Planetary scientists have used simulations of exoplanet atmospheres to determine the conditions that favour the emergence of a CO-rich gas envelope. The main source of carbon monoxide is the photolysis of CO2 by light, but CO can also come from volcanic emissions and the hydrothermal decomposition of formaldehyde H2CO2 in the ocean. Removal of CO from the atmosphere has occurred mainly through its reaction with hydroxyl radicals (OH) formed by photolysis of water vapour, and to a lesser extent through deposition on the planet’s surface.
CO accumulation appears to occur when carbon monoxide production exceeds its removal by hydroxyl radicals. This may occur due to higher levels of CO2 or in the presence of a reducing volcanic gas environment. At a temperature of 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 vapour in the atmosphere, which is the main source of OH radicals.
