paper

Photochemistry in hot H2-dominated exoplanet atmospheres

arXiv:2202.07943 · doi:10.3847/1538-4357/aaf79f

Abstract

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 Lyman-alpha 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 Lyman-alpha UV-radiation, confirming the possibility of forming photochemical hazes in hot-Jupiter exoplanet atmospheres with an enhanced C/O ratio compared to Solar.

32 pages, 3 tables, 10 figures

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