Steady-state model for methane condensation in Titan's troposphere

Abstract

In an attempt to better understand the physical basis for the large degrees of methane supersaturation inferred to be in Titan's troposphere from recent analyses of Voyager 1 IRIS data, a steady-state model has been developed in which the loss of methane at every level due to condensation and precipitation is exactly balanced by the replacement of methane vapor through eddy diffusion. Ethane ice crystals precipitating from the lower stratosphere serve as nucleation sites for methane condensation in the troposphere. Free parameters include a normalized methane abundance, eddy diffusion coefficient, and particle flux (equivalently an initial ethane particle radius); the downward ethane molecular flux is fixed from photochemical theory. Solutions are obtained for the altitude-dependences of methane particle size, degree of methane supersaturation, and methane mole fraction. It is found that the large degrees of methane supersaturation inferred from IRIS data require rather low initial ethane particle fluxes, corresponding to ethane particle radii ∼100 μm or so. A logical corollary to the theory suggests Titan's troposphere is stable against moist convection. From other analyses it is inferred that condensation and precipitation may occur cyclically with the season, and preferentially at the spring poles. It follows that there should be latitudinal gradients for both the degree of methane supersaturation in the upper troposphere, and methane mole fraction near the surface, with both quantities decreasing from equator to pole. This is consistent with the latitude-dependence for these quantities inferred from Voyager IRIS data.