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How did early Earth hold onto its oceans?

A new study published in the journal Physical Review Letters has investigated the origins of water on Earth. According to a team of scientists led by the Skolkovo Institute of Science and Technology (Skoltech) and Nankai University, a chemical compound that is now extinct could have helped preserve water deep underground in the era when massive asteroid collisions must have evaporated all the Earth’s surface water.

Surface water has an important role in stabilizing a planet’s climate over long periods of time, allowing life to emerge. Moreover, even small amounts of water under a planet’s surface dramatically increase rock plasticity, which is essential for plate tectonics – a process fundamentally shaping the planet’s continents and oceans, and driving earthquakes and volcanism.

While some scientists believe that water on Earth was seeded by comets, others argue that water originated deep within our planet’s mantle or even core. By using the crystal structure prediction method USPEX, the researchers discovered a stable compound that could lock up hydrogen and oxygen atoms within the Earth’s interior long enough, and then release them as water: magnesium hydrosilicate (Mg2SiO5H2), which consists of over 11 percent water and is stable at extremely high temperatures and pressures of more than two million atmospheres.

According to the scientists, for over 30 million years, while the Earth was frequently bombarded by asteroids, part of the Earth’s water was safely stored away in the form of hydrosilicates at the depths of what only later on became the planet’s iron core. By the time the core formed, the hydrosilicates had been pushed into low pressure areas, where they became unstable and decomposed, producing the magnesium oxide and silicate that now form the mantle.

This theory is “a story about how a material that existed for a brief moment on the planetary timescale had a massive impact on the Earth’s evolution,” said study co-author Artem Oganov, a material scientist at Skoltech. “This runs counter to the usual geological mindset, but come to think of it, an evolutionary biologist, for whom much of what we see today has evolved out of now-extinct species, would hardly be surprised, would they?”

These findings have important implications for understanding other celestial bodies too.  “Mars, for example, is too small to produce pressures necessary to stabilize magnesium hydrosilicate,” said Professor Oganov. “This explains why it is so dry and means that whatever water exists on Mars, it likely came from comets.”

In the case of very large planets from outside our solar system that can perhaps contain water and thus support life (the hypothetical “super-Earths”), pressures stabilizing the magnesium hydrosilicate can exist even outside the core, locking large amounts of water indefinitely. “As a result, super-Earths can have a much greater water content and still support the existence of exposed continents,” concluded study co-author Xiao Dong, a physicism at Nankai University.

By Andrei Ionescu, Staff Writer 

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