Retrieval and tentative identification of the 3 mum spectral feature in Titan's haze

International audienceRecently, an unidentified 3.3-3.4 mum feature found in the solar occultation spectra of the atmosphere of Titan observed by Cassini/VIMS was tentatively attributed to the C-H stretching mode of aliphatic hydrocarbon chains attached to large organic molecules, but without properly extracting the feature from adjacent influences of strong CH 4 and weak C 2H 6 absorptions ( Bellucci et al., 2009). In this work, we retrieve the detailed spectral feature using a radiative transfer program including absorption and fluorescent emission of both molecules, as well as absorption and scattering by haze particles. The spectral features of the haze retrieved from the VIMS data at various altitudes are similar to each other, indicating relatively uniform spectral properties of the haze with altitude. However, slight deviations observed near 127 km and above 300 km suggest inhomogeneity at these altitudes. We find that the positions of the major spectral peaks occur at 3.33-3.37 mum, which are somewhat different from the typical 3.3 mum aromatic or 3.4 mum aliphatic C-H stretches usually seen in the spectra of dust particles of the interstellar medium and comets. The peaks, however, coincide with those of the solid state spectra of C 2H 6, CH 4, and CH 3CN; and a broad shoulder from 3.37 to 3.50 mum coincides with those of C 5H 12 and C 6H 12 as well as those of typical aliphatic C-H stretches. This result combined with high-altitude (˜1000 km) haze formation process recently reported by Waite et al. (2007) opens a new question on the chemical composition of the haze particles. We discuss the possibility that the 3 mum feature may be due to the solid state absorption bands of these molecules (or some other molecules) and we advocate additional laboratory measurements for the ices of hydrocarbon and nitrogen-bearing molecules present in Titan's atmosphere for the identification of this 3 mum feature