The 3.4μm absorption in Titan’s stratosphere: Contribution of ethane, propane, butane and complex hydrogenated organics

Abstract

The complex organic chemistry harbored by the atmosphere of Titan has been investigated in depth by Cassini observations. Among them, a series of solar occultations performed by the VIMS instrument throughout the 13 years of Cassini revealed a strong absorption centered at 3.4 µm. Several molecules present in Titan's atmosphere create spectral features in that wavelength region, but their individual contributions are difficult to disentangle. In this work, we quantify the contribution of the various molecular species to the 3.4 µm band using a radiative transfer model. Ethane and propane are a significant component of the band but they are not enough to fit the shape perfectly, then we need something else. Polycyclic Aromatic Hydrocarbons (PAHs) and more complex polyaromatic hydrocarbons like Hydrogenated Amorphous Carbons (HACs) are the most plausible candidates because they are rich in C -H bonds. PAHs signature have already been detected above 900 km, and they are recognized as aerosols particles precursors. High similarities between individual spectra impede abundances determinations.

Exploring and Validating Exoplanet Atmospheric Retrievals with Solar System Analog Observations

“…Faux error bars were assigned at the level of 0.1 ppm, which results in a ratio of error bar size to spectral feature depth that is roughly comparable to those from JWST -relevant clearsky, solar metallicity, warm Neptune cases investigated by Greene et al (2016). Based on previous work analyzing VIMS Titan occultation data (Maltagliati et al 2015;Cours et al 2020), fits included volume mixing ratios for carbon monoxide, methane, acetylene (C 2 H 2 ), and propane (C 3 H 8 ). The fitted planetary radius was applied at the 10 mbar pressure level (Benneke & Seager 2012).…”

Distribution and intensity of water ice signature in South Xanadu and Tui Regio

“…We discretized the atmosphere into 70 layers uniformly distributed from ground level up to an altitude of 700 km, since it was found as an acceptable compromise between accuracy and computation time (McKay et al, 1989;Cours et al, 2020). In addition, for each layer we computed the average optical depths, phase functions and single scattering albedos for all VIMS-IR channels.…”

“…11-b), the value of the ratio is strictly greater than 1. Not all ratios have the same impact: we tend to have greater error bars at short wavelengths for the albedo, and there are known missing absorptions (Cours et al, 2020), like for ethane at 2.7-2.8  m (Maltagliati et al, 2015). Furthermore, we work with the VIMS data calibration RC-19 which is more recent, but less reliable for the 2.7-2.8 windows compared to the RC-17 as it tends to change the intensities values in this window by about 5 % (Clark et al, 2018).…”

Distribution and intensity of water ice signature in South Xanadu and Tui Regio

“…11-b), the value of the ratio is strictly greater than 1. Not all ratios have the same impact: we tend to have greater error bars at short wavelengths for the albedo, and there are known missing absorptions (Cours et al 2020), like for ethane at 2.7-2.8 µm (Maltagliati et al 2015). Furthermore, we work with the VIMS data calibration RC-19 which is more recent, but less reliable for the 2.7-2.8 windows compared to the RC-17 as it tends to change the intensities values in this window by about 5 % (Clark et al 2018).…”