Gaseous abundances and methane supersaturation in Titan's troposphere

Abstract

Various properties of Titan's troposphere are inferred from an analysis of Voyager 1 infrared spectrometer (IRIS) data between 200 and 600 cm −1. Two homogeneous spectral averages acquired at widely separated emission angles are chosen for the analysis. Both data sets are associated with northern low latitudes very close to that of the radio science ingress occultation point. Solutions require simultaneous nonlinear least-squares fits to the two IRIS data sets, coupled with iteration of the radio occultation refractivity data. Values and associated 1-σ uncertainties of several parameters are inferred from our analysis. These include mole fractions for molecular hydrogen (∼0.0011), argon (small), and methane near the surface (∼0.057). Solutions are also obtained for the hydrogen para-fraction (close to equilibrium, with considerable uncertainty), air temperature near the surface (∼93 K), surface temperature discontinuity (∼1 K), and maximum degree of methane supersaturation in the upper troposphere (∼ 1.5). Actual values for the above-mentioned parameters depend on the amount of ethane cloud near the tropopause. There is no evidence for methane clouds in the upper troposphere, nor is their presence compatible with large degrees of supersaturation. A wave number dependence for the stratospheric haze opacity is inferred similar to that found for a polymeric residue created in laboratory discharge experiments. This haze appears to be uniformly distributed with latitude between altitudes of 40 and 160 km, provided those nighttime data at southern high latitudes that are subject to possible systematic calibration errors are discounted. Assuming uniform haze distribution, both the air temperature and methane vapor mole fraction near the surface are symmetrically distributed about the equator, with lower values at higher latitudes. Either the tropopause temperature or the maximum degree of methane supersaturation is asymmetrically distributed about the equator. In either case, the data are consistent with a decrease of methane supersaturation toward the poles, which suggests an increase in mean annual precipitation at high latitudes compared with the equatorial region. If methane vapor is in saturation equilibrium with Titan's surface, the derived latitudinal gradient of the near surface methane vapor mole fraction implies that the liquid content of the surface is ethane-enriched near the poles.