Suppression of hydrodynamic escape of an H2-rich early Earth atmosphere by radiative cooling of carbon oxides

Abstract Radiative cooling by molecules is a crucial process for hydrodynamic escape, as it can efficiently remove the thermal energy driving the outflow, acquired through X-ray and extreme UV absorption. Carbon oxides, such as CO and CO 2 , and their photochemical products are anticipated to serve as vital radiative cooling sources not only in atmospheres dominated by carbon oxides but also in H 2 -rich atmospheres. However, their specific effects on the hydrodynamic escape, especially in H 2 -rich atmospheres, have been inadequately investigated. In this study, we conduct 1-D hydrodynamic escape simulations for H 2 -rich atmospheres incorporating CO, CO 2 , and their chemical products on an Earth-mass planet. We consider detailed radiative cooling processes and chemical networks related to carbon oxides to elucidate their impacts on the hydrodynamic escape. In the escape outflow, CO 2 undergoes rapid photolysis, producing CO and atomic oxygen, while CO exhibits photochemical stability compared to CO 2 . The H 2 oxidation by atomic oxygen results in the production of OH and H 2 O. Consequently, the hydrodynamic escape is significantly suppressed by the radiative cooling effects of CO, H 2 O, OH, and H 3 + even when the basal mixing fraction of CO and CO 2 is lower than ~ 0.01. These mechanisms extend the lifetime of H 2 -rich atmospheres by about one order of magnitude compared to the case of pure hydrogen atmospheres on early Earth, which also results in negligible escape of heavier carbon- and nitrogen-bearing molecules and noble gases.