Plausible condensates in Titan's stratosphere from Voyager infrared spectra

We have analyzed high-latitude Voyager 1 infrared spectra of Titan to investigate emission features, above the noise level, which are unidentified to this date and are probably not due to gaseous components. These are mainly located near 22q, 478, 504, 700, 760, and 816 cm(-1), of which the 478 cm(-1) feature has been previously identified as C4N2 Possible identifications of the five remaining features are investigated here by comparing the observations with laboratory spectra of potential ice condensate candidates: H2O, CO2, CH4, C2H2, C2H4, C2H6, C3H4, C3H8, C4H2, HCN, C2N2, HC3N, C4N2, CH3CN, as well as with spectra of some more complex nitriles and hydrocarbons, some of which correspond to original laboratory work. Radiative transfer calculations show that the feature observed near 220 cm(-1), which could be water ice, is about a factor of 2 too narrow to be compatible with the whole signature of solid H2O. In this paper we have also performed radiative transfer calculations for solid C4N2 and HC3N (confirming the presence of the first and suggesting the latter). By comparing with laboratory data, we find that C2H2, C2H5CN, and possibly HCN and C2H6 solids, are likely candidates, while eliminating other condensed forms of photochemical products in Titan's stratosphere. (C) 1999 Elsevier Science Ltd. All rights reserved.