Temperature effect on the far infrared absorption features of aromatic-based Titan aerosol analogs

Abstract:

The detection of benzene at ppm levels in Titan’s atmosphere^[1] by Cassini’s Ion and Neutral Mass Spectrometer (INMS) has enforced the idea that aromatic and heteroaromatic reaction pathways may play an important role in Titan’s atmospheric chemistry, especially in the formation of aerosols. Indeed, Trainer et al.^[2] showed that aromatic molecules are easily dissociated by ultraviolet radiation and can therefore contribute significantly to aerosol formation. Sebree et al. ^[3] used such a mixture of low concentration aromatic and/or heteroaromatic molecules (benzene, naphthalene, pyridine, quinoline and isoquinoline) to produce aerosol analogs and then analyzed their far infrared absorption spectra. Their study shows that such aerosols can reproduce some spectral features observed by Cassini’s Composite InfraRed Spectrometer (CIRS) in the far infrared below 500cm^{-1 [4]}. Aerosols absorption at such a low wavenumbers most likely results from lattice resonances within their structure^[4,5], and this might be influenced by the temperature of the sample. In this work we investigated the influence of temperature on the absorption spectra of the aerosol samples studied in Sebree et al.^[4]. We recorded spectra at 100K and 300K and this revealed variations in the total absorption with temperature, but no new absorption features were observed. Through this investigation we have also found an unexpected strong absorption band of cooled Silicon in the far infrared, which has never been reported in the literature.

[1] Waite et al. Science 316 (5826) : 870-875

[2] Trainer et al. ApJL 766: L4, 2013

[3] Sebree et al. Icarus 236: 146-152, 2014

[4] Anderson et al. Icarus 212: 762-778, 2011

[5] Gautier et al. Icarus 221: 320-327, 2012