Redox sensitivity of the surface conditions and climate state of the Hadean Earth

The Hadean (4.6-4.0 Ga) is one of the least understood eons in Earth’s history. Yet it was one of the most important, as it represents the formation of the planet and the establishment of the physical, chemical and geological conditions necessary for life. However, the only evidence from this period consists of zircons dated to 4.4 Ga, suggesting that there was an ocean of water on Earth at that time.

In this study, we examine the potential influence of the mantle redox state on the surface temperature and climate of Hadean Earth, both during and following the crystallisation of the global magma ocean after the Moon-forming impact, in order to determine the likelihood of an ocean of water existing. 

The redox state of the magma ocean significantly impacts the properties and sizes of Earth's reservoirs, controlling its internal structure, atmospheric evolution and, more broadly, surface habitability. We investigate the impact of redox conditions but also carbon, nitrogen, sulphur and total hydrogen content on the speciation and cycling of volatiles, redox reactions, radiative transfer in the atmosphere, and surface temperatures on early Earth. We model the coupled evolution of the mantle and atmosphere during and after magma ocean crystallisation. By linking geochemical processes in the mantle consistently with atmospheric composition and climate, we explore a wide range of redox states and bulk volatile inventories.

Our findings reveal that atmospheric composition, mantle crystallisation timescales, and post-crystallisation surface temperatures are highly sensitive to the redox state. In all scenarios, the resulting atmospheres lead to persistently high surface temperatures. These findings imply a significant interplay between the geochemical evolution of the mantle and the emergence of habitable conditions on the earliest Hadean Earth. 

To conclude, we find that forming oceans under typical volatile conditions through bottom-up crystallisation and mantle H2O outgassing alone is challenging. This differs from previous models in the literature and suggests that additional processes may have been necessary for liquid water oceans to have existed as early as 4.4 Ga.