The Titan Haze Simulation (THS) experiment on COSmIC. Part III. XANES study of laboratory analogs of Titan tholins

We have used the Titan Haze Simulation (THS) experimental set-up at NASA Ames’ COSmIC facility to produce four laboratory analogs of Titan’s aerosols, or tholins. These tholin samples were produced from four gas mixtures of initial compositions N2:CH4 (90:10 and 95:5) and N2:CH4:C2H2 (89.5:10:0.5 and 94.5:5:0.5) at 150 K, using a plasma discharge in the stream of a jet-cooled gas expansion. Here we present an ex situ X-ray absorption near-edge structure (XANES) spectroscopy analysis of these four tholin samples. The C- and N-XANES spectra show the presence of various functional groups (aromatic carbon, imines, nitriles, etc.) whose abundances are correlated with (i) the relative proportions between N2 and CH4, and (ii) the presence or absence of C2H2 in the initial mixtures. In particular, mixtures containing C2H2 result in the formation of tholins consisting of more aromatic structures and displaying larger relative amounts of imines and nitriles, but lower overall nitrogen content. XANES spectroscopy also allowed for the determination of the elemental C/N ratios for the four tholins, which were all found to be low (< 2.5) and consistent with those measured for tholins produced in other, independent experimental set-ups. The C/N ratios of tholins produced from gas mixtures that contained C2H2 (2.2–2.4) were found to be about twice as large as those of tholins produced from gas mixtures that did not contain C2H2 (0.9–1.3). These new results from the COSmIC/THS experimental set-up demonstrate the impact initial precursors have on the nitrogen chemistry during the formation of tholins, which may help in the interpretation of observations and the development of models of Titan’s atmosphere.