Planet Earth makes its own water from scratch deep in the mantle

By Andy Coghlan

Our planet may be blue from the inside out. Earth’s huge store of water might have originated via chemical reactions in the mantle, rather than arriving from space through collisions with ice-rich comets.

This new water may be under such pressure that it can trigger earthquakes hundreds of kilometres below Earth’s surface – tremors whose origins have so far remained unexplained.

That’s the upshot of a computer simulation of reactions in Earth’s upper mantle between liquid hydrogen and quartz, the most common and stable form of silica in this part of the planet.

“This is one way water can form on Earth,” says team member John Tse at the University of Saskatchewan in Canada. “We show it’s possible to have water forming in Earth’s natural environment, rather than being of extraterrestrial origin.”

The simple reaction takes place at about 1400 °C and pressures 20,000 times higher than atmospheric pressure as silica, or silicon dioxide, reacts with liquid hydrogen to form liquid water and silicon hydride.

Deep down

The latest work simulates this reaction under various temperatures and pressures typical of the upper mantle between 40 and 400 kilometres down. It backs up previous work by Japanese researchers who performed and reported the reaction itself in 2014.

“We set up a computer simulation very close to their experimental conditions and simulated the trajectory of the reaction,” says Tse.

But in a surprise twist, the simulation showed that the water forms within quartz but then can’t escape and so the pressure builds up.

“The hydrogen fluid diffuses through the quartz layer, but ends up forming water not at the surface, but in the bulk of the mineral,” says Tse. “We analysed the density and structure of the trapped water, and found that it is highly pressurised.”

According to the simulation, the pressure could reach as much as 200,000 atmospheres. “We observed the water to be at high pressure, which might lead to the possibility of induced earthquakes,” says Tse.

Quake trigger

The quakes could be triggered as the water finally escapes from the crystals. “However, further research is needed to quantify the amount of released water needed for triggering deep earthquakes,” says Tse.

Other researchers said it was plausible that this water caused deep quakes. “These results provide important insights into the reactions between quartz and hydrogen at high pressures,” says John Ludden, executive director of the British Geological Survey. “The formation and release of overpressured water may be a significant trigger in the deep lithosphere for ultra-deep earthquakes, sometimes located well below the crust and in the more rigid parts of deep continental plates.”

The findings may also inform how our planet got its water to start with.

Studies over the past few years have found evidence of several oceans’ worth of water locked up in rock, as far down as 1000 kilometres, questioning the assumption that water arrived from space after Earth’s formation. A study published this week, for example, based on isotopes from meteorites and Earth’s mantle, also found that water is unlikely to have arrived on icy comets after Earth formed, as has long been assumed.

Instead, all this research seems to suggest that much of our planet’s water may have come from within – although no one yet knows exactly how much.

Origin story

“As long as the supply of hydrogen can be sustained, one can speculate that water formed from this process could be a contributor to the origin of water during Earth’s early accretion,” says Tse. “Water formed in the mantle can reach the surface via multiple ways, for example, carried by magma in the form of volcanic activities.”

It is possible that water is still being made this way deep inside Earth today, and the same could be true of other planets.

The new simulation results are quite surprising “because rather than hydrogen bonding into the quartz crystal structure, it disrupts the structure completely by bonding with oxygen and forming water-rich regions below the surface”, says Lydia Hallis at the University of Glasgow, UK. “The study highlights how the minerals that make up Earth’s mantle can incorporate large amounts of water, and how Earth is probably ‘wet’ in some sense all the way down to its core.”

But despite the potential for the process to have created much of Earth’s water, Ludden thinks it may be small-scale and localised in comparison with the input of water from water-rich comets, meteorites and asteroids. “I think it’s reasonable to assume that much of the water came in this way,” he says.

Journal reference: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2016.12.031

Read more: Deepest water found 1000km down, a third of way to Earth’s core

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