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Millimeter and Submillimeter Heterodyne Observations of Titan: Retrieval of the Vertical Profile of HCN and the12C/13C Ratio☆

1997; Elsevier BV; Volume: 126; Issue: 1 Linguagem: Inglês

10.1006/icar.1996.5640

1090-2643

Taufiq Hidayat, A. Marten, Bruno Bézard, D. Gautier, Tobias Owen, H. E. Matthews, G. Paubert,

Planetary Science and Exploration

We report new observations of the (1–0) line of HCN at 88.63 GHz, emitted by Titan, made in May 1995 with the IRAM 30-m radiotelescope located at Pico Veleta (Spain). Compared to previous observations of Tanguyet al.(1990,Icarus85, 43–57), the signal-to-noise ratio has been improved by a factor of 5, permitting us to retrieve the vertical distribution of HCN in the 80–350 km altitude range. Using the production rate of HCN predicted from photochemical models, we derive a profile of the eddy diffusion coefficient up to 350 km altitude consistent with the observed HCN vertical distribution. This coefficient increases with height more rapidly than the standardn−0.5dependence in the stratosphere. The (4–3) line of H13CN at 345.34 GHz was subsequently observed with the JCMT on Mauna Kea (Hawaii) in June 1995. The analysis of the two spectra yields a12C/13C ratio between 70 and 120, consistent with the terrestrial value. Taking into account possible fractionation processes, we show that the C-bearing species which formed Titan should have had a carbon isotopic ratio in the same range.