Titan’s atmosphere from ISO mid-infrared spectroscopy

We have analyzed Titan observations performed by the Infrared Space Observatory (ISO) in the range 7–30 μm. The spectra obtained by three of the instruments on board the mission (the short wavelength spectrometer, the photometer, and the camera) were combined to provide new and more precise thermal and compositional knowledge of Titan’s stratosphere. With the high spectral resolution achieved by the SWS (much higher than that of the Voyager 1 IRIS spectrometer), we were able to detect and separate the contributions of most of the atmospheric gases present on Titan and to determine disk-averaged mole fractions. We have also tested existing vertical distributions for C2H2, HCN, C2H6, and CO2 and inferred some information on the abundance of the first species as a function of altitude. From the CH3D band at 1161 cm−1 and for a CH4 mole fraction assumed to be 1.9% in Titan’s stratosphere, we have obtained the monodeuterated methane-averaged abundance and retrieved a D/H isotopic ratio of 8.7−1.9+3.2 × 10−5. We discuss the implications of this value with respect to current evolutionary scenarios for Titan. The ν5 band of HC3N at 663 cm−1 was observed for the first time in a disk-averaged spectrum. We have also obtained a first tentative detection of benzene at 674 cm−1, where the fit of the ISO/SWS spectrum at R = 1980 is significantly improved when a constant mean mole fraction of 4 × 10−10 of C6H6 is incorporated into the atmospheric model. This corresponds to a column density of ∼ 2 × 1015 molecules cm−2 above the 30-mbar level. We have also tested available vertical profiles for HC3N and C6H6 and adjusted them to fit the data. Finally, we have inferred upper limits of a few 10−10 for a number of molecules proposed as likely candidates on Titan (such as allene, acetonitrile, propionitrile, and other more complex gases).