Plasma discharge in N 2 + CH 4 at low pressures: Experimental results and applications to Titan

Abstract

We report the yields of gaseous hydrocarbons and nitriles produced in a continuous flow, low-dose, cold plasma discharge excited in a 10% CH 4, 90% N 2 atmosphere at 295 K and pressures p of 17 and 0.24 mbar, and use the results to compute expected abundances of minor constituents in Titan's atmosphere. These experiments are, by design, relevant to the atmospheric chemistry induced by cosmic rays in Titan's troposphere and (at the lower pressure) to chemistry initiated by Saturnian magnetospheric electrons and other charged particle sources which excite stratospheric aurorae. At p = 17 mbar, 59 gaseous species including 27 nitriles are detected in overall yield 4.0 (C + N) atoms incorporated into products per 100 eV (heV). At p = 0.24 mbar, 19 species are detected, including six nitriles and three other unidentified N-bearing compounds; the yield is 0.79 (C + N)/heV, a mild decrease with pressure. The types of molecules formed change more markedly, with high degrees of multiple bonding at 0.24 mbar prevailing over more H-saturated molecules at 17 mbar. The molecules and yields at 0.24 mbar bear a striking resemblance to the minor constituents found in Titan's atmosphere, all of which are abundant products in the laboratory experiment. Using the altitude-integrated flux of charged particle energy deposition at Titan, the laboratory yields at p = 0.24 mb, and a simple eddy mixing model, we compute absolute stratospheric column abundances and mole fractions. These are found to be in very good agreement with the Voyager IRIS observations. Except for the primarily photochemical products, C 2H 6 and C 3H 8, the match is much better than that obtained by photochemical-kinetic models, demonstrating that properly designed laboratory experiments are directly applicable to modeling radiation-chemical processes in planetary atmospheres. On the basis of this agreement we expect CH 3CN (ethanenitrile = acetonitrile) CH 2CHCHCH 2 (1,3-butadiene), CH 2CCH 2 (1,2-propadiene = allene), and CH 2CHCCH (1-buten-3-yne) to be present at mol fractions X > 10 −9, and CH 2CHCN (propenenitrile), CH 3CHCH 2 (propene), and CH 3CH 2CN (propanenitrile) at X > 10 −10 in Titan's atmosphere.