Clouds, haze, and CH₄, CH₃D, HCN, and C₂H₂ in the atmosphere of Titan probed via 3 mum spectroscopy

Sang Joon Kim (1) , Thomas R. Geballe (2) , Keith S. Noll (3) , Régis Courtin (4, 5, 6)

1 Department of Astronomy and Space Science, Kyunghee University
2 Gemini Observatory
3 STScI - Space Telescope Science Institute
4 DESPA - Departement de recherche SPAtiale
5 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
6 Pôle Planétologie du LESIA

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 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 (˜ 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 (˜ 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, 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.

Domaines: Astrophysique [astro-ph]

Sang Joon Kim, Thomas R. Geballe, Keith S. Noll, Régis Courtin. Clouds, haze, and CH₄, CH₃D, HCN, and C₂H₂ in the atmosphere of Titan probed via 3 mum spectroscopy. Icarus, 2005, 173, pp.522-532. ⟨10.1016/j.icarus.2004.09.006⟩. ⟨hal-03786096⟩