An Ocean Within: Modeling the Entire Water Cycle

April 10, 2007

No other planet is quite like Earth. For one thing, it’s covered in water. Alien astronomers spying us from light years away would likely call our planet their equivalent of “Ocean.”

Now, some scientists say, that’s not the half of it. They have come to suspect that most of Earth’s water is not out in plain sight, but actually deep below ground.

Robert Bodnar, Distinguished Professor of Geosciences at Virginia Tech and recently inducted fellow of the American Association for the Advancement of Science, is one of them. He and his colleagues are developing a new computer model that attempts to represent the entire water cycle.

Their model contains all the bits you might remember from your high school earth science textbook—oceans, ice caps, surface water, shallow groundwater, clouds, evaporation, transpiration and so on. This is the water that resides in what geologists call the “exosphere.”

Cartoon depiction of Bodnar's water cycle model. Grey slabs of ocean crust carry water down into the mantle. Click to Expand.

But Bodnar said, you can’t leave out the water in the “geosphere.” This is water hidden deep inside Earth, from the rocky crust through the mantle and perhaps even right down to the molten iron core. It doesn’t exist as a liquid ocean, but rather locked up as countless molecules of hydrogen and oxygen inside the crystals that make up rocks. That hasn’t stopped some people from calling it Earth’s interior ocean.

Bodnar, selected as the Jackson School of Geoscience’s Allday Endowed Lecturer, presented his ideas to the school community March 22 in a talk titled “The Earth’s Water Cycle: From the Clouds to the Core.”

Bodnar called his water cycle model a “steady state” model in that the water that goes in to each domain (say, the oceans) equals the water that goes out. Once the scientists are satisfied that the model does a good job of representing the real world water cycle, they plan to throw it out of balance.

“What if more water is melting from glaciers and ice sheets than is being added to those places through precipitation?” speculated Bodnar. Kicking the model in this way might yield new insights about how global warming could alter the water cycle.

Down to the Last Drop

It is thought that slabs of rocks and minerals subducted (pushed down) by the collision of tectonic plates carry water down into Earth’s mantle and perhaps even as deep as the core.

Robert Bodnar (left) receives a plaque from Richard Kyle honoring him as the Allday Endowed Lecturer.

In 1987, Joseph Smyth, a geologist from the University of Colorado at Boulder published a paper in the journal American Mineralogist suggesting that a type of mineral containing magnesium and silicon, residing in a 100 kilometer thick rind within Earth’s mantle, could hold an enormous amount of water.

Over the last 20 years, he and other researchers have discovered that other minerals in the mantle are capable of storing water. They have used laboratory and seismic studies to try to determine just how much water is down there.

The speed at which sound travels through the mantle suggests that the rocks in the area of the mantle known as the transition zone contain on the order of 0.5 percent water (by weight). These seismic studies agree more or less with Smyth’s conjecture.

Because of the size of Earth’s mantle and its remoteness, scientists may never know exactly how much deep water there is. But based on several lines of evidence, Smyth believes Earth’s mantle might hold as much as five times as much water as Earth’s oceans.

Flow chart showing the Exosphere and the Geosphere and how they are linked by subduction (which transfers water from the Exosphere to the Geosphere) and volcanism (which transfers water from the Geosphere to the Exosphere).

Bodnar said even if the minerals inside the mantle contain just 10 percent of their capacity for water (a conservative estimate), the mantle would hold three times as much water as there is above ground.

Making Earth’s Skin Crawl

Understanding how much water is below our feet, as well as how it got there and how it changes over time, might help scientists better understand other important processes that make our planet such a dynamic and vivacious place.

For example, Bodnar said plate tectonics might be impossible without deep water. Rocks containing water bend and melt at a lower temperature. And molten rock in Earth’s interior lubricates gliding plates, increases heat flow and allows convection.

Some scientists, including Peter Ward and Donald Brownlee at the University of Washington, have suggested that without plate tectonics, among other things, Earth would be unable to support complex life.

Volcanism and subduction (both driven by plate tectonics) help moderate Earth’s atmospheric temperatures through feedback loops that add and remove carbon dioxide from the atmosphere. Without them, Earth might resemble a runaway greenhouse like Venus or a chilly wasteland like Mars.

Earth and Moon as photographed by the Mariner 10 spacecraft in 1973. Striking features of the Earth visible to the passing spacecraft include blue oceans and white clouds, showing Earth to be truly a water world.

Convection (you guessed it, also related to plate tectonics) draws heat from Earth’s interior to the surface. This convection might be necessary to keep the molten iron core spinning and to create Earth’s magnetosphere, a global magnetic field that protects us from harmful particles from the sun and cosmic rays.

Take away water and plate tectonics, said Bodnar, and Earth would be unrecognizable. On a screen behind him, an enchanting image of Earth and the moon from the Mariner 10 space probe was projected.

“Earth is blue and alive looking, the moon looks grey and dead,” said Bodnar. “The difference is water.”

by Marc Airhart

For more information about the Jackson School contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.