Our planet's water arrived late — in the final 15% of Earth's formation

In a new study, chemical signatures reveal Earth's formation from dry, rocky materials, with life-giving water and other essential volatiles arriving later than expected.
Sade Agard
The glimpse of Earth's possible formation showing internal structure.
The glimpse of Earth's possible formation showing internal structure.


Our planet initially accreted from hot and dry materials, suggesting that the crucial ingredient for the evolution of life—water—arrived later in Earth's history, according to a study published in Science Advances on July 5.

The research holds significance for our understanding of planet formation — a realm that continues to spark vigorous scientific debate. Moreover, it provides valuable insights into the building blocks of other terrestrial planets, such as Mercury and Venus, which are also believed to have originated from similarly dry materials.

How do scientists use lava to understand Earth's structure?

Although humans cannot physically venture into the depths of our planet, rocks from deep within the Earth can reach the surface naturally in the form of volcanic lavas.

These lavas originate from "parental magmas" that can come from various depths within Earth. For instance, the upper mantle begins approximately 15 kilometers beneath the surface and extends around 680 kilometers downward. 

On the other hand, the lower mantle starts from 680 kilometers deep. It stretches to the core-mantle boundary, about 2,900 kilometers beneath our feet. Scientists can chemically compare these magmas to comprehend Earth's layers' distinct characteristics.

Since Earth's formation occurred gradually through the accumulation of materials, samples obtained from the lower and upper mantle offer distinct clues about the processes that unfolded during Earth's accretion.

In this new study, the researchers found that our planet was predominantly made up of dry and rocky materials during its early stages. 

By examining chemical signatures deep within the Earth, they observed a distinct absence of volatile substances, such as water and iodine, which quickly evaporate.

In contrast, when they analyzed samples from the upper mantle, they detected a significantly higher presence of volatiles. These substances' proportions were three times greater than those in the lower mantle.

Building on these chemical ratios, Weiyi Liu, a graduate student at Caltech and the study's lead author, developed a model that provided valuable insights. 

Studying Venus and Mercury could tell us how Earth formed

The model revealed that Earth formed from hot, dry, and rocky materials. It also suggested that the addition of essential volatiles crucial for life, including water, occurred relatively late in Earth's formation, specifically during the last 15 percent or even less of the planet's development.

"Space exploration to the outer planets is really important because a water world is probably the best place to look for extraterrestrial life," said Francois Tissot, assistant professor of geochemistry and Heritage Medical Research Institute Investigator, in a press release. 

 "But the inner solar system shouldn't be forgotten. There hasn't been a mission that's touched Venus's surface for 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 understand better how terrestrial planets such as Earth formed."