Retrieval and tentative indentification of the 3 μm spectral feature in Titan's haze

Abstract

Recently, an unidentified 3.3–3.4μm 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 CH4 and weak C2H6 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 127km and above 300km suggest inhomogeneity at these altitudes. We find that the positions of the major spectral peaks occur at 3.33–3.37μm, which are somewhat different from the typical 3.3μm aromatic or 3.4μm 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 C2H6, CH4, and CH3CN; and a broad shoulder from 3.37 to 3.50μm coincides with those of C5H12 and C6H12 as well as those of typical aliphatic C–H stretches. This result combined with high-altitude (∼1000km) 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 3μm 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 3μm feature.