Titan's Atmosphere from Voyager Infrared Observations: IV. Latitudinal Variations of Temperature and Composition

Abstract

We have analyzed nine Voyager 1 infrared spectral averages covering Titan's disk from 53°S to 70°N. By use of radiative transfer modeling we have determined the thermal profiles and mean molecular abundances in the stratosphere of the species with signatures in the region 200-1500 cm-1. Temperature latitudinal variations were found 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°S (the warmest region) and 70°N, whereas the temperature drops only by ∼3 K from 5° to 53°S. Mean molecular fractions, associated with atmospheric 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 north (total enhancement of > 30). For all the other molecules, variations in composition exist mainly between the equator and the north polar region. Ethane, acetylene, and propane show a moderate enrichment by about a factor of two. C4H2, C2H4, C3H4 show significantly higher mole fractions at latitudes > 50°N (by factors of ∼7-15). C2N2 and HC3N, undetected southward of 50°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 ∼2.5 ± 0.3. Coldest temperatures, found at high northern latitudes, are associated with enhanced gas concentration and haze opacity, and this may be caused by more efficient radiative cooling (Bézard, B., A. Coustenis, and C. P. McKay 1995, Icarus 113,267-276). The observed latitudinal variations in hydrocarbons 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 photochemical models.