Planetary scientists simulate iron oxidation by giant impact during early Earth’s atmospheric evolution

Redox processes caused by giant impacts in the atmosphere and magma ocean may have played a crucial role during Earth’s evolution. However, the absence of rock records from the time or period makes it difficult to understand these processes.

In a report published in Scientists progressJinhuyuk Choi and a research team of planetary scientists in Seoul, Germany and Korea presented experimental results capable of simulating giant reactions caused by impact between iron and volatile substances, using electron lasers free x-rays.

The scientists used the X-ray free electron laser pump to oxidize iron to wüstite and reduce volatiles to hydrogen and carbon monoxide. Oxidation of iron occurred to form hydrides and siderites, implying a redox limit. The results shed light on the process of creating a reduced atmosphere which is at the origin of the emergence of prebiotic organic molecules on the early Earth.

Evolution of the Earth and origin of life

The first giant impact that led to the formation of the Moon more than 4.5 billion years ago was a catastrophic determinant of Earth’s evolution. The overall chemical mixing and redox process that occurred in the vapor atmosphere and magma ocean due to this giant impact effect led to outgassing and intermediate formation in the reduced atmosphere; a prerequisite for the origin of life.

While various studies have proposed plausible scenarios underlying redox processes in Earth’s early mantle and atmosphere, researchers have attributed the formation of prebiotic organic species to the planet’s low oxygen fugacity. The Earth’s mantle has been oxidized to current levels since the Archean age to contain water, carbon dioxide and nitrogen. To carry out numerical simulations reproducing the first terrestrial conditions, researchers had to develop appropriate experimental methods to validate the reactions induced by giant impacts.

The moon-forming impact may have vaporized Earth’s silicate mass to form a vapor atmosphere and expanded the magma ocean to cause global mixing between proto-Earth materials and the differentiated impactor. Planetary scientists postulate that the impact of the formation of the Moon would have induced vigorous chemical reactions between the differentiated compounds of the impactor and the proto-Earth, leading to the beginning of life.

X-ray free electron lasers as a structural probe

Since X-ray free electron lasers are the brightest artificial light source in the energy regime of X-rays produced from undulator magnets. The team incorporated ultrashort pulsed laser-like structures generated from self-amplified spontaneous emission.

In this work, Choi and his colleagues used X-ray free electron lasers to pump and probe a pre-compressed mixture of heavy iron, volatile water, and carbon dioxide to simulate chemical reactions between the metal core of the impactor and the volatile substances present in the proto-Earth. The results provided experimental evidence for giant impact-driven iron oxidation, leading to the first evolutionary pathways necessary for the origin of life.

Simulation of the environment induced by a giant impact

During the experiments, scientists used various materials and estimated the temperature of the iron sheet when irradiated by a single X-ray free electron laser pulse from the deposited energy to match the energy d impulse absorbed by the irradiated sample. The energy density increased the moment the pressure lasted for picoseconds, by laser shock compression.

Although the difference in time scale between a giant impact and its experimental simulation still exists, the energy of the X-ray pump covered much of the conditions caused by a giant impact.

Choi and the team further determined the pressure and temperature of the X-ray probe pulse and determined the effect on the iron-water system. When the team probed the samples with an additional pulse some time after each train of pulses, the resulting reactions produced additional hydrogen as a secondary oxidation product.

Additionally, Choi and his team performed X-ray free electron lasers on the iron-carbon dioxide system, where the ferrous oxide reacted further with CO.₂ to form siderite from consecutive pulses.

Microscopic observations of recovered samples

The scientists gained new insights into the experiments’ reaction pathway after probing the cross sections of the recovered samples using focused ion beam and electron microscopy.

To understand the role of silicate in giant impact-driven reactions, the team performed an in situ laser heating experiment on the iron-water-silicate system. They noted that the presence of silicate did not impact iron oxidation or the production of reduced species. While the amount of water and carbon dioxide present on Earth before the formation of the Moon was highly controversial, there is a hypothesis that the reactants would fully participate in the reactions induced by the giant impact.

Outlook

In this way, Jinhuyuk Choi and colleagues proposed a scheme for the giant redox processes induced by the impacts of the primitive atmosphere and the Earth’s mantle. They observed that the extent of iron oxide formation and iron hydrogenation was inversely correlated with the pressure produced by the reaction between iron and water. X-ray free-electron laser pump-probe experiments on pre-compressed iron, mixed with volatiles, experimentally simulated giant impact-driven reactions in the magma ocean.

The research team estimated the amounts of oxidized iron species and reduced volatiles. The team supported the Theia hypothesis during the work, which describes a collision between proto-Earth and an astronomical body called Theia. The results explain the temporal and global conformation of the oxidized mantle and reduced atmosphere to facilitate the emergence of life on the early Earth.

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