Exchange between a magma ocean and vapor produced Earth’s earliest atmosphere. Its speciation depends on the oxygen fugacity (fO₂) set by the Fe³⁺/Fe²⁺ ratio of the magma ocean at its surface. Here, we establish the relationship between fO₂ and Fe³⁺/Fe²⁺ in quenched liquids of silicate Earth-like composition at 2173 K and 1 bar. Mantle-derived rocks have Fe³⁺/(Fe³⁺+Fe²⁺) = 0.037 ± 0.005, at which the magma ocean defines an fO₂ 0.5 log units above the iron-wüstite buffer. At this fO₂, the solubilities of H-C-N-O species in the magma ocean produce a CO-rich atmosphere. Cooling and condensation of H₂O would have led to a prebiotic terrestrial atmosphere composed of CO₂-N₂, in proportions and at pressures akin to those observed on Venus. Present-day differences between Earth’s atmosphere and those of her planetary neighbors result from Earth’s heliocentric location and mass, which allowed geologically long-lived oceans, in-turn facilitating CO₂ drawdown and, eventually, the development of life.

岩浆海洋与蒸气之间的交换产生了地球最早的大气层。其形态取决于由岩浆海洋在其表面的Fe ³⁺ / Fe ²⁺比率设定的氧逸度（f O ₂）。在这里，我们建立了在2173 K和1 bar的硅酸盐类地球组成的淬火液体中f O ₂和Fe ³⁺ / Fe ²⁺之间的关系。来自地幔的岩石具有Fe ³⁺ /（Fe ³⁺ + Fe 2 ⁺）= 0.037±0.005，在此岩浆海在铁-白钨矿缓冲区上方限定了f O ₂ 0.5 log单位。在这个fO ₂，在岩浆海洋中HCNO物种的溶解度产生了富含CO的气氛。H ₂ O的冷却和冷凝将导致由CO ₂ -N ₂组成的益生元陆地大气，其比例和压力类似于在金星上观察到的压力。目前，地球大气与其行星邻居的大气层之间的差异是由于地球的日心中心位置和质量所致，从而使地质长寿的海洋得以生存，进而促进了CO _2的下垂，并最终促进了生命的发展。