Published in the journal AGU Advances, it suggested that back when the planet was a molten ball of magma, its mantle held far less water than it does today. The mantle is the thick, semi-solid layer that sits between the planet's dense, super-heated inner layer called the core and its thin outer one called the crust.
Lead researcher Junjie Dong, a graduate student of mineral physics at Harvard University in Massachusetts, explained that over billions of years, the mantle's water storage capacity gradually increased as the temperatures in that layer cooled. This may be why the mantle today holds more water.
That suggests the water was somewhere else. "And the most likely reservoir is the surface," said Dong.
Earth was once a "water world"
Water is stored within the mantle in the form of hydroxy groups. These are simple structures consisting of an oxygen and hydrogen atom bonded together. In particular, water is stored in two high-pressure forms of olivine, an important rock-forming mineral. Its two forms are hydrous wadsleyite and ringwoodite.
Current samples of hydrous wadsleyite collected deep inside the mantle may contain three percent water and current samples of ringwoodite may contain around one percent water.
Researchers of previous studies have subjected these two minerals to the high pressures and temperatures of the mantle of the present Earth to figure out their water storage capacities.
But rather than limit their analysis to just pressures and temperatures, the researchers compiled and used all available mineral physics data to calculate the water storage for the two minerals across a wider range of pressures and temperatures, including those scientists believe to have existed billions of years ago.
Their analysis showed that hydrous wadsleyite and ringwoodite have lower water storage capacities at higher temperatures. Since ancient Earth was much warmer internally then than it is today, it follows that the water storage capacity of its mantle now is higher than it once was.
In particular, the mantle's water storage capacity today is 1.86–4.41 times the modern surface ocean mass. In all, the difference in water storage capacity is significant even with conservative calculations.
So if the water stored in the mantle today is much greater than its storage capacity roughly three billion years ago, then it is entirely possible that the planet was once covered by water.
Michael Walter, director of the Geophysical Laboratory at the Carnegie Institution of Washington who wasn't involved in the study, said the paper makes intuitive sense. "It's a simple idea that could have important implications."
More geological evidence of a water-bound planet
These findings support those of a previous study by scientists from the University of Colorado Boulder. They studied an ancient slab of the ocean floor that was turned on its side.
Inside the slab were chemical signatures that offered clues about the seawater that covered the Earth roughly 3.2 billion years ago. They found that seawater then contained a stable, environmental isotope of oxygen, called oxygen-18. The abundance of this isotope suggests Earth had less land then than it does today.
To sum up, studies like these could help scientists answer important questions about the Earth, such as when life began on the planet. (Related: Meteorite in Michigan may hold clues to origin of life on Earth.)
Scientists are also still torn over whether life began on land or on water. So if the planet was, in fact, engulfed by water billions of years ago, that may settle the long-standing debate.
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