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

• Published in: Planetary and space science Vol. 88; pp. 93 - 99
• Main Authors: Sim, Chae Kyung, Kim, Sang Joon, Courtin, Régis, Sohn, Mirim, Lee, Dong-Hun
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2013; Elsevier
• Subjects: Astrophysics; Atmosphere; Composition; Haze; Infrared; Physics; Spectroscopy; Titan; Spectroscopy; Composition; Infrared; Titan; Atmosphere; Haze
• ISSN: 0032-0633; 1873-5088
• DOI: 10.1016/j.pss.2013.10.003

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 CH4 ro-vibrational bands, collision-induced absorption by N2–N2 pairs, and H2–N2 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 2.1–2.8μm range, with peaks near 2.30 and 2.35μm, and the relative amplitude of these peaks changing with altitude. The gross spectral shape of the τ-spectra is found similar to the typical 2-μm absorption spectra of the alkane group of hydrocarbon (CnH2n+2) ices. The τ-spectra retrieved at 2μm and those previously retrieved at 3μm by Kim et al. (2011) are simultaneously reproduced by combinations of 2- and 3-μm 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 τ-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. ●Spectral structure of 2-μm Titanian haze has been retrieved from VIMS data.●The retrieved τ-spectra show variations, and spectral peaks changes with altitude.●The gross spectral shape of τ-spectra is similar to the spectra of alkane ices.●The 2 and 3μm spectra of the haze are also reproduced by alkane and nitrile ices.●Further lab measurements of various ice spectra will help firm identification.