HR: 08:00h
AN: P21D-01 INVITED    [Abstracts]
TI: Titan's atmospheric composition: from Voyager to Cassini and beyond
AU: * Coustenis, A
EM: athena.coustenis@obspm.fr
AF: LESIA, Paris-Meudon Observatory, 5, place Jules Janssen, Meudon, 92195, France
AB: Titan's atmosphere was revealed by the Voyager missions in the 80s. The trace composition was in particular inferred from infrared spectra by the V1/IRIS Spectrometer. ISO gave us an opportunity to further explore this exciting milieu in 1997 (Coustenis et al., 1998; 2003) and brought the discovery of new molecules : H₂O and C₆H₆. Our understanding of Titan's atmospheric chemical composition has recently been enhanced by the data returned by the Cassini instruments. Spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft have been processed from the Titan flybys spanning three years now since SOI (Flasar et al., 2005; Teanby et al., 2006, Vinatier et al., 2006; Nixon et al., 2006; Coustenis et al., 2007). The spectra characterize various regions on Titan from 85°S to 80°N with a variety of emission angles. We have studied the emission observed in the CIRS detector arrays (covering the 10-1500 cm^-1 spectral range with apodized resolutions of 2.54 or 0.53 cm^-1). We have used temperature profiles retrieved from the inversion of the emission observed in the methane 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., 2007a). The composite spectra show several signatures of previously identified molecules: hydrocarbons, nitriles, H₂O and CO₂. Besides these well-known trace species, a firm detection of benzene (C₆H₆) is provided by CIRS at 674 cm^-1 and allows for the study of its latitudinal variations. No longitudinal variations were found for any of the gases. Information is retrieved on the meridional variations of the trace constituents and tied to predictions by dynamical-photochemical models (Hourdin et al., 2004; Lavvas et al., 2007). 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. We have also identified the presence of C₂HD at 678 cm^-1 for the first time (Coustenis et al., 2007b, in preparation). Constraints are also set on the vertical distribution of C₂H₂. However successful, the Cassini-Huygens mission has brought new enquiries that can only be answered by future missions to Titan. Such a mission, a collaboration between ESA and NASA in the spirit of Cassini, was recently proposed by the TANDEM Consortium in response to ESA's Cosmic Vision Call. References : Coustenis et al., 1989, Icarus 80, 54; Coustenis et al., 1998, A & A 336, L85-L89; Coustenis et al., 2003, Icarus 161, 383; Coustenis et al., 2007a, Icarus 1889, 35-62; Flasar et al., 2005, Science 308, 975 ; Hourdin et al., 2004, J. Geophys. Res. 109, E1205; Nixon et al., 2007, Icarus, in press; Lavvas et al., 2007, Plan. Space Sci., in press; Teanby et al., 2006, Icarus 181, 243; Vinatier et al., 2006, Icarus, 188, 120.
DE: 5210 Planetary atmospheres, clouds, and hazes (0343)
DE: 5405 Atmospheres (0343, 1060)
DE: 5410 Composition (1060, 3672)
DE: 5464 Remote sensing
DE: 5470 Surface materials and properties
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting