The Chemical Composition Of Titan's Stratosphere from Cassini/CIRS spectra
Résumé
Titan's atmospheric composition was known through Voyager and ISO data (such as the detection of water vapor (Coustenis et al., 1998) and the chemical composition as a disk-average (Coustenis et al., 2003). More recently, data recorded by Cassini/CIRS became available during the Titan flybys (Flasar et al., 2005; Teanby et al., 2006, Coustenis et al., 2006). The spectra characterize various regions on Titan from 85°S to 80°N with a variety of emission angles. We study the emission observed in the mid-infrared CIRS detector arrays (covering roughly the 600-1500 cm-1 spectral range with apodized resolutions of 2.54 or 0.53 cm-1). The composite spectrum shows signatures of hydrocarbons, nitriles and CO2. A firm detection of benzene (C6H6) is provided by CIRS at 674 cm-1 and allows for the study of its latitudinal variation. We have used temperature profiles retrieved from the inversion of the emission observed in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere (Coustenis et al., 2006). No longitudinal variations were found for the gases. Molecules showing a significant enhancement at northern latitudes are the nitriles (HC3N, HCN) and the complex hydrocarbons (C4H2, C3H4). The D/H ratio on Titan was also determined from the CH3D band at 8.6 micron and found to be about 1.2 ±0.2 10-4. Constraints are also set on the vertical distribution of C2H2. We also search to identify some remaining features unaccounted-for by the gas model, such as at 678 and 700 cm-1.