Experimental Investigation of the Photochemical Production of Hydrocarbons in Warm Giant Exoplanet Atmospheres

1. Introduction

For exoplanets with T < ~1500 K, photochemistry can seriously affect the atmospheric gas-phase composition [1] — on the one hand by destructing major molecules such as carbon monoxide (CO), water (H₂O), or methane (CH₄) and on the other hand by enhancing the formation of more complex species such as acetylene (C₂H₂), hydrogen cyanide (HCN), heavier hydrocarbons or nitriles with more carbon atoms such as benzene (C₆H₆) [2, 3]. These disequilibrium processes have been considered when analyzing some observational data, highlighting that, in the case of highly irradiated exoplanets, photochemistry may be responsible for an observed chemical composition departing from the one predicted by thermochemical models [4, 5]. In addition, numerous observations suggest that aerosols are ubiquitous in a large variety of exoplanet atmospheres [6-8], including giant exoplanets. However, the nature (condensate clouds or photochemical hazes) of these aerosols and their properties remain largely unconstrained by these observations.

Laboratory experiments are important to advance our understanding of photochemical processes and aerosols properties in exoplanet atmospheres. In our previous studies, we investigated experimentally the influence of photochemistry on the composition and the formation of photochemical aerosols in hot giant exoplanet atmospheres with T > 1000 K and different C/O ratios [9, 10]. Here we will present the results of new laboratory experiments focusing on warm atmospheres (T < 1000 K), for which CH₄ is expected to be the main carbon carrier [3] instead of CO for the higher temperatures that we investigated previously. This particularity may be more favorable to a more efficient formation of hydrocarbons such as C₂H₂ or ethane (C₂H₆), making these planets good candidates to detect tracers of atmospheric photochemistry [3].

2. Material and Methods

To simulate the photochemistry and the formation of aerosols in warm giant exoplanet atmospheres, we used the Cell for Atmospheric and Aerosol Photochemistry Simulations of Exoplanets (CAAPSE) experimental setup [9]. A scheme of the setup is presented in Figure 1.

The cell was filled at room temperature with 15 mbar of either a H₂:CH₄:N₂ (99%:0.5%:0.5%) or H₂:CH₄:H₂O gas mixture with (98.4%:0.8%:0.8%). These compositions were chosen based on the main atmospheric constituents predicted for an exoplanet temperature of 500 K and a solar C/O ratio of 0.54 [3]. The gases were heated at 5 K minute^-1 to oven temperatures ranging from room temperature (~295 K) to 1073 K. After attaining the desired temperature, the gas mixture was irradiated with UV photons at 121.6 nm (Ly_α) and 140-160 nm using a hydrogen microwave discharge lamp separated from the cell by a MgF₂ window.

The evolution of the gas mixture composition was monitored using infrared spectroscopy in transmission.

3. Results and Discussions

We found that photochemistry led to significant modifications in the gas-phase composition resulting in the consumption of CH₄ and the formation of different photochemical products. The main hydrocarbon product is C₂H₆ in every studied condition while C₂H₂ and propane (C₃H₈) have also been detected in smaller amounts. In addition, we observed that the methane consumption efficiency and the hydrocarbon production yields vary significantly with the temperature. When the temperature increases, the methane consumption and the hydrocarbon production decrease. Finally, our results highlight that the production of hydrocarbons was more efficient in the experiments performed with the H₂:CH₄:N₂ gas mixture than in the ones made with the H₂:CH₄:H₂O gas mixture.

In the case of giant planet atmospheres with methane as the main carbon carrier, our results suggest that products of organic photochemistry, such as hydrocarbon molecules (C₂H₂, C₂H₆) and maybe photochemical organic aerosols, are more likely to be observed in planets with lower atmospheric temperatures and lower water amounts.

Acknowledgments

The research work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. This work was supported by the NASA Exoplanet Research Program. B.F. thanks the Université Paris-Est Créteil (UPEC) for funding support (postdoctoral grant).

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