Titan's atmosphere from voyager infrared observations: I. The gas composition of Titan's equatorial region

Abstract

The abundances of minor atmospheric constituents in Titan's equatorial region have been inferred from Voyager 1 infrared spectra. A temperature profile for the stratosphere was retrieved from the best fit of the 7.7-μm methane band in which the emission originates mainly from pressure levels between 0.01 and 3 mbar (150 and 400 km). Assuming uniform vertical distributions for all the absorbers above their condensation levels, the analysis of three different selections yields stratospheric mole fractions for the following species: C 2H 2 (2.2 +0.7 −0.9 × 10 −6), C 2H 4 (9.0 +3 −5 × 10 −8), C 2H 6 (1.3 +0.5 −0.7 × 10 −5), C 3H 4 (4.4 +1.7 −2.1 × 10 −9), C 3H 8 (7.0 +4 −4 × 10 −7), C 4H 2 (1.4 +0.6 −0.7 × 10 −9), HCN (1.6 +0.4 −0.6 × 10 −7), and CO 2 (1.4 +0.3 −0.5 × 10 −8). Upper limits were obtained for the abundances of C 2N 2 (1.5 × 10 −9) and HC 3N (1.5 × 10 −9). An altitude-dependent profile of CO 2 was tested against observations but no conclusive information as to the vertical distribution of carbon dioxide could be extracted. The formation of emission lines for all of the minor components is found to originate from pressure levels between 1 and 20 mbar (75 and 200 km). There is no evidence for longitudinal variations in the atmospheric composition. The results are finally compared to previous studies and to current photochemical models.