Tholin formation and accumulation due to hypersonic high-temperature flows in methane-containing atmospheres

We report on another tholins formation mechanism relying on the high energies typically available in hypersonic flows. The considered conditions are representative of vehicle or meteorite entry in atmospheres containing the required precursor molecules. It is shown that the involved high temperatures can break the chemical bonds in methane molecules, thereby releasing carbon atoms in the gas mixture. These can combine with the other species and form various compounds with a variable degree of carbon depending on the local conditions in which the reaction takes place. Notably, the variability in composition also depends on the specific path followed by molecules in the gas flow. The presence of sudden expansions or localized shock waves in the current and the ensuing variations in pressure and temperature, have a non-negligible effect on the nature of the organic residues. These have been characterized by an array of chemical and spectroscopic techniques: atmospheric pressure photoionization mass spectrometry (APPI-MS), elemental analysis, infrared absorption (FT-IR), UV-visible absorption and fluorescence. Moreover, some effort was provided to correlate them with the thermochemical and fluid-dynamic conditions occurring in the hypersonic wind tunnel used for the experiments. Overall, such results suggest a novel possible route to the formation of tholins, which is still poorly known and hitherto scarcely investigated.