10. The Faint Young Sun Problem

Geologic evidence for liquid oceans on the Earth nearly 4.5 billion years ago and astrophysical evidence for an early Sun 25% less luminous than present constitute the “Faint Young Sun” paradox. It appears that the most likely solution to this problem is higher levels of greenhouse gases in the atmosphere. However, the concentration of carbon dioxide required throughout the Hadean and Archean conflict with geologic data and would have been difficult to maintain given the reactivity of CO₂ with silicates. Reduced gases have been suggested, but largely rejected because of their photochemical instability or because of implied production of an “antigreenhouse” haze. Hydrothermal production of ammonia and methane from an organic compound rich ocean (and submarine organic sediments) can solve the faint young sun problem.