31st Annual Meeting of the DPS, October 1999
Session 41. Titan: Physics
Contributed Oral Parallel Session, Thursday, October 14, 1999, 8:30-9:50am, Sala Pietro d'Abano

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[41.02] Constraints on Eddy Diffusion and the Efficiency of Methane Nucleation in Titanís Troposphere from an Energetic Equilibrium Model

L. Guez (NRC/NASA ARC), C. P. McKay (NASA ARC), P. Bruston, F. Raulin (Univ. Paris 12/LISA)

A one-dimension steady-state cloud model for Titanís atmosphere has been set up in which methane condenses on aerosols settling from the upper atmosphere. The possibly difficult nucleation of methane is modeled by allowing methane to condense on only a subset of the aerosols. Thus, the fraction of aerosols which serve as condensation nuclei, alpha, is a free parameter of the model. The other important free parameter is the tropospheric eddy diffusion coefficient, K. We use the radiative transfer and aerosol models from McKay et al. (1989, Icarus 80, 23) and, in a first approach, the background atmospheric profiles from Lellouch et al. (1989, Icarus 79, 328). We require that the methane energy flux (associated to latent heat exchanges) be at most of the same order of magnitude than the net thermal infrared and net solar radiative fluxes. We can then constrain K and alpha. If alpha >~ 0.1 then K must be <~ 0.01 m2 s-1. Modifying the temperature profile within acceptable margins (constrained by Voyager radio-occultation) does not change the above constraint on K and alpha. In a second approach, we try to obtain stronger constraints on K and alpha by requiring the atmosphere to be in energetic equilibrium and looking for the best fit to the radio-occultation temperature profile. With alpha ~ 1, regardless of the K value, the fit is not satisfactory. A good fit might be obtained with alpha << 1 and K ~ 0.1 m2 s-1, when supersaturation of methane induces greenhouse warming of the surface and release of latent heat warms the middle troposphere. This work was performed while L. G. held a National Research Council-NASA ARC research associateship.


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