Modeling Titan's Thermal Infrared Spectrum for High-Resolution Space Observations

Abstract

The infrared spectrum of Titan has been modeled between 7 μm and 1 mm at a spectral resolution of 0.5 cm -1, using up-to-date information on the satellite's thermal/compositional structure and recent spectroscopic parameters, to illustrate the capabilities of future space mission instruments (such as the short-wavelength spectrometer/long-wavelength spectrometer aboard the Infrared Space Observatory and the composite infrared spectrometer to fly aboard Cassini) in terms of detectability of new species and improvement of actual temperature/abundance determinations. In addition, specific parts of the spectrum have been modeled at 0.03-cm -1 resolution—representative of the ISO/Fabry-Pérot mode—and synthetic spectra have been generated to account for the horizontal viewing measurements that will be performed by CIRS. Notably, the submillimeter range (10-200 cm -1) of Titan's spectrum is simulated as it will be observed for the first time, with rotational lines of HCN, CO, H 2O (as yet undetected), and CH 4 clearly appearing at these frequencies. The combined analysis of the methane rotational lines and of its v 4 band at 7.7 μm should yield constraints on both the temperature and the methane abundance in Titan's atmosphere. Our calculations show that the thermal spectrum recorded by ISO in a few hours' integration time should allow us to retrieve the vertical distributions of the most abundant species after methane and possibly detect for the first time water vapor, benzene, allene, and other heavier trace molecules, if their abundances are in the range of the laboratory expectations. The Fabry-Pérot mode of ISO, very expensive in terms of time, will allow us to retrieve the vertical distributions of some of the less abundant species from selected small spectral regions. While ISO data are representative of Titan's disk average, CIRS, with comparable capabilities in the vertical viewing mode, will complete this information with spatially resolved measurements.