Titan Atmospheric Chemistry Revealed by Low-Temperature N₂–CH₄ Plasma Discharge Experiments

Abstract

Chemistry in Titan's N2-CH4 atmosphere produces complex organic aerosols. The chemical processes and the resulting organic compounds are still far from understood, although extensive observations, laboratory, and theoretical simulations have greatly improved physical and chemical constraints on Titan's atmosphere. Here, we conduct a series of Titan atmosphere simulation experiments with N2-CH4 gas mixtures and investigate the effect of initial CH4 ratio, pressure, and flow rate on the production rates and composition of the gas and solid products at a Titan relevant temperature (100 K) for the first time. We find that the production rate of the gas and solid products increases with increasing CH4 ratio. The nitrogen-containing species have much higher yield than hydrocarbons in the gas products, and the N/C ratio of the solid products appears to be the highest compared to previous plasma simulations with the same CH4 ratio. The greater degree of nitrogen incorporation in the low temperature simulation experiments suggests temperature may play an important role in nitrogen incorporation in Titan's cold atmosphere. We also find that H2 is the dominant gas product and serves as an indicator of the production rate of new organic molecules in the experiment, and that CH2NH may greatly contribute to the incorporation of both carbon and nitrogen into the solid particles. The pressure and flow rate affect the amount of time of the gas mixture exposed to the energy source and therefore impact the N2-CH4 chemistry initiated by the plasma discharge, emphasizing the influence of the energy flux in Titan atmospheric chemistry.

Organic Hazes as a Source of Life’s Building Blocks to Warm Little Ponds on the Hadean Earth

“…The plasma does not significantly affect the neutral gas temperature, which remains approximately at room temperature. The exposure time of the gas to the plasma is approximately 2 s (He et al 2022).…”

The Importance of Producing and Characterizing Laboratory Analogs of (Exo)Planetary Atmospheric Aerosols

“…For the study of the gas phase, mass spectrometry and gas chromatography-mass spectrometry (GC-MS) can be used to monitor and characterize the molecular products that result from the chemistry (e.g., Sciamma-O'Brien et al (2010; Gautier et al (2011);Raymond et al (2018); Dubois et al (2019aDubois et al ( ,b, 2020; He et al (2022b) for Titan applications; and He et al (2020a,b) for exoplanet applications). In the case of Titan simulations, the experimental mass spectra can be compared to INMS or CAPS (Waite et al 2007;Crary et al 2009;Coates et al 2009Coates et al , 2010 as well as be used to constrain photochemical models that have been developed to try and understand the chemical processes occurring in Titan's atmosphere (e.g., Vuitton et al (2007Vuitton et al ( , 2019; Krasnopolsky (2009Krasnopolsky ( , 2012; Dobrijevic et al (2014Dobrijevic et al ( , 2016, and other studies referenced in, e.g., Hörst (2017) and MacKenzie et al ( 2021)).…”

“…Initial work on the Titan tholins looked at their chemical composition and the impact of the addition of carbon monoxide on composition using NMR and GC-GC-MS (He et al 2017;Sebree et al 2018). The gas phase products of these experiments were also looked at more recently through mass spectrometry (He et al 2022b). Much of the PHAZER work on Titan tholins has focused on material properties of the particles, with a particular focus on understanding how those properties might impact sand transport on Titan and also the particles' abilities to act as condensation nuclei based on their surface energies.…”

“…One major difference between these measurements and most previous measurements is that these experiments were conducted at Titan relevant temperatures (100 K). Additional work is underway to understand the impact of temperature on Titan experiments including changes to the gas phase composition (see, e.g., He et al (2022b)).…”

Nonadiabatic quantum dynamics explores non-monotonic photodissociation branching of N2 into the N(4S) + N(2D) and N(4S) + N(2P) product channels

“…[15][16][17][18][19] A combination of lab experiments in plasma and computer simulations shows possible reactions of the activated forms of N 2 with CH 4 and C 2 H 4 in Titan's atmosphere. [20][21][22] To understand the mechanism of these reactions and their complexity, one should have a closer look at the electronic structure of N 2 .…”