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E.L. Barth, O.B. Toon (LASP, Univ. of Colorado)
A time-dependent microphysical model was run to study the evolution of ethane ice clouds in Titan's atmosphere. The model simulates nucleation, condensational growth, evaporation, coagulation, and transport of particles. For a critical saturation of 1.15 (a lower limit, determined by laboratory experiments), we find that ethane clouds can be sustained between altitudes of 8 and 50 km. Growth due to coalescence is inefficient, limiting the peak in the size distribution (by number) to 20 microns. Because cloud particles are sparse, they are optically thin, nor do they remove enough tholin to significantly alter Titan's albedo at infrared wavelengths. Rainout due to methane cloud formation raises the altitude of the cloud bottom and may eliminate ethane clouds entirely if rainout occurs up to 30 km altitude. However, clouds formed from other gases in Titan's atmosphere could be sustained even with rainout up to 30 km.
This work was funded by L.W. Esposito through the grant JPL 961196.