Inside EARTH

Would you like to crack Earth open like an egg to see what's inside? All you'd need is a really big bomb and about 100,000 tons of molten iron.

According to scientist David Stevenson, the first step to peering inside Earth is to set off a nuclear bomb. The explosion should create a giant crack in Earth's crust, the rocky outer layer that covers our round planet like the skin on a peach. As soon as the crack is formed, fill it with the hot liquid iron. Then, drop a grapefruit-sized probe into the iron. Because the iron is so heavy, it should keep pushing the crack downward, and the probe should travel with it, sending data back to scientists above ground.

Stevenson's idea sounds pretty crazy. But he says that it's not impossible. And, anyway, the point of writing about it was mostly to get people asking questions. We send spaceships to the moon and beyond. Submersibles snap pictures of the darkest, deepest ocean. But we know surprisingly little about what's inside Earth.

Sure, scientists have drilled a few miles down, but that's just a tiny fraction of the nearly 4,000 miles between where you stand on the surface and Earth's center. The temperature inside Earth is so hot and the pressure is so great that no one yet has managed to dig very far.

All sorts of fascinating things might be down there, from materials that could fuel the cars of the future to microscopic beings that offer clues about life in outer space. Luckily, scientists do have methods for figuring out what's inside Earth, without actually cracking it open.

Wave Readers

Scientists have learned most of what they know about our planet's insides from earthquakes. An earthquake can happen when the pieces that make up Earth's crust shift suddenly, causing sound waves to travel outward. We feel some of these waves as vibrations that shake the ground, while others travel deep into Earth.

By measuring how fast these waves move and the paths they take, scientists have figured out that the waves are traveling through different layers, changing speed and direction as they do. Researchers even have a good idea what the layers are made of. Thanks largely to seismic waves, scientists think that Earth's core is mostly iron. But, materials we know well from Earth's surface don't look or act the same when subjected to inner Earth's high temperatures and pressures.

A Tight Squeeze

Think of the gray point on a pencil. That's graphite. Now picture the clear, sparkly stone in a wedding ring. That's diamond. Graphite is silver gray, diamond is usually clear. Graphite is soft, diamond is the hardest substance on Earth. Would you be surprised to learn that they're both made entirely of the same thing-carbon atoms? And that a diamond is really just graphite that's been heated and squeezed deep inside Earth?

In her laboratory at Stanford University, Wendy Mao uses a cool tool called a diamond anvil cell to put the squeeze on samples of iron and other materials to see how they react. The tool is made of two diamonds like those you'd see in a fancy ring, wider at the top and tapering to a tip. A tiny sample-the biggest is only about as wide as a human hair-is placed between the tips of the two diamonds. Then the tool squeezes and squeezes the diamonds until the pressure on the sample is about what it would face in Earth's core.

"Because diamond is transparent it makes a very good window," says Mao. "We can use lasers or x-rays to see what's going on with the sample. And we use infrared lasers to heat up a sample to very high temperatures."

Very high indeed. Lasers can heat a sample to 5,000 degrees Kelvin, which is just a little cooler than the surface of the sun.

Mao's work is helping scientists confirm what they've hypothesized about the inside of Earth from looking at seismic waves. But that's not all What Mao has learned has exciting possibilities for life above ground as well. She has discovered that under certain extreme conditions of pressure and temperature, water forms ice cages that trap hydrogen inside. "It's an interesting way of storing hydrogen that could maybe be used as alternative energy," Mao says.

How? You may have heard that the oil, gas, and coal we depend on to power cars and factories are in danger of running out. Scientists have long been interested in hydrogen as an alternative to fossil fuels, but they haven't figured out a good way to store it. If they can recreate the ice cages Mao discovered, that may solve the problem and pave the way for cars to run on hydrogen power.

Mining for Microbes

Scientist Tullis Onstott works in gold mines, but he isn't looking for precious metals. Instead, he mines for tiny organisms so unusual they may be more like Martians than anything living on Earth.

The mines in South Africa where Onstott looks for microorganisms are a mess of tunnels some three miles beneath Earth's surface. That's about as deep as a person can go, and just getting to work is quite a journey.

First, Onstott and his coworkers climb into a cage that sinks 180 stories in four minutes.