MILLIMETER AND SUBMILLIMETER HETERODYNE OBSERVATIONS OF TITAN, RETRIEVAL OF THE VERTICAL PROFILE OF HCN AND THE C-12 C-13 RATIO/

We report new observations of the (1-0) line of HCN at 58.63 GHz, emit ted by Titan, made in May 1995 with the IRAM 30-m radiotelescope locat ed at Pico Veleta (Spain). Compared to previous observations of Tanguy et al. (1990, Icarus 85, 43-57), the signal-to-noise ratio has been i mproved by a factor of 5, permitting us to retrieve the vertical distr ibution of HCN in the 80-350 km altitude range. Using the production r ate 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 wit h height more rapidly than the standard n(-0.5) dependence in the stra tosphere, The (4-3) line of (HCN)-C-13 at 345.34 GHz was subsequently observed with the JCMT on Mauna Kea (Hawaii) in June 1995. The analysi s of the two spectra yields a C-12/C-13 ratio between 70 and 120, cons istent with the terrestrial value. Taking into account possible fracti onation processes, we show that the C-bearing species which formed Tit an should have had a carbon isotopic ratio in the same range. (C) 1997 Academic Press.