TITANS THERMAL EMISSION-SPECTRUM - REANALYSIS OF THE VOYAGER INFRAREDMEASUREMENTS

We have modeled the far-infrared spectrum of Titan between 200 and 600 cm(-1), including the fine structure of the H-2-N-2 and H-2-CH4 dimer s around 355 and 585 cm(-1) respectively. A selection of 373 Voyager I RIS spectra recorded at low and mid-latitudes provides the observation al basis for our analysis. The opacity model is significantly improved over previous work by taking into account recent ab initio calculatio ns of the collision-induced absorption by N-2-CH4 pairs, as well as la boratory measurements of the H-2-N-2 dimer transitions. In addition to the collision-induced gaseous absorption, the radiative transfer mode l includes scattering and absorption by stratospheric haze particles a nd potential tropospheric condensation clouds. We investigate the poss ible presence of argon, mainly through its influence on the thermal pr ofile retrieved from the Voyager radiooccultation measurements. We fin d the following results: (1) The observations are best fit with signif icant methane supersaturation in the troposphere (up to a factor of tw o) and no condensation cloud present. (2) If a condensation cloud is p resent we find that it must have an optical depth less than 0.7 with a mean particle radius of about 50 mu m and be located very near the mi nimum temperature level around 40 km, significantly higher than the ex pected condensation level. (3) In any case, the CH4 mole fraction at t he cold trap is 0.017-0.045, the H-2 mole fraction is 1.0 +/- 0.4 x 10 (-3), the argon mole fraction is Less than 0.06 (3 sigma), and the ima ginary index of refraction of the haze material at 600 cm(-1) is 30-80 % that of tholins. (C) 1995 Academic Press, Inc.