A study by Caltech found that the early Earth was formed of hot, dry materials, indicating the late arrival of water during Earth’s formation. This study used magmas from different layers of the Earth’s interior, providing unique information about the formation of the planet.

A new study from Caltech suggests that the early Earth formed from hot, dry materials, implying that water arrived late in Earth’s formation. The research, offering clues from different layers of the mantle, posits that major additions of volatiles only occurred during the final stages of Earth’s formation, impacting theories of how terrestrial planets formed.

Billions of years ago, in the giant disk of dust, gas, and rocky material that circled around our young sun, larger and larger bodies coalesced to eventually form the planets, moons, and asteroids we see today. Scientists are still trying to understand the processes by which planets, including our home planet, formed.

One way for researchers to study the formation of the Earth is to examine the magmas rising from the depths of the planet’s interior. The chemical signatures of these samples contain a record of when and what materials came together to form the Earth, analogous to how fossils give us clues to Earth’s biological past.

Now, a study by Caltech shows that the early Earth built up from hot, dry materials, indicating that our planet’s water – the crucial component for the evolution of life – must have arrived late in the history of Earth’s formation.

The study, involving an international team of researchers, was conducted in the laboratories of François Tissot, assistant professor of geochemistry and researcher at the Heritage Medical Research Institute; and Yigang Zhang from the University of the Chinese Academy of Sciences. An article describing the research was recently published in the journal Scientists progress. Caltech graduate student Weiyi Liu is the first author of the paper.

Although humans have no way of getting inside our planet, rocks deep within the earth can naturally rise to the surface as lava. The parent magmas of these lavas can come from different depths of 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) to the core-mantle boundary about 2,900 kilometers (1,800 miles) below our feet.

Like sampling different layers of a cake – the frosting, the filling, the sponge – scientists can study magmas from different depths to understand the different “flavors” of Earth’s layers: the chemicals found inside and their relationships to each other.

Because Earth’s formation was not instantaneous and instead involved materials that accreted over time, samples from the lower mantle and the upper mantle give different clues to what happened over time during Earth’s accretion. In the new study, the team found that early Earth was mostly composed of dry, rocky materials: chemical signatures from deep within the planet showed a lack of so-called volatiles, which are easily evaporated materials like water and iodine.

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

The study is a crucial contribution to theories of planet formation, a field that has undergone several paradigm shifts in recent decades and is still characterized by vigorous scientific debate. In this context, the new study makes important predictions about the nature of the constituent elements of other terrestrial planets – Mercury and Venus— which would presumably have formed from such dry materials.

“Space exploration to the outer planets is really important because an aquatic world is probably the best place to search for extraterrestrial life,” Tissot says. “But the inner solar system should not be forgotten. No mission has touched the surface of Venus for nearly 40 years, and there has never been a mission to the surface of Mercury. We need to be able to study these worlds to better understand how terrestrial planets such as Earth formed.

Reference: “I/Pu reveals that the Earth is mainly accreted from poor volatile differentiated planetesimals” by Weiyi Liu, Yigang Zhang, François. LH Tissot, Guillaume Avice, Zhilin Ye and Qing-Zhu Yin, July 5, 2023, Scientists progress.
DOI: 10.1126/sciadv.adg9213

Besides Liu and Tissot, the co-authors are Zhang from the University of the Chinese Academy of Sciences; Guillaume Avice from the University of Paris Cité, Institute of Earth Physics in Paris; Zhilin Ye of the Chinese Academy of Sciences; and Qing-Zhu Yin of the University of California, Davis. Funding was provided by the Chinese Academy of Sciences, the National Science Foundation, a Packard Science and Engineering Fellowship, the Heritage Medical Research Institute and Caltech.