Modeling of a low-temperature plasma glow discharge to characterize the atmospheric ion chemistry of Saturn's moon Titan David Dubois 1 , A. W. Raymond 2 , E. Sciamma-O’Brien 1 and F. Salama 1 1 NASA Ames Research Center, Moffett Field, CA, USA. 2 NASA Jet Propulsion Laboratory, Caltech, Pasadena, USA Titan, Saturn’s largest moon, is the only moon in the solar system to have a dense atmosphere. This atmosphere is mainly composed of nitrogen (N 2 ) and methane (CH 4 ). The organic haze surrounding Titan is produced from the gas phase molecular precursors that result from a complex organic chemistry in the upper atmosphere due to the photolysis and radiolysis of N 2 and CH 4 . These precursors consist of hydrocarbon radicals ( e.g., CH 2 , CH 3 ) and more complex hydrocarbons, nitriles and polycyclic aromatic hydrocarbons [1]. The Cassini spacecraft unveiled the presence of large positively- and negatively-charged species between 950-1500 km in Titan’s upper atmosphere. These measurements highlighted the important role of charged molecular species in Titan’s atmospheric chemistry. Many laboratory experiments mimicking Titan’s atmospheric conditions (temperature, chemical composition, energy source) have investigated the chemical pathways involved in Titan’s atmospheric chemistry [3]. In parallel, photochemical and microphysical models have substantially advanced our understanding of the chemistry and microphysics occurring in Titan’s ionosphere [4,5]. However, these models have faced obstacles due to limited reaction rate data. In the study presented here, we investigated Titan's low-temperature (150 K) gas phase chemistry using both numerical and experimental work: (1) the Titan Haze Simulation (THS) experiment developed on the COsmic Simulation Chamber (COSmIC) at NASA Ames Research Center allows simulating Titan’s atmospheric chemistry at low temperature (150 K) using an abnormal glow plasma discharge generated in the stream of a jet-cooled gas expansion [6]; and (2) a 1D chemical network model using a fluid mechanical framework is employed to simulate the ion-neutral chemical reactivity occurring at low temperature in the COSmIC/THS [7]. Our study focuses on N 2 -CH 4 -based gas mixtures relevant to Titan's upper atmosphere. We have incorporated updated reaction rates into our numerical model and expanded on the plasma parameter space from previous studies to assess the sensitivity of changing plasma conditions on the resulting ion chemistry. C/N elemental composition of the gas-phase products and comparisons with recently published solid-phase C/N ratios of the Titan aerosol analogs produced in COSmIC/THS will be presented. This is particularly important in order to characterize the molecular precursors leading to the formation of these organic aerosols. The sensitivity of our calculations with source voltage variations and their impact on the chemistry will also be discussed. Finally, the implications of these results will be compared to other laboratory and numerical simulations demonstrating the importance of low-temperature plasma chemistry experiments combined with modeling to improve our understanding of cold planetary environments. References 1. J. H. Waite Jr et al., Science, 316, 870 (2007). 2. V. A. Krasnopolsky, Icarus, 236 (2014). 3. M. L. Cable et al., Chemical Reviews, 112 (2012).
4. P. Lavvas et al., PNAS, 110 (2013). 5. V. Vuitton et al., Icarus, 324 (2019). 6. E. Sciamma-O’Brien et al., Icarus, 243 (2014). 7. A. W. Raymond et al., The Astrophysical Journal, 853 (2018).
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