The size of the Earth is key to its habitability

The Earth is oxidised inside, which makes it possible for humans to inhabit its surface. The origin of this oxidised interior has been a long-standing scientific puzzle. Compared to other planetary bodies, such as Mars, the Moon or asteroids, the Earth’s rocks are significantly more oxidised. Through volcanic degassing, this has sustained Earth’s oxidised and habitable surface. How and when this oxidized interior arose remains poorly understood.

According to planetary scientists, the early Earth was nearly entirely molten, a stage known as the ‘magma ocean’, in which the metal core segregated from the silicate mantle. A key question, therefore, is how the silicate Earth oxidated after descending from the seemingly reduced, metal-saturated, magma ocean.

Owing to Earth’s size, quantifying the oxidation state of silicate in a deep magma ocean of Earth is difficult. It requires quenching glasses at core-formation conditions, thought to be approximately 30-50 GPa and > 2500 K. This can only be done in laser heated diamond avil cells (LH-DAC). However, LH-DAC experiments produce tiny samples, and quantifying their oxidation state by measuring the ferric iron in the silicate glasses is a complex analytical challenge.  

Now scientists from China University of Geosciences Beijing, University of Minnesota, University of Bristol, Australian National University, Smithsonian Institution, Carnegie Institution for Science, and ESRF have developed a way to control and monitor the oxygen fugacity in LH-DAC experiments. They optimised the sample synthesis and post-processing procedure, which produced homogeneous glass samples, a few tens of micrometres across.

“The unique resonance beamline at ESRF ID18 was the only feasible tool that we could use to quantify the iron redox state accurately in these tiny samples”, explains Hongluo Zhang, researcher in the China University of Geosciences and corresponding author of the publication.

The Earth’s size is key   

Thanks to the ferric iron concentration measurements done at the ESRF, the team built a new thermodynamic model. This showed that the ferric iron concentration formed in Earth’s magma ocean stage was sufficient to account for the modern bulk silicate Earth redox budget.

The key factor stabilising this relatively oxidised load was the very high pressures prevailing in the magma ocean. ”Therefore, the Earth’s relatively oxidised state originated at its very birth, and is a product of the size of the planet”, says Zhang.

Part of the team on ID18. Credits: H. Zhang.

These novel results illustrate how the size of a planet may guide its subsequent evolution and habitability, providing improved understanding of how the Earth came to its present condition, as well as what we might expect for the habitability of exoplanets of different sizes.

The next step will be checking how much of the effect on the redox evolution there is after the magma ocean stage.

Reference:

Zhang H.L., et al, Science Advances, 16 Oct 2024, Vol 10, Issue 42. DOI: 10.1126/sciadv.adp1752

Text by Montserrat Capellas Espuny