Clouds, haze, and CH 4, CH 3D, HCN, and C 2H 2 in the atmosphere of Titan probed via 3 μm spectroscopy

Abstract

Using 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 μm 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 μm interval the analysis yields strong evidence for the existence of a cloud deck or optically thick haze layer at about the 10 mbar (∼ 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 μm. These cloud and haze components must be transparent at 2.87–2.92 μm, where analysis of the CH 3D spectrum demonstrates that Titan's surface is glimpsed through a second cloud deck at about the 100 mbar (∼ 50 km) level. Through a combination of areal distribution and optical depth this cloud deck has an effective transmittance of ∼ 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 ∼ 10 −5 , 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 μm.