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L.A. Mayo (Raytheon ITSS), R.E. Samuelson (NASA Goddard Space Flight Center)
Considering only absorption and emission, Samuelson and Mayo (1991) determined the vertical distribution of aerosol opacity in the thermal infrared associated with Titan's north polar hood using the Voyager I IRIS dataset. In addition to a uniformly distributed aerosol haze between 40 and 300 km, they inferred a second component in the lower stratosphere where hydrocarbon condensation is expected. Shortly thereafter, Samuelson (1992) considered the role of scattering and inferred that the wavenumber dependence of opacity for the second component matched that of a thin hydrocarbon cloud composed of moderately large particles. With further modeling, Mayo and Samuelson (1997) infered an upper limit to particle size of around 10 microns. These particles then are strong candidates as nucleation centers for condensing methane out of the troposphere which would result in the observed mole fraction variation of methane with height.
Ethane being the most abundant volatile in the lower stratosphere by almost an order of magnitude is strongly implicated as a candidate for this process. Additionally, it is known from laboratory measurements that methane will condense out and adhere to ethane condensate particles. However, only gaseous ethane has been observed empirically in infrared spectra. Previous attempts to firmly identify condensed ethane as the seed nuclei in this process have been somewhat incomplete having not fully accounted for collisionally induced absorption. This study, represents a more thorough analysis of the IRIS north polar hood spectra, as it includes calculations of opacity in the troposphere due to collisional absorption from N2, H2, and CH4 using absorption coefficients derived from laboratory measurements. Abundances for the ethane condensate cloud are inferred. The impact of these results on seasonal variations is discussed. The impact on latitudinal variations in methane abundances as infered by Samuelson, Nath, and Borysow (1997) are also examined.