DYNAMICAL INFLUENCE ON THE ISOTOPIC ENRICHMENT OF CH3D IN THE OUTER PLANETS

We investigate the influence of atmospheric dynamics on the isotopic e nrichment factor of monodeuterated methane, f(z) = (D/H)(CH4)/(D/H)(H2 ), in the outer planets. We explore the influence of dynamics using th ree classes of models. In the first two, we briefly discuss the simple approaches that have previously been used to account for dynamics in the determination of f(z). We discuss the strengths and limitations of these approximations. Then, we combine the best portions of the first two models to produce a third model that we feel more realistically d escribes mixing on the outer planets. Using different limiting assumpt ions about convective vertical velocities inside the outer planets, we place rough upper and lower limits on possible values of the isotopic enrichment factor at a pressure level of 1 bar for each planet. We fi nd that the isotopic enrichment factor in the observable regions of th e outer planets is sensitive to the dynamical model and parameters tha t are assumed, and thus a measured value of f(z) in the upper, observa ble levels of the atmosphere could constrain the dynamical regime at d eeper levels. (C) 1996 Academic Press