Isomer specific photochemistry in Titan’s atmosphere

Titan’s atmosphere is an active organic laboratory instigated by the photolysis of its main components, N₂ and CH₄, and terminating with the formation of photochemical hazes (Hörst et al. 2017, Coustenis et al. 2021). Its complex chemical inventory has been characterized through observations with multiple space-born and ground-based observatories and with dedicated space missions such as Cassini-Huygens (NASA-ESA-ASI) that explored the Saturnian system from 2004 to 2017 providing an unprecedented view of Titan’s complexity. The characterization of the neutral inventory has revealed a plethora of hydrocarbons with up to 6 carbons atoms as well as multiple nitrile species (Nixon 2024). The corresponding ion characterization has revealed multiple ions up to mass of 100 Da as well as larger macromolecules up to 10 000 Da/q that are considered the embryos of haze formation (Waite et al. 2007). To interpret this complexity, photochemical models include hundreds of chemical species involved in chemical networks of ion-neutral processes containing thousands of reactions (Vuitton et al. 2019, Loison et al. 2019, Willacy et al. 2022). However, despite this profound complexity these networks contain limited information on the different isomers that could be formed for a given stoichiometric structure. For small hydrocarbons up to two carbon atoms isomerization is not a major issue but the possible number of structural forms rapidly increases from three carbon atoms and above. While observations have identified the main isomers for small mass hydrocarbons, an understanding for the abundance of minor isomers is useful for exploring their possible detection in Titan’s atmosphere. Moreover, isomers not typically considered in photochemical models can partake in chemical processes that lead to rapid chemical growth and foster different pathways of chemical evolution (Thomas et al. 2019). This is particularly important given recent advance in spectroscopic observations sensitivity that allow observational constraints on the less abundant isomers, unlocking the potential for powerful constraints on the chemical schemes.

 

In this work we explore the photochemistry of isomers in Titan’s atmosphere. We first investigate isomers of C₃H_x stoichiometry (Fig. 1) as for those there are observational constraints. We then progressively expand the investigation to larger molecules. Our simulations demonstrate two main mechanisms controlling the formation of the different isomers from neutral processes. Forward mechanisms lead to the growth of isomers by the chemical reaction of smaller molecules/radicals and is drastically limited by the low temperature conditions in Titan’s atmosphere. Backward mechanisms due to the photodissociation of larger molecules that allows for the formation of isomers that are not accessible through molecular collisions alone. A third mechanism involves reactions with ions or ion recombination for the formation of different isomers. Information for these mechanisms is not always available. Particularly, details for the photodissociation channels and yields for different isomers from either experimental or theoretical studies becomes sparser with increasing molecular mass. We will present the dominant mechanisms for different isomers and discuss the current assumptions/limitations in the estimation of their abundances.

 

Figure 1. Simulated mole fractions of different isomers of hydrocarbons with 3 carbon atoms in Titan’s atmosphere.

 

References

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