Analyses of the gas and solid phases of Titan aerosol simulants produced in the COSmIC/THS experiment

Titan’s atmosphere, composed mainly of N2 and CH4, is the siege of a complex chemistry, which occurs at temperatures lower than 200K and leads to the production of heavy molecules and subsequently solid aerosols that form the haze surrounding Titan. The Titan Haze Simulation (THS) experiment has been developed at the NASA Ames COSmIC facility to study Titan’s atmospheric chemistry at low temperature. The chemistry is simulated by plasma in the stream of a supersonic expansion. With this unique design, gas is jet-cooled to Titan-like temperature 150K) before inducing the chemistry by plasma, and remains at low temperature in the plasma discharge 200K measured by optical emission spectroscopy). Different N2-CH4-based gas mixtures can be injected in the plasma, with or without the addition of trace elements present on Titan. Both the gas phase and solid phase products resulting from the plasma-induced chemistry can be monitored and analyzed using different in situ and ex situ diagnostics. Here we present and discuss the results of two recent studies. A mass spectrometry analysis of the gas phase has demonstrated that the COSmIC/THS experiment can be used to study the first and intermediate steps as well as specific chemical pathways of Titan’s atmospheric chemistry due to the short residence time of the gas in the plasma discharge. The more complex chemistry, when adding trace elements to the initial N2-CH4 mixture, has been confirmed by an extensive study of the solid phase: scanning electron microscopy images show much bigger grains produced in N2-CH4-C6H6 mixtures (1-5 µm) compared to N2-CH4 mixtures (0.1-0.5 µm), and NMR supports a growth evolution of the chemistry when adding acetylene to the N2-CH4 mixture, resulting in the production of more complex (possibly double/triple or nitrogen-hydrogen) hydrogen bonds than with a simple N2-CH4 mixture. Raman spectroscopy, IR spectroscopy, and gas-chromatography mass spectrometry results will also be discussed. These complementary studies show the potential of COSmIC/THS to better understand the first steps of Titan’s chemistry and the origin of aerosol formation.