In the past of the Earth there is the mystery of the origin of the oceans
In addition to being the very important substance for the origin of life as we know it, surface water is important for stabilizing the climate of a planet for long periods of time, allowing the 'evolution. Even small amounts of water deep below the surface are known to dramatically increase rock plasticity, which is essential for plate tectonics - a process that shapes continents and oceans and drives earthquakes and volcanism. But despite its enormous importance for the evolution of rocky planets like ours, we don't know where the Earth's water comes from.
"Some scientists thought that our water was sown by comets, but this source it appears to be very limited - the isotopic composition of water in comets is very different from that on Earth, ”says Professor Artem R. Oganov of Skoltech, co-author of the study.
If the water did not come from above, it must have come from below, from the depths of the mantle or even from the core of the Earth. But how could it survive the first 30 million years or so in Earth's history, when the planet was very hot and was relentlessly bombarded with asteroids and even suffered a catastrophic collision with a planet the size of Mars? These processes must have evaporated part of the Earth and what remained was melted at least several hundred kilometers below, removing the water. Until now, scientists did not know of a stable compound that could block hydrogen and oxygen atoms inside the planet long enough and then release them as water.
Oganov collaborated with a group of led scientists by Professor Xiao Dong of Nankai University, China, and together they used Oganov's USPEX crystal structure prediction method to discover a compound that fits the bill: magnesium hydrosilicate, with the formula Mg2SiO5H2, which is more than 11% water by weight and is stable at pressures of over 2 million atmospheres and extremely high temperatures. Such pressures exist in the core of the Earth. But everyone knows that the core is a ball of metal - mostly iron - so the elements that make up the magnesium hydrosilicate are simply not available there, right?
"Wrong. At the time there was no core. At the beginning of its existence, the Earth had a more or less uniformly distributed composition, and it took about 30 million years since the planet was formed to filter to its center, pushing silicates into what we now call the mantle " , Oganov explained.
This means that for 30 million years, some of the Earth's water has been safely stored in the form of hydrosilicates at the depths of its present core. During that time the Earth withstood the heaviest phase of the asteroid bombardment. When the core formed, the hydrosilicates had been pushed into low pressure areas, where they became unstable and decomposed. This produced the magnesium oxide and magnesium silicate that make up the mantle today, and water, which began its 100-million-year journey to the surface.
“Meanwhile, the Earth it was being hit by asteroids and even a protoplanet, but the water was safe, because it hadn't made its way to the surface yet, ”adds Oganov.
Researchers say their study shows how much they can sometimes human intuitions may be defective. No one had thought of silicates at core pressures, because the constituent atoms were presumably not there. And even then, people would not have expected a hydrosilicate to be stable under central conditions, because extreme temperatures and pressures were believed to "squeeze" the water out of the mineral. Yet accurate modeling based on quantum mechanics has proven otherwise.
"It is also a story about how a material that existed for a brief moment on the planetary time scale had a huge impact on the evolution of the Earth", continues the materials scientist. “This is at odds with the usual geological mentality, but come to think of it, an evolutionary biologist, for whom much of what we see today has evolved from extinct species, wouldn't be surprised, would it?”
The new hypothesis of the origin of water also has implications for other celestial bodies. "Mars, for example, is too small to produce the pressures needed to stabilize magnesium hydrosilicate," says Oganov. “This explains why it is so dry and it means that any water that exists on Mars probably comes from comets.”
Or, consider planets outside our solar system. “To be habitable, an exoplanet must have a stable climate, which requires both continents and oceans. So there must be water, but not too much, ”adds Xiao Dong. "There was an estimate that for an Earth-like planet of any size to be habitable, it should have no more than 0.2% water by weight. Our results imply that for large Earth-like planets, called "super-Earths", the story is probably different: in such planets, the pressures that stabilize the magnesium hydrosilicate must also exist outside the nucleus, blocking large amount of water indefinitely. As a result, super-Earths can have a much higher water content and still support the existence of exposed continents. "
It also has implications for a planet's magnetosphere. “At temperatures above 2,000 degrees Celsius, magnesium hydrosilicate will conduct electricity, with hydrogen protons acting as charge carriers. This means that our hydrosilicate will contribute to the magnetic fields of super-Earths ", explains Oganov, adding that the list of consequences of the new hypothesis continues indefinitely.
