Recent Advances at the NASA Ames COSmIC/THS Facility: Study of Titan’s Upper Atmosphere Reactivity and Gas Phase Organic Precursors

The Cassini spacecraft measured the molecular masses of positively- and negatively-charged species between 950-1500 km in the upper atmosphere of Saturn’s largest moon, Titan. These measurements highlighted the role of charged molecular species in Titan’s atmospheric chemistry, which eventually leads to the formation of the organic haze layers surrounding Titan. Photochemical and microphysical models have substantially advanced our understanding of the chemistry and microphysics occurring in Titan’s ionosphere. In parallel, laboratory experiments have examined chemical pathways involved in Titan’s atmospheric chemistry.

In the study presented here, we investigate Titan’s low-temperature (150 K) gas phase chemistry using both experimental and numerical work: (1) the Titan Haze Simulation (THS) experiment developed on the COsmic Simulation Chamber (COSmIC) at NASA Ames Research Center simulates Titan’s atmospheric chemistry at low temperature in the laboratory using an abnormal glow plasma discharge generated in the stream of a jet-cooled gas expansion [1]; and (2) a 1D chemical network model using a fluid mechanical framework simulates numerically the chemical reactivity occurring in the COSmIC/THS [2]. Our study focuses on N₂-CH₄-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 [3] 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 [4] will be presented. The sensitivity of our calculations with source voltage variations and its 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 plasma chemistry experiments and modelling to improve our understanding of planetary environments.

References: [1] Sciamma-O’Brien et al., Icarus (2014). [2] Raymond et al., ApJ. (2018). [3] Dubois et al., submitted. [4] Nuevo et al. Icarus (2022).

Acknowledgments: This work is supported by the NASA Postdoctoral Program and the NASA SMD SSW and APRA R&A Programs. A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA (80NM0018D0004).