Photochemistry in Hot H2-dominated Exoplanet Atmospheres

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. The evolution of gas-phase molecular composition was quantitatively monitored with infrared spectroscopy and mass spectrometry. We found that H-2/CO gas compositions can change significantly from thermal equilibria compositions when irradiated with Ly alpha photons at temperatures ranging from 600 to 1500 K. Carbon dioxide and water were found to be the main products caused by photolysis, while the 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 supersolar 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 alpha UV radiation, confirming the possibility of forming photochemical hazes in hot-Jupiter exoplanet atmospheres with an enhanced C/O ratio compared to solar.