Detection Opportunity for Aromatic Signature in Titan’s Aerosols in the 4.1–5.3 μm Range

Abstract

The Cassini / Huygens mission provided new insights on the chemistry of the upper atmosphere of Titan. The presence of large molecules and ions (>100's of amu) detected by Cassini at high altitude was not expected, and questions the original assumptions regarding the aerosol formation pathways. From recent laboratory studies, it has been shown that the inclusion of trace amounts of aromatic species drastically impacts the chemistry of aerosol formation and induces observable changes in the properties of the aerosols. In this Letter we focus on the effect of one of the simplest nitrogenous aromatics, pyridine (C 5 H 5 N), on the infrared signature of Titan’s aerosol analogs. We introduce initial gas mixtures of (i) N 2 –C 5 H 5 N (100%/250 ppm), (ii) N 2 –CH 4 –C 5 H 5 N (99%/1%/250 ppm), and (iii) N 2 –CH 4 (99%/1%) in a cold plasma discharge. The material produced, herein called tholins, is then analyzed by mid-infrared spectroscopy. When adding pyridine in the discharge, the tholins produced present an aromatic signature in the 4.1–5.3 μ m (1850–2450 cm −1 ) spectral region, attributed to overtones of aromatic C–H out-of-plane bending vibrations. We also observe a spectral shift of the nitrile and iso-nitrile absorption band with the inclusion of pyridine in the gas mixture. These results could help to investigate the data obtained at Titan by the Cassini /Visual and Infrared Mapping Spectrometer (VIMS) instrument in the 1–5 μ m infrared window.