Thermal characterization of Titan's tholins by simultaneous TG-MS, DTA, DSC analysis

Three samples of Titan's tholins synthesized in laboratory under simulated Titan's conditions and presenting different degrees of exposure to ambient atmosphere have been used to study in detail their thermal behavior using thermogravimetry coupled with a mass spectrometer (TG-MS), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The degradation of Titan's tholins under inert atmosphere follows a three-step consecutive decomposition: a drying stage (>150 °C) where moisture is desorbed, this stage indicated the high hydrophilicity of the tholins; a second stage, the main pyrolysis stage (150-575 °C) where endothermic decomposition begins releasing mainly ammonia, HCN, acetonitrile, and methane over a broad temperature range. Few other hydrocarbon fragments such as ethylene and propane are released but no cyclic molecules, aliphatic or aromatic, are observed. The last stage (>575 °C) is the carbonization of the material leading to a non-crystalline graphitic residue. The thermal degradation under oxygen atmosphere shows the same stages as in argon, with a shift of the thermogravimetric peaks toward lower temperatures indicating a lower thermal stability. The last stage in this case is an oxidative combustion of the char residue. This research concludes that even if Titan tholins, subjected to air contamination for few minutes to several years (varying with the storage conditions) transform to produce different C/N and C/O ratios and thermal stabilities, they undergo the same thermal degradation phases and products. This suggests that the studied three tholins have a similar main chemical structure which does not alter by the air exposure. We discuss on the possible nature of this structure.