Redox Revolutions on Earth and Beyond: Implications for Life Detection on Extrasolar Worlds

The discovery of thousands of exoplanets has ignited interest in searching their atmospheres for signatures of life. O₂ looms large in these plans, which begs the question: What is the relationship between the evolution of oxygenic photosynthesis and the accumulation of O₂ in the atmosphere of an Earth-like world? Is O₂ accumulation rapid and inevitable once biological production begins? Alternatively, might there be worlds on which O₂ never accumulates even if it is biologically produced at rates comparable to those on Earth?

We can elucidate the planetary controls on O₂ by studying Earth's ancient history. Much work focuses on the Great Oxidation Event ("GOE") at the start of the Proterozoic, after which pO₂ was irreversibly > 0.001 atm. However, many lines of evidence indicate a smaller oxygenation event ~100 Ma earlier. Additional evidence of mild environmental oxidation - probably by O₂ - is found throughout the Archean. This emerging evidence suggests that the GOE is best regarded as the key turning point in a broader "First Redox Revolution" (FRR) of the Earth system characterized by two or more "Archean Oxidation Events" (AOE).

What kept the FRR in check until the GOE? It increasingly appears that the solid Earth played a key role. In particular, recent examinations of oxygen fugacity (f_O2) during the formation of Precambrian basalts and komatiites suggest that f_O2 of large volumes of the mantle underwent a secular increase of ~ 1.3 log₁₀units from 3.5 to 2.4 Ga. The cause(s) of this increase in f_O2 remain unclear but are important because biological O₂ production is ultimately balanced by consumption by reaction with reductants from Earth's interior. Models incorporating this f_O2 data and δ¹³C constraints on secular change in O₂ production from photosynthesis show that mantle redox evolution can account for a shift from net O₂ consumption to net O₂ production at about 2.5 Ga. Before this time, O₂ sinks overwhelmed O₂ sources. After this time, the probability is > 95% that O₂ sources overwhelmed O₂ sinks.

These findings suggest that modest differences in mantle compositions or tectonics might substantially alter the timing of surface oxygenation on other worlds, highlighting the need for better understanding of how solid planet processes might operate on other nominally "Earth-like" worlds.