The 3.4μm Absorption in Titan’s Stratosphere: Contribution of Ethane, Propane, Butane and Complex Hydrogenated Organics

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 generate 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 3.4μm band using a radiative transfer model. Ethane and propane are significant components of the band but they are insufficient to reproduce perfectly its shape. 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 precursors of aerosol particles. High similarities between individual spectra impede the derivation of abundances.