Titan's Thermal Emission Spectrum: Reanalysis of the Voyager Infrared Measurements

Abstract

We have modeled the far-infrared spectrum of Titan between 200 and 600 cm-1, including the fine structure of the H2-N2 and H2-CH4 dimers around 355 and 585 cm-1 respectively. A selection of 373 Voyager IRIS spectra recorded at low and mid-latitudes provides the observational basis for our analysis. The opacity model is significantly improved over previous work by taking into account recent ab initio calculations of the collision-induced absorption by N2-CH4 pairs, as well as laboratory measurements of the H2-N2 dimer transitions. In addition to the collision-induced gaseous absorption, the radiative transfer model includes scattering and absorption by stratospheric haze particles and potential tropospheric condensation clouds. We investigate the possible presence of argon, mainly through its influence on the thermal profile retrieved from the Voyager radiooccultation measurements. We find the following results: (1) The observations are best fit with significant methane supersaturation in the troposphere (up to a factor of two) and no condensation cloud present. (2) If a condensation cloud is present we find that it must have an optical depth less than 0.7 with a mean particle radius of about 50 μm and be located very near the minimum temperature level around 40 km, significantly higher than the expected condensation level. (3) In any case, the CH4 mole fraction at the cold trap is 0.017-0.045, the H2 mole fraction is 1.0 ± 0.4 × 10-3, the argon mole fraction is less than 0.06 (3σ), and the imaginary index of refraction of the haze material at 600 cm-1 is 30-80% that of tholins.