The two-micron spectral characteristics of the Titanian haze derived from Cassini/VIMS solar occultation spectra

摘要

Vertically-resolved spectral characteristics of the Titanian haze in the 2-μm 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 CH_4 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 τ-spectra) for the haze show marked variations in the 2.1-2.8 μm range, with peaks near 2.30 and 2.35 μm, and the relative amplitude of these peaks changing with altitude. The gross spectral shape of the T-spectra is found similar to the typical 2-μm absorption spectra of the alkane group of hydrocarbon (C_nH_(2n+2)) ices. The T-spectra retrieved at 2 μm and those previously retrieved at 3 μm by Kim et al. (2011) are simultaneously reproduced by combinations of 2- and 3-μm absorbance spectra of alkane ices such as CH_4, C_2H_6, C_5H_(12), C_6H_(16), with the addition of a nitrile ice, CH_3CN. These combinations are neither unique nor limited and need more fine-tuning to fit the detailed features of the T-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.