Deep beneath our feet, in the scorching mantle, iron leads a double life. It can be divalent or trivalent, like a coin that lands heads or tails. Which side dominates determines how much oxygen stays in the planet's rocky shell and how much sinks into the core. But what happens to iron when pressure climbs to millions of atmospheres has remained a mystery.
To peer into those depths, scientists took glasses similar in composition to basalts and peridotites and enriched them with a special isotope, iron-57. The samples were then compressed in a diamond anvil cell, a device where two diamonds press on the sample, to 174 gigapascals, which is like stacking Mount Everest on a fingernail. Using synchrotron radiation (extremely bright X-rays), they tracked the spectra — unique fingerprints that reveal the state of iron atoms.
This transition is reversible: when pressure is released, everything snaps back. But while it lasts, low-spin divalent iron becomes more stable, and trivalent iron begins to reduce to divalent. As a result, the fraction of oxidized iron in the mantle first increases with depth, peaks at a pressure of about 40 gigapascals, and then drops.
The most intriguing part is that this peak coincides with the pressure at which the core and mantle are thought to have separated on the young Earth. Had the pressure been slightly lower or higher, the mantle's oxidation would have been different. Perhaps this 'golden' pressure gave Earth its unique chemistry, and with it, the conditions for life.
as written by the authors arXiv:2606.15472
The Earth's mantle has elevated Fe$$^{3+}$$ relative to those of other rocky bodies, yet the oxidation- and electronic state of iron at extreme pressures is poorly known. We present in-situ energy-domain synchrotron M\"ossbauer spectra of $$^{57}$$Fe-enriched silicate glasses at 298 K from 1 bar to 174 GPa in a diamond anvil cell. Glasses were synthesised with Fe$$^{3+}$$/[Fe$$^{3+}$$ + Fe$$^{2+}$$] from 0.02 $$\pm$$ 0.02 to 1.00 $$\pm$$ 0.02, as determined by colourimetry. While pure Fe$$^{3+}$$-basaltic glass shows minimal changes up to 174 GPa, the spectra of Fe$$^{2+}$$-peridotitic and basaltic glasses are fit by two doublets, D$$_1$$ and D$$_2$$. At 1 bar, their relative intensities are $$\sim$$92 % and $$\sim$$8 %, respectively, but the integral area ratio, D$$_2$$/(D$$_1$$ + D$$_2$$), reaches 0.65 by 172 GPa. Because this transition is reversible with pressure and no metallic iron is detected, the D$$_2$$ feature is Fe$$^{2+}$$ low spin (LS), whereas D$$_1$$ is Fe$$^{2+}$$ high spin (HS). Consequently, the Fe$$^{3+}$$/[Fe$$^{3+}$$+Fe$$^{2+}$$] of planetary mantles reach a maximum near $$\sim$$40 GPa, before decreasing at higher pressures due to the stabilisation of Fe$$^{2+}_{\mathrm{{LS}}}$$. This peak coincides with estimated core-mantle equilibrium on Earth, implying that its uniquely oxidised mantle and habitable state may result from core formation within a Goldilocks pressure range. Secondary atmospheres are predicted to transition from H$$_2$$-rich for Moon-sized bodies, to CO-rich for Earth-like planets and H$$_2$$- and CH$$_4$$-bearing around super-Earths and sub-Neptunes.
The pressure of 174 gigapascals achieved in the experiment is more than 1.7 million times atmospheric pressure. At such pressure, diamond anvils sometimes shatter, even though the sample is smaller than a grain of sand.
In Arthur C. Clarke's novel 'Rendezvous with Rama,' an alien spacecraft has an internal structure resembling a planet with a core and mantle, where pressure and chemistry create a self-contained ecosystem.