From rock to water: Studying the origins of water on Earth

Imagine water seeping up from the solid ground beneath our feet. Then imagine water falling from the sky — not as rain, but as a giant rock crashing into Earth. Both seem like ridiculous scenarios, but they closely relate to real theories about what may have happened billions of years ago. The origins of water on our planet have baffled scientists for decades. The scalding temperatures of proto-Earth’s surface 4.55 billion years ago prevented water from existing as a liquid. With the lack of an atmosphere, the young planet could not prevent water molecules from evaporating into space. Even today, scientists do not have a definitive theory. Popular theories speculate space as the origin of water, pointing toward asteroids as the main contributor. However, a recent study conducted by researchers from Nankai University and Skoltech found new evidence that may suggest a different story.

Meteorite impacts have long stood as a means for water to travel from space to Earth. These scientists found that carbonaceous chondrites — meteorites with large carbon stores that formed during the early years of the solar system — contained large amounts of water. One test conducted on the chondrites looked at their hydrogen isotopes, which are atoms of hydrogen that contain different amounts of neutrons. Unfortunately, when scientists compared the isotopic hydrogen ratios of these chondrites with that of water on Earth, they found these ratios do not match. This suggests that the chemical composition of the water from carbonaceous chondrites differs too much from Earth’s water for meteorite impacts to be the main water contributor. However, meteorite studies may still help us understand the origins of water. Recent studies conducted by researchers at the University of Lorraine found that enstatite-heavy chondrites had a composition (including their isotopic hydrogen ratio) similar to Earth’s mantle layer. They furthermore demonstrated that enstatite chondrites contained a large enough hydrogen composition to fill up Earth’s oceans and more. Their findings have a large implication; Earth was formed from material similar to enstatite chondrites. If true, then the mantle layer, with its similar composition to enstatite chondrites, could have stored large amounts of water on proto-Earth. Perhaps the origins of water lay underneath the surface rather than extraterrestrially.

Meteorite studies may still help us understand the origins of water.

Earth has not always been separated neatly into its known layers  (crust, mantle, and core). The moment in proto-Earth’s age before the core-mantle separation was important for water storage. Physicists from Nankai University and Skoltech discovered magnesium hydrosilicate compounds — which existed before the separation of the core and mantle — that could store water inside Earth. The magnesium hydrosilicates, alpha-Mg₂SiO₅H₂ and beta-Mg₂SiO₅H₂, likely resided in abundance within proto-Earth, widely distributed across its inner terrain. Without the core, they inhabited a deeper level of Earth where higher pressure was present. The same researchers conducted tests on the compounds and found that both variants were stable at high pressures and temperatures similar to those found in proto-Earth at the time. These conditions would have allowed the hydrosilicates to stably store the components of water — oxygen and hydrogen — underground while the atmosphere of proto-Earth was unsuitable for liquid water.

However, Mg₂SiO₅H₂ could not last forever. As Earth’s core grew, it slowly pushed up the silicates from their residence underneath the ground. At shallower depths, further from the center of Earth, there was lower pressure, leading to the destabilization of both alpha-Mg₂SiO₅H₂ and beta-Mg₂SiO₅H₂. As a result, the hydrosilicates decomposed into magnesium silicate (MgSiO₃), magnesium oxide (MgO), and water (H₂O). Molecules of water would slowly diffuse to Earth’s surface where it would eventually shape the biosphere—allowing the emergence, survival, and evolution of life.

Molecules of water would slowly diffuse to Earth’s surface where it would eventually shape the biosphere—allowing the emergence, survival, and evolution of life.

While not all scientists agree with how water came to reside on Earth, with the silicate theory being one of many, they agree that water’s presence heavily influences Earth’s internal activities. Its ability to soften (or even break apart) rocks, lower their friction and decrease their melting temperatures helped generate magma as well as impact the movements of tectonic plates and consequent earthquakes. Deeper in Earth, water reacts with iron to form iron oxides and pyrites; dissociation of these pyrites was believed to have contributed to the oxidation of the atmosphere — an important evolutionary event for life. Needless to say, the origins of this molecule could also be considered the origins of life on our planet, shaping the terrain and the nature around us.

Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.035703

Science (2020). DOI: 10.1126/science.aba1948

National Science Review (2020). DOI: 10.1093/nsr/nwz071

Science (2020). DOI: 10.1126/science.abc1338