Follow the Carbon: Laboratory Studies of 13C-Labeled Early Earth Haze Analogs

While the Sun was still young and faint before the rise of molecular oxygen 2.4 Ga, early Earth might have been kept warm by an atmosphere containing the greenhouse gases methane and carbon dioxide in abundances greater than what is found on Earth today. It has been suggested that an atmosphere containing approximately 1000 ppmv methane and carbon dioxide could provided the needed greenhouse warming for liquid water to exist at the surface. Laboratory and modeling studies suggest that an atmosphere containing methane and carbon dioxide could lead to the formation of significant amounts of organic haze due to photochemical reactions initiated by Lyman-α (121.6 nm) excitation. Chemical mechanisms proposed to explain the chemistry rely on methane as the source of carbon in these hazes and treat carbon dioxide as a source of oxygen only. In the present work, we use isotopically labelled precursor gases to examine the source of carbon in photochemical haze formed in a CH4/CO2/N2 atmosphere. We generate haze analogs in the laboratory by far-UV irradiation of analog atmospheres containing permutations of 1,000 ppmv unlabeled and 13C-labeled methane and carbon. Products in the particle phase were analyzed by both unit mass resolution and high-resolution (m/Δm=5,000) aerosol mass spectrometry. Results indicate that carbon from carbon dioxide accounts for 20% (×5%) of the total carbon contained in the hazes. These results have implications for the geochemical interpretations of inclusions found in Archaean rocks on Earth, and for the astrobiological potential of other planetary atmospheres.