Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth

<jats:p> The low O <jats:sub>2</jats:sub> content of the Archean atmosphere implies that methane should have been present at levels ∼10 <jats:sup>2</jats:sup> to 10 <jats:sup>3</jats:sup> parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH <jats:sub>4</jats:sub> is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH <jats:sub>4</jats:sub> implies that hydrogen escapes to space (↑space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O <jats:sub>2</jats:sub> and H, and methanogenesis transfers the H into CH <jats:sub>4</jats:sub> . Hydrogen escape after CH <jats:sub>4</jats:sub> photolysis, therefore, causes a net gain of oxygen [CO <jats:sub>2</jats:sub> + 2H <jats:sub>2</jats:sub> O → CH <jats:sub>4</jats:sub> + 2O <jats:sub>2</jats:sub> → CO <jats:sub>2</jats:sub> + O <jats:sub>2</jats:sub> + 4H(↑space)]. Expected irreversible oxidation (∼10 <jats:sup>12</jats:sup> to 10 <jats:sup>13</jats:sup> moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized. </jats:p>