Vertically-resolved spectral characteristics of the Titanian haze in the 2-mu m wavelength range were derived from solar occultation spectra measured by Cassini/VIMS on January 15, 2006. At the various altitudes probed by the solar occultation measurements, we reproduced the observed spectra using a radiative transfer program including absorption by CH4 ro-vibrational bands, collision-induced absorption by N-2-N-2 pairs, and H-2-N-2 dimers, as well as absorption and scattering by the haze particles. The retrieved optical depth spectra (or tau-spectra) for the haze show marked variations in the 2.1-2.8 mu m range, with peaks near 2.30 and 235 mu m, and the relative amplitude of these peaks changing with altitude. The gross spectral shape of the tau-spectra is found similar to the typical 2-mu m absorption spectra of the alkane group of hydrocarbon (CnH2n+2) ices. The tau-spectra retrieved at 2 mu m and those previously retrieved at 3 mu m by Kim et al. (2011) are simultaneously reproduced by combinations of 2- and 3-mu m absorbance spectra of alkane ices such as CH4, C2H6, C5H12, C6H14, with the addition of a nitrile ice, CH3CN. These combinations are neither unique nor limited and need more fine-tuning to fit the detailed features of the tau-spectra. There is a need for additional laboratory measurements of absorbance and indices of refraction for a wider variety of hydrocarbon and nitrile ices in the temperature range relevant to Titan. (C) 2013 Elsevier Ltd. All rights reserved.
