| The Big Squeeze | |
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An old Tom Swift book I read as a kid began with a memorable scene. Our hero launched a huge self-propelled boring machine straight down into the Earth. He and his white-coated staff sat down to wait while it drilled down a few thousand miles—I forget how long it took, a paragraph or two anyway. Presently a gigantic fountain of white-hot molten iron shot up from the Earth's core high in the air, delivering an endless supply of metal for American civilization. "Gee willikers!" Tom erupted, or something like that.
What a dream! And alas, totally absurd. Only a few dozen kilometers or so down, the material of the Earth is glowing a deep red and quickly fills in any opening. Rock at that depth is still a solid, but it flows plastically like glacier ice, and Tom Swift could never drill there to go wildcatting for iron gushers, let alone do any proper mantle research.
But geoscientists today are doing marvelous things with high-pressure, high-temperature lab apparatus. Their advances have been impressive. Presses that were state-of-the-science 20 years ago are cheap enough today to use in manufacturing industrial diamonds. There are even a few being used in high schools, as I mentioned in Part 2. Reproducing the conditions of the upper mantle is practically child's play now.
The state of the science today is good enough to subject samples to any pressure and (for brief periods) any temperature found in the mantle—that's up to about 135 gigapascals (20 million pounds per square inch) and several thousand degrees Celsius. And we can watch and analyze those samples while they're being squeezed and heated, thanks to our friend from Part 2, the diamond.
Diamonds are the hardest, most rigid substance we can use. The diamond anvil cell consists of two gem-grade diamonds with their faces pressing against each other. A little metal ring gasket sits between them so as to create a tiny chamber that holds a sample measured in milligrams. An apparatus capable of lower-mantle pressures is small enough to hold in your hand.
This system has another great advantage besides its size—diamond is transparent. We can observe the sample directly, we can heat up the sample by shining lasers on it, and we can aim X-rays through the diamonds to study the physical changes in the sample.
Today we can reach pressures almost as great as those at the Earth's center (about 360 GPa), but the process is difficult. The diamonds themselves deform at these pressures, along with the metal parts around them. Also, because diamond is a superb conductor of heat, it's hard to keep a sample very hot. A 1984 lab accident involving a misdirected laser showed, though, that if we want to, we can actually melt diamonds. But that's undesirable, not just because it can wreck the equipment but also because diamonds still cost a lot of money. Enough of the anvils explode into dust as it is, so experimenters have to leave the room during pressure trials.
More and more research labs are getting diamond-cell presses, but geologists have to stand in line. Solid-state physicists and other specialists are putting frozen gases, water, solutions, industrial materials and exotic mixtures into the diamond cell to learn more about the basic physics of matter.
One way to study even higher pressures is to find something stronger than diamond (there are some candidates). Another is through shock-wave experiments. This is hard-core science indeed—basically, you smash things together with high explosives (very carefully!) and study them in the minuscule instant before they're totally destroyed. Some remarkable experiments of this type, still largely classified, were done during nuclear bomb tests.
These studies get us well beyond earthly pressures and into the range of conditions found deep inside stars. They are useful for deep-Earth science, but the diamond anvil cell is a laboratory hero because it can hold samples for long periods and release them unharmed.
We're learning a great deal about the mantle from this research—but discovering the mantle's composition, behavior, and history may be the single most complex scientific problem ever undertaken. It will take millions more high-pressure experiments, cross-fertilized with the chemical and seismological and computational studies described earlier in this series, to bring clarity to the depths of Earth. Still, I can lay out some of the preliminary answers in Part 6.
In the early 1980s I saw a talk on a study where the researchers put iron metal in the diamond cell next to a few grains of silicate minerals. The news was that at deep-mantle conditions, the two substances reacted with each other. The speaker pointed to an enlargement on the screen and said, "now if we take the iron as a model of the core and the silicate as the mantle . . ." and the audience of scientists chuckled, because the whole sample was maybe a millimeter across. But that tiny experiment was absolutely revolutionary, as I hope to explain in Part 6.
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