Photochemistry has the potential to substantially impact the atmospheric composition of exoplanets with consequences on the radiative transfer, thermal structure and dynamics of the atmospheres, particularly in UV-rich stellar environments. Here, we present the results of a first laboratory experimental simulation of photochemistry in carbon-rich exoplanet atmospheres at elevated temperatures. Evolution of gas-phase molecular composition was quantitatively monitored with infrared spectroscopy and mass spectrometry. We found that H2/CO gas compositions can change significantly from thermal equilibria compositions when irradiated with Ly-α photons at temperatures ranging from 600 K to 1500 K. Carbon dioxide and water were found to be the main products caused by photolysis, while formation of methane was also observed to a lesser extent. We find that photochemistry efficiency is strongly correlated with increasing temperature. Our finding that water is efficiently produced by photochemistry in a super Solar C/O=1 environment, representing C enhancement relative to solar values C/O ratio = 0.54, has significant implications for the interpretation of many exoplanet transmission spectra. We also find the formation of an organic solid condensate at 1500 K and under Ly-α UV-radiation, confirming the possibility of forming photochemical hazes in hot-Jupiter exoplanet atmospheres with an enhanced C/O ratio compared to Solar.

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We have conducted laboratory experiments to study the chemistry in hot Jupiter atmospheres with C/O ratio of 0.35. We have compared our results with the ones obtained previously for atmospheres with a C/O ratio of 1 to investigate the influence of the C/O ratio on the chemistry and formation of photochemical organic aerosol. We found that the C/O ratio and the gas mixture compositions strongly influence the pathways responsible for the formation of CO2.Thermochemical reactions are primarily responsible for the formation of CO2 in low C/O ratio atmospheres, while photochemistry is the dominant process in high C/O ratio atmospheres even if the final CO2 concentration is the same in both cases. Our results show that low C/O atmospheres at the thermochemical equilibrium contain a higher water abundance, while high C/O atmospheres are significantly depleted in water. However, in low C/O atmospheres, the water abundance is not affected by UV photolysis, while our previous work demonstrated that significant amount of water can be produced in high C/O ratio atmospheres. This contrast in water production suggests that photochemistry should be considered when interpreting exoplanet transit spectra. Finally, we did not observe the formation of a detectable amount of non-volatile photochemical aerosols in low C/O atmospheres, in contrast to our previous study. We infer that for C/O ratio < 1, water likely inhibits organic growth and aerosol formation, suggesting that photochemical organic aerosols are likely to be observed in planets presenting a carbon enrichment compared to their host stars.

The goal of this experimental study is to investigate how the presence of atmospheric CO 2 affects the optical properties of organic photochemical aerosols. To this end, we add CO 2 to a N 2 :CH 4 gas mixture used in a plasma typically used for Titan studies. We produce organic thin films (tholins) in plasmas where the CO 2 / CH 4 ratio is increased from 0 to 4. We measure these films via spectrometric ellipsometry in the ultraviolet, visible and near-infrared (270 nm to 2 µm). The ellipsometry parameters are fitted with a Tauc-Lorentz model used for optically transparent materials, to obtain the thickness of the thin film, its optical band gap, and the refractive indices. According to this optical mode, oxidized organic aerosols are transparent in the visible up to the near ultraviolet (k = 0 from 900 -400 nm), while the fully reduced organic aerosol absorbs in the visible as well as in the near UV (k = 0 from 900 -500 nm). As the CO 2 /CH 4 ratio is quadrupled, the position of the UV absorption resonance is shifted from ∼177 nm to 264 nm, and its strength is also quadrupled: oxidized tholins absorb more efficiently in the middle UV with respect to the far UV for reduced tholins. Our laboratory wavelength-tabulated refractive indices provide further constraints to atmospheric models of the early Earth and Earth-like exoplanets including photochemical hazes formed under increasingly oxidizing conditions.

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