Latitudinal variations in Titana[tm]s methane and haze from Cassini VIMS observations

We analyze observations taken with Cassinia[tm]s Visual and Infrared Mapping Spectrometer (VIMS), to determine the current methane and haze latitudinal distribution between 60A degree S and 40A degree N. The methane variation was measured primarily from its absorption band at 0.61I14m, which is optically thin enough to be sensitive to the methane abundance at 20a"50km altitude. Haze characteristics were determined from Titana[tm]s 0.4a"1.6I14m spectra, which sample Titana[tm]s atmosphere from the surface to 200km altitude. Radiative transfer models based on the haze properties and methane absorption profiles at the Huygens site reproduced the observed VIMS spectra and allowed us to retrieve latitude variations in the methane abundance and haze. We find the haze variations can be reproduced by varying only the density and single scattering albedo above 80km altitude. There is an ambiguity between methane abundance and haze optical depth, because higher haze optical depth causes shallower methane bands; thus a family of solutions is allowed by the data. We find that haze variations alone, with a constant methane abundance, can reproduce the spatial variation in the methane bands if the haze density increases by 60% between 20A degree S and 10A degree S (roughly the sub-solar latitude) and single scattering absorption increases by 20% between 60A degree S and 40A degree N. On the other hand, a higher abundance of methane between 20 and 50km in the summer hemisphere, as much as two times that of the winter hemisphere, is also possible, if the haze variations are minimized. The range of possible methane variations between 27A degree S and 19A degree N is consistent with condensation as a result of temperature variations of 0a"1.5K at 20a"30km. Our analysis indicates that the latitudinal variations in Titana[tm]s visible to near-IR albedo, the north/south asymmetry (NSA), result primarily from variations in the thickness of the darker haze layer, detected by Huygens DISR, above 80km altitude. If we assume little to no latitudinal methane variations we can reproduce the NSA wavelength signatures with the derived haze characteristics. We calculate the solar heating rate as a function of latitude and derive variations of a arrow right 410a"15% near the sub-solar latitude resulting from the NSA. Most of the latitudinal variations in the heating rate stem from changes in solar zenith angle rather than compositional variations.