Laboratory Investigations of Titan Haze Formation: In Situ Measurement of Gas and Particle Composition

摘要

Prior to the arrival of Cassini-Huygens, aerosol production in Titan'satmosphere was believed to begin in the stratosphere where chemical processesare predominantly initiated by FUV radiation. However, measurements taken byCassini UVIS and CAPS indicate that haze formation initiates in thethermosphere where there is a greater flux of EUV photons and energeticparticles available to initiate chemical reactions, including the destructionof N2. The discovery of previously unpredicted nitrogen species in Titan'satmosphere by Cassini INMS indicates that nitrogen participates in thechemistry to a much greater extent than was appreciated before Cassini. Thedegree of nitrogen in the haze is important for understanding the diversity ofmolecules present in Titan's atmosphere and on its surface. We have conducted aseries of simulation experiments using either spark discharge or FUV photons toinitiate chemistry in CH4/N2 gas mixtures (0.01% CH4/99.99% N2 to 10% CH4/90%N2). We obtained in situ real-time measurements using an HR-ToF-AMS to measurethe particle composition as a function of particle size and a PIT-MS to measurethe composition of gas phase products. These two techniques allow us toinvestigate the effect of energy source and initial CH4 concentration on thedegree of nitrogen incorporation in both the gas and solid phase products. Theresults presented here confirm that FUV photons produce not only solid phasenitrogen bearing products but also gas phase nitrogen species. We find that inboth the gas and solid phase, nitrogen is found in nitriles rather than aminesand both the gas phase and solid phase products are composed primarily ofmolecules with a low degree of aromaticity. The UV experiments reproduce theabsolute abundances measured in Titan's stratosphere for a number of gas phasespecies including C4H2, C6H6, HCN, CH3CN, HC3N, and C2H5CN. [Edited for length]