SIMULATIONS OF TITANS BRIGHTNESS BY A 2-DIMENSIONAL HAZE MODEL

We have used a 2-D microphysics model to study the effects of atmosphe ric motions on the albedo of Titan's thick haze layer. We compare our results to the observed variations of Titan's brightness with season a nd latitude. We use two wind fields; the first is a simple pole-to-pol e Hadley cell that reverses twice a year. The second is based on the r esults of a preliminary Titan GCM. Seasonally varying wind fields, wit h horizontal velocities of about 1 cm sec(-1) at optical depth unity, are capable of producing the observed change in geometric albedo of ab out 10% over the Titan year. Neither of the two wind fields can adequa tely reproduce the latitudinal distribution of reflectivity seen by Vo yager. At visible wavelengths, where only haze opacity is important, u pwelling produces darkening by increasing the particle size at optical depth unity. This is due to the suspension of larger particles as wel l as the lateral removal of smaller particles from the top of the atmo sphere. At UV wavelengths and at 0.89 mu m the albedo is determined by the competing effects of the gas and the haze material. Gas is bright in the UV and dark at 0.89 mu m. Haze transport at high altitudes con trols the UV albedo and transport at low altitude controls the 0.89-mu m albedo. Comparisons between the hemispheric contrast at UV, visible , and IR wavelengths can be diagnostic of the vertical structure of th e wind field on Titan. (C) 1996 Academic Press, Inc.