Titan's atmosphere from ISO mid-infrared spectroscopy

Abstract

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 C ₂H ₂, HCN, C ₂H ₆, and CO ₂ and inferred some information on the abundance of the first species as a function of altitude. From the CH ₃D band at 1161 cm ^-1 and for a CH ₄ 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 ν ₅ band of HC ₃N 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 C ₆H ₆ is incorporated into the atmospheric model. This corresponds to a column density of ∼ 2 × 10 ¹⁵ molecules cm ^-2 above the 30-mbar level. We have also tested available vertical profiles for HC ₃N and C ₆H ₆ 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).