Planetary scientists have applied simulations of exoplanet atmospheres to determine the conditions favorable for the formation of a CO-rich gas envelope. The main source of carbon monoxide is the photolysis of CO2 under the influence of light, but CO can also originate from volcanic gas emissions and hydrothermal decomposition of formaldehyde H2CO2 in the ocean. The removal of CO from the atmosphere occurs mainly through its reaction with hydroxyl (OH) radicals produced from the photolysis of water vapor, and to a lesser extent through deposition on the surface of the planet.
It turns out that CO accumulation occurs when carbon monoxide production exceeds its removal by hydroxyl radicals. This may occur due to higher CO2 levels or the presence of the reducing environment of volcanic gases. At 277 Kelvin, suitable conditions occur when the partial pressure of CO2 exceeds 0.2 bar. At higher temperatures (300 Kelvin), even higher levels of CO2 and volcanic gases are required due to the increased amount of water vapor in the atmosphere, which is the main source of OH radicals.
