Scientists may have uncovered a new explanation for how Earth acquired its water.
Our planet’s oceans are unlike anything else known in the solar system and are essential for life. Yet researchers say Earth may not have started out as the blue world we know today.
According to scientists at the University of New Mexico, Earth could have been entirely dry when it first formed.
Their new theory suggests that much of the water now covering the planet may have been produced from Earth’s own rocky building blocks.
As the young planet grew, rocky pebbles may have fallen through a hydrogen-rich atmosphere, triggering chemical reactions.
Under intense heat, iron oxide in the rocks reacted with hydrogen, producing water and pure iron. That iron would later have contributed to the formation of Earth’s metallic core.
Laboratory experiments indicate that this process could have generated the equivalent of hundreds of oceans of water.
Co-author Professor Zachary Sharp, a planetary scientist at the University of New Mexico, said: ‘Looking out over a large body of water, it is hard to imagine that that water may have been produced 4.5 billion years ago in a hell-like atmosphere, but it very well may have.’
Researchers say Earth may once have been completely dry, then produced its own water through reactions between its hot atmosphere and falling ‘pebbles’ (artist’s impression)
Earth’s abundance of liquid water is well established, but its origin has long presented astronomers with a difficult puzzle.
Scientists believe Earth and the other planets formed from a swirling disc of dust and rock known as the protoplanetary disc.
Near the disc’s centre, where the inner planets eventually emerged, powerful solar radiation would have broken most water molecules apart into hydrogen and oxygen.
The challenge, therefore, is explaining how water came to exist on Earth in such large quantities.
‘Previous ideas about the source of water to Earth rely on an external source, such as comets or meteorites,’ Professor Sharp said.
These so-called ‘icy pebbles’ would have formed in the colder outer regions of the solar system before moving inward towards the developing Earth.
That scenario would require the young planet to be struck by a vast number of comets during its formation, a possibility some experts consider unlikely.
Professor Sharp and his colleagues instead argue that most of Earth’s water may have formed locally, through reactions involving rocky minerals and hydrogen.
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As Earth formed, rocky silicate pebbles fell through a hot, hydrogen-rich atmosphere. Iron oxide within the rocks reacted with hydrogen to produce water and pure metallic iron, which helped form the planet’s core (illustrated)
Many scientists now think planets grew as small rocky pebbles gradually accumulated into larger bodies, a process called pebble accretion.
Once the growing planet reached sufficient size, it developed a hot, high-pressure atmosphere rich in hydrogen. That atmosphere could react with iron oxide in the incoming pebbles and generate water.
To investigate the idea, the researchers heated rock samples to between 1,200°C and 1,400°C (2,190–2,550°F) while exposing them to streams of hydrogen. They also conducted comparison tests using helium.
In their paper, published in Nature Geoscience, the team reported that 80 per cent of the iron oxide in samples exposed to hydrogen was converted into metallic iron, with water forming within minutes.
The samples exposed to helium showed no comparable reaction, indicating that the hot hydrogen atmosphere was essential to the process.
Professor Sharp said: ‘We had suspected and hoped that we would produce iron metal and water in our reduction experiments, but we really had no way of knowing how fast and how much.
‘We were quite surprised to find that extrapolation of our work suggests that hundreds of oceans’ worth of water were likely produced in this way.’
The findings suggest that this ancient ‘pebble rain’ may have played a crucial role in creating the oceans that eventually made life possible on Earth.
To investigate the theory, scientists heated silicate mineral samples to extreme temperatures and monitored whether they produced iron and water. Pictured: Rock samples viewed under a scanning electron microscope after heating
The theory could also have important implications for the search for life beyond our solar system.
If many rocky planets formed through pebble accretion, they may have undergone similar water-producing reactions.
Instead of relying on ice or water-rich minerals arriving from space, a growing planet could potentially create its own oceans from the materials used to build it.
As a result, liquid water may be common on far more rocky, Earth-sized planets than previously thought.
‘The idea of pebble accretion is still relatively new. It’s only been around for the last decade. But it seems to solve many problems that exist with the existing paradigm for planet formation,’ Professor Sharp said.
‘And now we see that it also can explain why our planet is habitable. By extension, it is reasonable to expect that other Earth-sized bodies circling other stars would also have water and possibly life.’
