Caltech reveals new details about Earth’s formation

A new study from the California Institute of Technology indicates that the early Earth formed from hot, dry material, which means that water arrived late in the formation of the Earth. The research, which presents evidence from different mantle layers, posits that major additions of volatiles occurred only during Earth’s final stages of formation, influencing theories of planet formation.

Billions of years ago, in the giant disk of dust, gas, and rocky material that orbited our young Sun, larger and larger bodies combined to eventually produce the planets, moons, and asteroids we see today. Scientists are still trying to understand the processes through which planets, including our own, were formed.

One way researchers can study how Earth formed is by examining the magma that flows from deep within the planet’s interior. The chemical signatures from these samples contain a record of when and what materials came together to form Earth — similar to how fossils give us clues about Earth’s biological past.

Now, a study from the California Institute of Technology shows that the early Earth accumulated hot, dry material, suggesting that our planet’s water—a key ingredient for the development of life—must have arrived much later in Earth’s formation.

The study, involving an international team of researchers, was conducted in the laboratories of François Tissot, associate professor of geochemistry and investigator of the Heritage Medical Research Institute; and Yigang Zhang of the Chinese Academy of Sciences. A paper describing the research was recently published in the journal Science advances. Caltech graduate student Wei Liu is the first author of the paper.

Although humans have no way of traveling into our planet’s interior, rocks deep within the Earth can naturally make their way to the surface in the form of lava. The parental magma of these lavas can originate from different depths within the Earth, such as the upper mantle, which begins about 15 kilometers (9 miles) below the surface and extends for about 680 kilometers; or the lower mantle, which extends from a depth of 680 kilometers (425 miles) all the way to the core-mantle boundary at about 2,900 kilometers (1,800 miles) below our feet.

Like sampling different layers of a cake—frosting, filling, and sponge—scientists can study magma emerging from different depths to understand the different “flavors” of Earth’s layers: the chemicals within them and their ratios in relation to each other.

Because the formation of the Earth was not instantaneous, and instead involved the accumulation of material over time, samples from the lower mantle and upper mantle give different clues to what was happening over time as the Earth accumulated. In the new study, the team found that early Earth consisted mainly of dry, rocky material: chemical signatures from deep within the planet showed no presence of so-called volatiles, easily evaporating substances such as water and iodine.

In contrast, samples from the upper mantle revealed a higher proportion of volatiles, three times that from the lower mantle. Based on these chemical proportions, Liu created a model that showed that the Earth is composed of hot, dry rocky materials, and that a major addition of volatiles essential to life, including water, occurred only during the last 15 percent (or less) of Earth’s formation.

The study is a crucial contribution to theories of planetary formation, a field that has undergone several paradigm shifts in recent decades and is still marked by vigorous scientific debate. In this context, the new study makes important predictions for the nature of the building blocks of the other terrestrial planets – Mercury and Venus– which is expected to have been formed from similar dry materials.

“Space exploration for exoplanets is really important because the watery world is probably the best place to look for extraterrestrial life,” says Tissot. But the inner solar system should not be forgotten. There has not been a mission to touch the surface of Venus in nearly 40 years, and there has never been a mission to the surface of Mercury. We need to be able to study those worlds to better understand how terrestrial planets like Earth formed. “

Reference: “I/Pu Earth reveals mainly volatile poor contrast planets” by Weiyi Liu, Yigang Zhang, and François. LH Tissot, Guillaume Avice, Zhilin Ye, and Qing-Zhu Yin, July 5, 2023, Available here. Science advances.
DOI: 10.1126/sciadv.adg9213

In addition to Liu and Tissot, the co-authors are Zhang from the Chinese Academy of Sciences. Guillaume Avice of the City University of Paris, Institute of Physics in the World, Paris; Qilin Ye of the Chinese Academy of Sciences; and King Chu Yin of the University of California, Davis. Funding was provided by the Chinese Academy of Sciences, the National Science Foundation, the Packard Fellowship of Science and Engineering, the Heritage Medical Research Institute, and the California Institute of Technology.