Relative abundances of CO2, CO, and CH4 in atmospheres of Earth-like lifeless planets

TL;DR

This study uses a theoretical atmospheric chemistry model to explore how the relative abundances of CO2, CO, and CH4 vary on Earth-like lifeless planets, revealing conditions that lead to CO-rich atmospheres and characteristic chemical gaps.

Contribution

It introduces a systematic analysis of atmospheric carbon chemistry on Earth-like planets, identifying conditions for CO runaway and the existence of a chemical gap in lifeless planetary atmospheres.

Findings

Elevated CO2 levels cause CO photochemical instability.

Higher volcanic outgassing of reduced C triggers CO runaway.

A distinct chemical gap exists in the atmospheres of such planets.

Abstract

Carbon is an essential element for life on Earth, and the relative abundances of major carbon species (CO2, CO, and CH4) in the atmosphere exert fundamental controls on planetary climate and biogeochemistry. Here, we employed a theoretical model of atmospheric chemistry to investigate diversity in the atmospheric abundances of CO2, CO, and CH4 on Earth-like lifeless planets orbiting Sun-like (F-, G-, and K-type) stars. We focused on the conditions for the formation of a CO-rich atmosphere, which would be favorable for the origin of life. Results demonstrated that elevated atmospheric CO2 levels trigger photochemical instability of the CO budget in the atmosphere (i.e., CO runaway) owing to enhanced CO2 photolysis relative to H2O photolysis. Higher volcanic outgassing fluxes of reduced C (CO and CH4) also tend to initiate CO runaway. Our systematic examinations revealed that anoxic atmospheres of Earth-like lifeless planets could be classified in the phase space of CH4/CO2 versus CO/CO2, where a distinct gap in atmospheric carbon chemistry is expected to be observed. Our findings indicate that the gap structure is a general feature of Earth-like lifeless planets with reducing atmospheres orbiting Sun-like (F-, G-, and K-type) stars.