Self-oxidation of the atmospheres of rocky planets with implications for the origin of life

TL;DR

This study models how rocky planet atmospheres self-oxidize over time, becoming CO2 and H2O dominated, which influences the potential pathways for the origin of life and the availability of prebiotic molecules.

Contribution

It demonstrates that atmospheric self-oxidation occurs regardless of initial composition, affecting prebiotic chemistry and emphasizing CO2 as the primary carbon source for early metabolism.

Findings

Atmospheric H escapes within tens of millions of years due to stellar XUV radiation.

Rocky planet atmospheres tend to become CO2 and H2O dominated over time.

Lightning is inefficient at producing HCN, but NO and dust may supply fixed nitrogen.

Abstract

Rocky planets may acquire a primordial atmosphere by outgassing of volatiles from their magma ocean. The distribution of O between H$_2$O, CO and CO$_2$ in chemical equilibrium subsequently changes significantly with decreasing temperature. We explore here two chemical models: one where CH$_4$ and NH$_3$ are assumed to be irrevocably destroyed by photolysis, and one where these molecules persist. In the first case, we show that CO cannot co-exist with H$_2$O, since CO oxidizes at low temperatures to form CO$_2$ and H$_2$. In both cases, H escapes from the thermosphere within a few ten million years by absorption of stellar XUV radiation. This escape drives an atmospheric self-oxidation process whereby rocky planet atmospheres become dominated by CO$_2$ and H$_2$O, regardless of their initial oxidation state at outgassing. HCN is considered a potential precursor of prebiotic compounds and RNA. Our oxidizing atmospheres are inefficient at producing HCN by lightning. Instead, we demonstrate that lightning-produced NO, which dissolves as nitrate in the oceans, and interplanetary dust particles may be the main sources of fixed nitrogen to emerging biospheres. Our results highlight the need for origin-of-life scenarios where the first metabolism fixes its C from CO$_2$, rather than from HCN and CO.