The bowels of the early and hot Earth saved water for us – how is this possible?
Water is not only inextricably linked with the emergence of terrestrial life, but also provided the conditions for its evolution. After all, for about three billion years, life existed and developed exclusively in the oceans, which would not exist if there were no more or less stable climate on the planet. In addition, even small amounts of water in the interior of the Earth soften rocks – a necessary condition for plate tectonics, which in turn is responsible for the shape of continents and oceans, earthquakes and volcanic activity – all that determined the shape of our Earth. Despite such a large role of water in the evolution of living and non-living things on Earth, it is still not entirely clear why there is so much water on Earth.
According to one hypothesis, comets could bring water to us, but the isotopic composition of terrestrial and comet water is different. Another hypothesis says that water was released from the bowels of the earth. But then the question arises of how a primordial ocean could survive the turbulent first tens of millions of years in Earth’s history, when it was hot, heavily bombarded by asteroids, and even collided with an ancient protoplanet. All these cataclysms were supposed to melt the upper few hundred kilometers of the earth’s crust and permanently evaporate water from the surface of the planet.
But if water still hid somewhere in the depths of the Earth, there must be a chemical substance capable of holding water molecules for a long time at high temperature and colossal pressure for millions of years. And then release it in a more peaceful era.
As they write in the magazine Physical Review Letters researchers from Nankai Xiao Dong University, together with colleagues from Skoltech, magnesium hydrosilicate Mg₂SiO₅H₂ is suitable for the role of such a compound. It contains 11% water by mass and is stable at pressures of more than 2 million atmospheres and extremely high temperatures – just like in the Earth’s core. But here comes the next logical question. In the center of the Earth, as we know, there is a metal ball, consisting mainly of iron, and there is no smell of any hydrous magnesium silicate. So where does the water go then?
As one of the authors of the work, Professor Artyom Oganov, says, at the initial stage of the Earth’s existence, it could not have had any formed core. The chemical composition of the young planet was homogeneous from the surface to the very depths. It took about 30 million years for iron to concentrate in the center of the Earth, forming a core and displacing silicates from there into the mantle. If this is true, then during the first 30 million years, during the most catastrophic phase of the asteroid bombardment, part of the Earth’s water was safely hidden at the depth of the current core in the form of hydrosilicates. And when the process of core formation ended, the hydrosilicates were forced out of the central region of the planet into a zone of lower pressure, where they turned out to be unstable and underwent decay. This is how magnesium oxide and silicate were formed – the mantle now consists of them – and water, the gradual rise of which to the surface took about another 100 million years.
A new hypothesis of the origin, or rather the preservation of terrestrial water, gives a new look at the fate of water on other planets. For example, Mars is smaller than the Earth, so the pressure inside its core is less and magnesium hydrosilicate is unstable in it. There was no way for the water to “sit out” the bombardment in a safe place, which is why there is so little water on Mars, and the water that exists now may have been brought just by comets. What about exoplanets? Inside the massive terrestrial planets – the so-called super-Earths – the high pressure that stabilizes the hydrous magnesium silicate exists outside the core. Therefore, their bowels are theoretically capable of holding even larger volumes of water than the Earth. And, perhaps, the conditions for the evolution of life on them are no less favorable than on our planet.