The millimeter spectrum of Titan: Detectability of HCN,HC3N,and CH3CN and the CO abundance

The flux emitted by Titan's disk in millimeter lines of HCN, HC₃N, CH₃CN, and CO is calculated by means of a radiative transfer formulation which takes into account the sphericity of the atmosphere. It is demonstrated that the plane-parallel approximation for radiative transfer is no longer valid, especially in the core of emission lines, when Titan is not spatially resolved. The antenna temperatures which would be measured by large radiotelescopes observing Titan at frequencies of (1?0) and (2?1) transitions of CO, of (1?0), (2?1), and (3?2) transitions of HCN, and of selected transitions of HC₃N and CH₃CN in the range 80–300 GHz are calculated. The observability of these transitions is investigated. It is concluded that there is the possibility of inferring the vertical stratospheric distribution of these species from line shape measurements to be achieved with existing or forthcoming radioastronomical instrumentation. The determination of the CO abundance by D. O. Muhleman, G. L. Berge, and R. T. Clancy (1984, (Science (Washington, D.C.), 223, 393–396) from measurements at 115.3 GHz in two 200 MHz bands, is reinterpreted by means of this radiative transfer formulation. A CO mixing ratio between 3 × 10^?5 and 18 × 10^?5, with a most plausible value of 7.5 × 10^?5, is found.