CH4 NONLOCAL THERMODYNAMIC-EQUILIBRIUM IN THE ATMOSPHERES OF THE GIANT PLANETS

CH4 vibrational relaxation in the upper stratospheres of the Giant Planets has been examined. A model was developed for the thermalization of solar energy absorbed under nonlocal thermodynamic equilibrium (non-LTE) conditions in the CH4 band groups centered at λ {reversed tilde equals} 1.7, 2.3, and 3.3 μm. Assuming radiative equilibrium and employing a non-LTE radiative transfer algorithm from J. S. Hogan (1968, Ph.D. dissertation), a range of model atmospheres was produced, reflecting uncertainties in CH4 collisional relaxation rates and their temperature dependences. Nominal models for the Giant Planets indicate that the ratios S/B (source function/Planck function) at 1300 cm-1 (near the center of the CH4 ν4 fundamental) begin to depart significantly from unity at P - 0.1 mbar total pressure. In the formulation used here, this ratio drops to roughly 0.8 at P = 0.01 mbar. The results show large departures from LTE at lower pressures: for example, S/B < 0.5 at 1 μbar in all models. Rough estimates of CH4 7.7-μm non-LTE emission resulting from scattered sunlight suggest that it is a negligible component of the 7.7-μm flux emanating from all four planets, with the possible exception of Uranus. At 0.1 mbar, all of the non-LTE models are within 2°K of the LTE reference models. Differences between extreme non-LTE and reference LTE models increase steadily upward, however, reaching about ±20°K at 0.1 μbar (near the tops of the models). For Uranus, temperature profiles based on stellar occultation measurements are considerably more complex than the models: some of the data exhibit temperature variations having smaller vertical wavelengths and, at some levels, larger amplitudes. These comparisons, however, encompass many uncertainties, both in the non-LTE computations and in the contemplation of a data set that is of good quality but very complex. Requirements for improving the theoretical models are discussed. © 1990.