A new study from the Cal Institute of Technology shows that the Earth’s earliest times consisted of hot, dry materials, meaning water arrived late in Earth’s formation. The study, which offers clues from different layers of the mantle, influences theories on the formation of terrestrial planets, suggesting that significant additions of volatiles occurred only during the final stages of Earth’s formation.

Billions of years ago, in the giant disk of dust, gas, and rock orbiting our young Sun, larger and larger bodies finally came together to form the planets, moons, and asteroids we see today. Scientists are still trying to understand the processes by which planets form, including our home planet.

One way to study how the earth was formed is to study the magma that flows from deep within the planet. The chemical signatures of these specimens contain a record of the time and nature of the materials that came together to form the Earth—similar to how fossils give us clues about Earth’s biological past.

Now, research from the California Institute of Technology shows that the earliest times on Earth consisted of hot, dry materials; This suggests that water on our planet, a crucial component for the evolution of life, must have appeared late in Earth’s history.

The study, involving an international research team, was carried out in the laboratories of François Tissot, associate professor of geochemistry and researcher at the Heritage Institute for Medical Research; and Yigan Zhang of the University of Chinese Academy of Sciences. An article describing the work was recently published in a journal. Science Advances. Caltech graduate student Weiyi Liu is the first author of the paper.

While there is no way for humans to travel deep into our planet, rocks deep in the earth can naturally come to the surface as lava. The main magmas of these lavas may come from various 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 mi) to the core-mantle interface about 2,900 kilometers (1,800 mi) below our feet.

Just like sampling the different layers of a cake (frosting, filling, sponge), scientists can study magma from different depths to understand the different “flavors” of Earth’s layers: the chemicals they contain and how they relate to each other.

Because the Earth’s formation was not instantaneous, but instead involved materials that accumulated over time, samples from the lower and upper mantle provide different clues to what happened over time during Earth’s accretion. In the new study, the team found that early Earth consisted mostly of dry rocky materials: chemical signatures from deep within the planet showed that it was absent from so-called volatile substances that easily evaporate, such as water and iodine.

In contrast, the upper mantle samples revealed a higher proportion of volatile matter, three times that of the lower mantle. Based on these chemical relationships, Liu created a model showing that the Earth is composed of hot, dry, rocky materials, and that significant additions of essential volatile substances for life, including water, occur only during the last 15 percent (or less) of Earth’s formation.

The research makes a crucial contribution to the theory of planet formation, a field that has undergone several paradigm shifts in recent decades and is still characterized by heated scientific debate. In this context, the new study makes important predictions about the nature of the building blocks of other terrestrial planets (Mercury and Venus), which are expected to consist of similar dry materials.

“Space exploration to distant planets is really important because the aquatic world is probably the best place to look for extraterrestrial life,” says Tissot. “But don’t forget the inner solar system. No mission has touched the surface of Venus for nearly 40 years, and no missions have been to the surface of Mercury. We need to be able to study these worlds to better understand how terrestrial planets like Earth were formed.” Source