Clouds, haze, and CH<SUB>4</SUB>, CH<SUB>3</SUB>D, HCN, and C<SUB>2</SUB>H<SUB>2</SUB> in the atmosphere of Titan probed via 3 mum spectroscopy

International audienceUsing synthetic spectra derived from an updated model atmosphere together with a continuum model that includes contributions from haze, cloud and ground, we have re-analyzed the recently published (Geballe et al., 2003, Astrophys. J. 583, L39-L42) high-resolution 3 mum spectrum of Titan which contains newly-detected bands of HCN (in emission) and C 2H 2 and CH 3D (in absorption), in addition to previously detected bands of CH 4. In the 3.10-3.54 mum interval the analysis yields strong evidence for the existence of a cloud deck or optically thick haze layer at about the 10 mbar (Ë 100 km) level. The haze must extend well above this altitude in order to mask the strong CH 4 lines at 3.20-3.50 mum. These cloud and haze components must be transparent at 2.87-2.92 mum, where analysis of the CH 3D spectrum demonstrates that Titan's surface is glimpsed through a second cloud deck at about the 100 mbar (Ë 50 km) level. Through a combination of areal distribution and optical depth this cloud deck has an effective transmittance of Ë 20%. The spectral shape of Titan's continuum indicates that the higher altitude cloud and haze particles responsible for suppressing the CH 4 absorptions have a largely organic make-up. The rotational temperature of the HCN ranges from 140 to 180 K, indicating that the HCN emission occurs over a wide range of altitudes. This emission, remodeled using an improved collisional deactivation rate, implies mesospheric mixing ratio curves that are consistent with previously predictions. The stratospheric and mesospheric C 2H 2 mixing ratios are Ë10, considerably less than previous model predictions (Yung et al., 1984), but approximately consistent with recent observational results. Upper limits to mixing ratios of HC 3N and C 4H 2 are derived from non-detections of those species near 3.0 mum