmospheric models, owing to its general nature and relative city. Yet, it requires physical quantities for which experimental values do not exist. It is shown bow surface enthalpies of solids and contact an gles may be linked to other material properties which are within reach of laboratory experiments, mainly ultraviolet absorption spectra of s olid 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 Ti tan, though we question the ability of the critical rate concept to ma ke predictions for the condensation altitudes. A possible consequence of difficult methane nucleation is periodic evolution of the lower atm osphere, on a timescale of the order of 10(2) years. (C) 1997 Elsevier Science Ltd.