Inner Earth May Hold More Water Than the Seas

By Ben Harder
for National Geographic News
March 7, 2002
Molten rocks deep in the earths interior may be surprisingly wet, Japanese researchers say. From lab experiments, they have concluded there may be more H2O deep underground than in all oceans, lakes, and rivers combined.

The scientists first heated "mineral cocktails" to a white-hot 1600 degrees Celsius (2900 degrees Fahrenheit) and squeezed them until the pressure reached more than three million pounds per square inch (200,000 kilograms per square centimeter). Then they cooked the samples for an hour.

The experiments replicated the environment and conditions deep in the Earth.

Based on what they witnessed in their lab, the researchers concluded that more water probably exists deep within the Earth than is present on Earth's surface—as much as five times more.

"Our results suggest that the lower mantle can potentially store considerable amounts of water," said Motohiko Murakami of the Tokyo Institute of Technology, where the experiments were conducted.

"The presence of water in the crystal structure of [deep-Earth] minerals would be expected to soften the minerals and change their flow behavior," he added. That, in turn, could affect how the innards of the planet mix and shift over time, and could indirectly affect conditions and forces near the surface, such as plate tectonics.

Wet Rocks

Far beneath the seas, in the lower mantle, rocks exist at temperatures and pressures similar to those recreated in the Tokyo lab. The research team wanted to determine how much water might be in that region of Earth's interior, which they did by studying the nature of the chemical reactions in their tabletop mini-mantle.

The results indicated that the lower mantle has a lot of water, they reported March 8 in the journal Science.

Murakami and his colleagues reached their conclusion based on how much water they managed to dissolve under the experiment's extreme conditions in several types of material that make up much of the lower mantle.

They used heat and pressure—25.5 gigapascals of it, or more than 250,000 times natural atmospheric pressure at sea level—to create four mineral compounds that exist in the lower mantle.

These minerals—manganese perovskite, calcium perovskite, magnesiowustite, and stishovite—were produced as a result of the reactions among the chemical ingredients the researchers had placed inside a pressurizing multi-anvil apparatus. When water was added, some of it was absorbed into the newly formed minerals.

In different trials, the scientists slightly varied the proportions of the ingredients they used, and added or removed trace quantities of certain metallic compounds that they suspect mix in small amounts with more abundant materials in the lower mantle.

The trials resulted in different amounts of water being absorbed into the mantle-like matter. But in each case, water made up at least 0.19 percent of the material's mass.

That doesn't sound like a lot, but for Murakami and his team, the finding was a watershed.

A Waveless Waterworld

Earth's oceans make up just 0.02 percent of the planet's total mass. This means the vast lower mantle could contain many times more water than floats on the planet's surface.

The Japanese experiments don't guarantee that that's the case, of course, because the researchers haven't actually measured the mantle. No one is ever likely to get a direct sample of material from the fiery mantle itself. But by simulating mantle-like conditions in the lab, Murakami and his colleagues have demonstrated that a water-rich inner Earth is plausible.

Just how water-rich it is depends on the amount of trace "impurities" in the minerals. Compounds such as aluminum and iron "could dramatically change the solubility of water in these minerals," Murakami explained.

Other research has suggested that a zone between the mantle and the crust also contain a great deal of water, the Japanese researchers noted. If so, there could be more than ten times the amount of water inside the planet as there is on its surface.

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