Conference Agenda

The Earth’s present-day N₂-O₂ atmosphere is the result of time-integrated processes, testament to a distinct evolutionary history to its planetary neighbours. However, the causes leading to oxygenation of the Earth’s crust and atmosphere remain elusive. Here, we determine dependence of the Fe³⁺/Fe²⁺ ratio on the oxygen fugacity (fO₂) of peridotitic and basaltic glasses, relevant for the Earth’s mantle at extreme pressures using X-Ray Absorption Near-Edge Structure and Mössbauer spectroscopy. We show that the Earth’s magma ocean, prior to crystallisation, would have produced a CO₂-N₂ Venus-like atmosphere following cooling. The elevated Fe³⁺/Fe²⁺ (~0.1) of the Earth stems from the fact that its core formed at pressures of 40 - 50 GPa, above which we find the transition of Fe²⁺ from high- to low spin results in stabilisation of ferrous over ferric iron and hence more reducing atmospheres. Therefore, the uniquely oxidised mantle and atmosphere of the Earth may result from core formation within a Goldilocks pressure range that stabilises Fe³⁺. Atmospheres around rocky plan are predicted to transition from H₂-rich around Moon-sized bodies, to CO/CO₂-rich for Earth-like planets and H₂- and CH₄-bearing around super-Earths.