Influence of C/O Ratio on Hot Jupiter Atmospheric Chemistry

We conducted laboratory experiments to study the chemistry in hot Jupiter atmospheres with a C/O ratio of 0.35. We 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 a 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 nonvolatile photochemical aerosols in low C/O atmospheres, in contrast to our previous study. We infer that for a 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.