TITANS ATMOSPHERE FROM VOYAGER INFRARED OBSERVATIONS .4. LATITUDINAL VARIATIONS OF TEMPERATURE AND COMPOSITION

We have analyzed nine Voyager 1 infrared spectral averages covering Ti tan's disk from 53 degrees S to 70 degrees N. By use of radiative tran sfer modeling we have determined the thermal profiles and mean molecul ar abundances in the stratosphere of the species with signatures in th e region 200-1500 cm(-1). Temperature latitudinal variations were foun d in accordance with Flasar and Conrath (1990, Icarus 85, 346-354). A maximal temperature decrease of 17 K at the 0.4-mbar level (225 km of altitude) is observed between 5 degrees S (the warmest region) and 70 degrees N, whereas the temperature drops only by similar to 3 K from 5 degrees to 53 degrees S. Mean molecular fractions, associated with at mospheric levels between 4 and 9 mbar, were derived from the best fit of the infrared data. The CO2 abundance remains constant from pole to pole within error bars. HCN shows a steady increase from south to nort h (total enhancement of > 30). For all the other molecules, variations in composition exist mainly between the equator and the north polar r egion. Ethane, acetylene, and propane show a moderate enrichment by ab out a factor of two. C4H2, C2H4, C3H4 show significantly higher mole f ractions at latitudes > 50 degrees N (by factors of similar to 7-15). C2N2 and HC3N, undetected southward of 50 degrees N, show at least an order of magnitude enhancement near the north pole. The stratospheric haze opacity at wavenumbers larger than 600 cm(-1) was found to show a north-to-south enhancement of similar to 2.5 +/- 0.3. Coldest tempera tures, found at high northern latitudes, are associated with enhanced gas concentration and haze opacity, and this may be caused by more eff icient radiative cooling (Bezard, B., A. Coustenis, and C. P. McKay 19 95, Icarus 113, 267-276). The observed latitudinal variations in hydro carbons and nitriles may be related to seasonal and spatial variations of the solar flux (Yung, Y. L. 1987, Icarus 72, 468-472). The present results set constraints for the future development of 2-D seasonal ph otochemical models. (C) 1995 Academic Press, Inc.