Mapping of Titan's tropopause and surface temperatures from Voyager IRIS spectra

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We have derived temperatures at the tropopause and near the surface of Titan from a set of radiance measurements made by the Voyager IRIS instrument at 300 and 510 cm −1 (33.3 and 19.6 μm ). At 510 cm −1 , Titan's atmosphere is relatively transparent and the surface contribution to the measured radiance is approximately 60%. On the other hand, the 300 cm −1 radiance is mostly sensitive to the tropopause temperature, as well as to the degree of methane supersaturation in the upper troposphere. The retrievals are based on a simple inversion scheme of the radiative transfer equation and rely on a model of the atmospheric opacity derived by Courtin et al. (Icarus 114 (1995) 144). Although the longitude–latitude coverage afforded by the IRIS dataset is severely limited, as is also the surface area covered by the IRIS footprints (∼22%), we were able to extract longitudinal and latitudinal profiles of the tropopause and near-surface (brightness) temperatures. We find that the zonal distributions of the tropopause and near-surface temperatures are fairly homogenous. The meridional distribution of the tropopause temperature shows only a slight increase ( ∼1 K ) from −60° to 60°. On the other hand, the surface brightness temperature varies significantly and symmetrically from equator to pole (2.5– 3.2 K ). Less then one-third of this decrease can potentially be attributed to meridional variations of the stratospheric thermal structure. This confirms earlier results obtained from more limited selections of the IRIS data, both in terms of symmetry and amplitude. The role that could be played by meridional variations of the supersaturated distribution of methane is explored, although no definite answer can be derived in this respect. A significant variation of the bulk temperature gradient between the surface and the tropopause is suggested, the troposphere at high latitudes appearing more stable with respect to moist convection than in the equatorial regions. Finally, a very tentative result concerns an area where the near-surface (brightness) temperature shows a significant differential with respect to the extrapolation of the latitudinal distribution of surface temperatures. Possible interpretations of this differential include: a low emissivity surface feature with 25% contrast, an actual surface cold spot (with a temperature contrast of ∼3 K ), a transient tropospheric cloud of optical thickness τ∼1, or an elevated area of height ∼5 km . Further high spatial resolution data, especially those that will be collected by the Cassini orbital mission, will be necessary to discriminate between the various possibilities.