Self-Oxidation of the Atmospheres of Rocky Planets with Implications for the Origin of Life.

Abstract

Rocky planets may acquire a primordial atmosphere by the outgassing of volatiles from their magma ocean. The distribution of O between H₂O, CO, and CO₂ in chemical equilibrium subsequently changes significantly with decreasing temperature. We consider here two chemical models: one where CH₄ and NH₃ are assumed to be irrevocably destroyed by photolysis and second where these molecules persist. In the first case, we show that CO cannot coexist with H₂O, since CO oxidizes at low temperatures to form CO₂ and H₂. In both cases, H escapes from the thermosphere within a few 10 million years by absorption of stellar XUV radiation. This escape drives an atmospheric self-oxidation process, whereby rocky planet atmospheres become dominated by CO₂ and H₂O regardless of their initial oxidation state at outgassing. HCN is considered a potential precursor of prebiotic compounds and RNA. Oxidizing atmospheres are inefficient at producing HCN by lightning. Alternatively, we have demonstrated that lightning-produced NO, which dissolves as nitrate in oceans, and interplanetary dust particles may be the main sources of fixed nitrogen in emerging biospheres. Our results highlight the need for origin-of-life scenarios where the first metabolism fixes its C from CO₂, rather than from HCN and CO.