Titan's atmosphere from voyager infrared observations: III. Vertical distributions of hydrocarbons and nitriles near Titan's North Pole

Abstract

The temperature structure and vertical distributions of minor stratospheric constituents in Titan's north polar region have been inferred from the study of Voyager 1 infrared spectra. The data were recorded at grazing incidence over the limb, and cover an approximate vertical range of 100–300 km. The upper stratospheric temperature is retrieved from an analysis of the 7.7-μm methane band in which the emission originates mainly from pressure levels between 0.01 and 5 mbar (120 and 375 km). We find the thermal profile at 70°N to be significantly colder (by ∼ 20°K) than the equatorial one. Vertical distributions of the following absorbers were derived from the best fits obtained in the spectral region 200–1400 cm −1 for three different data sets (corresponding to three different altitudes above the satellite's surface): C 2H 2, C 4H 2, C 2H 6, C 3H 4, HCN, HC 3N, and C 2N 2. The mixing ratios of these species increase with height, suggesting their formation in the upper stratosphere and above. The abundances of C 3H 8 and C 2H 4 could only be obtained near the 1.5-mbar level. When compared to the abundances derived at the equator, we find the nitriles HC 3N and C 2N 2 and some of the hydrocarbons (C 2H 4, C 3H 4, and C 4H 2) to be considerably enhanced in the north polar region. The upper limit of the CO 2 abundance obtained near the north pole indicates a depletion of this gas with respect to the equator (by a factor of at least ∼2). Current available photochemical models are insufficient to interpret these observations.