If there is no chemical production of CO2, all the CO2 in the current Martian atmosphere would be removed by photolysis in about 3,500 years.[1] The hydroxyl radicals (OH) produced from the photolysis of water vapor, together with the other odd hydrogen species (e.g. H, HO2), can convert carbon monoxide (CO) back to CO2. The reaction cycle can be described as:[36][37]