[Previous] | [Session 20] | [Next]
E. L. Barth (Southwest Research Institute), O. B. Toon (LASP/University of Colorado)
A time-dependent microphysical model is used to study the evolution of ice clouds in Titan's atmosphere. A variety of cloud compositions are studied, including pure ethane clouds, pure methane clouds, and mixed methane-ethane clouds (all with tholin cores). The abundance of methane cloud particles is limited by the number of ethane nuclei rather than the number of tholins. Condensation of methane onto these mixed cloud particles is sufficient to keep the methane close to saturation. Typical methane supersaturations are of order 0.06 on the average, however dynamically induced temperature changes can produce time varying supersaturations. Cloud production does not require a continuous surface source of methane. However, clouds produced by mean motions are not the visible methane clouds seen in recent ground-based and Cassini observations. Ethane clouds in the troposphere almost instantaneously nucleate methane to form mixed clouds. However, a thin ethane `haze' remains just above the tropopause for some scenarios and the mixed clouds at the tropopause remain \leq~50% ethane by mass. Also, evaporation of methane on the mixed cloud particles near the surface leaves a thicker layer of ethane cloud particles at an altitude near 10~km.
Short-lived optically thick clouds can be created sporadically by dynamically driven atmospheric cooling. Horizontal quasi-barotropic motions are more likely to drive the supersaturation creating these clouds than are vertical motions. We expect to find these optically thick, mostly methane clouds at the pole where they can be observed by instruments on the Cassini orbiter.
Funding for this project was provided by Cassini science grant JPL 961196.
[Previous] | [Session 20] | [Next]
Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.