Diamonds from outer space - diamonds found in meteor craters; includes notes on how diamonds are formed in the earth

Science World, Oct 4, 1996 by Chana Freiman Stiefel

What do you get when a massive comet collides with Earth? Billions of diamonds! Scientists really "dig" this dazzling phenomenon.

Science isn't always glitter and glitz. But in a while, scientists discover something truly spectacular--like diamonds! Recently, geologists (earth scientists) discovered billions of diamonds where no one before. had thought to search. They found the diamonds in craters, basins pounded into Earth's surface when speeding rocks from outer space smash into our planet.

Traditionally, mining companies dig up diamonds from deep below Earth's surface. But the newly discovered diamonds are a sparkling surprise. They lie encased in rock right on the surface, says Virgil. Sharpton, a geophysicist at the Lunar and Planetary Institute in Houston. Scientists have dubbed their discovery "impact diamonds" because they form during impacts of comets, asteroids, or meteors.

Russian scientists discovered. the first impact diamonds about 20 years ago, says Richard A.F. Grieve, al research scientist at the Geological Survey of Canada. A group of prospectors had been analyzing rocks near a crater in Siberia. While viewing the rocks under a microscope, they realized the rocks contained diamonds. The Russians kept their treasure secret for two decades.

But now the word is out, thanks to greater communication with Russian scientists since the end of the Cold War. And over the past decade, geologists have uncovered impact diamonds in more than half a dozen giant craters around the globe.

Despite the huge size of these treasure troves, scientists don't expect to strike it rich any time soon. "Most of the impact diamonds are smaller than the head of a pin," Sharpton says. Some are microscopic--tinier than viruses! As one scientist jokes, "They'd make good engagement rings for an ant, but Liz Taylor wouldn't be very impressed."

Still, scientists are excited because the impact diamonds tell a tale about Earth's history.

COSMIC COLLISIONS

Believe it or not, our planet is constantly being bombarded with rocky objects from outer space, Sharpton explains. "Most of the objects that come close to Earth are so small they either burn up in our atmosphere or rain down as small meteorites.

"It's very rare," Sharpton continues, "that you find an object large enough to make it through Earth's atmosphere and strike the Earth with such velocity that it produces a crater." But the 150 impact craters scientists have mapped around the globe over the past 50 years are evidence that such collisions do occur. (The most recent major impact was probably some 50,000 years ago.)

Though scientists discover only three to five craters each year, hundreds more may dot the planet. These include many craters that wind and water have eroded, or worn away, over time.

That's where impact diamonds come in. Finding them is like finding "'fingerprints' of an impact event," Sharpton says. It's as if the diamonds hold up a sign that says, "A cosmic collision happened here."

UNDER PRESSURE

The diamonds are also signposts of the enormous heat and pressure that result when a large space rock crashes, says Grieve. Intense heat and pressure, after all, are essential to diamond formation. So is the element carbon, diamond's main ingredient.

These materials and conditions certainly exist where diamonds usually form, deep within our planet (see diagram, opposite page). Some 160 kilometers (100 miles) beneath the crust, in the mantle. Earth's thickest layer, temperatures soar as high as 1,000[degrees]C (1,832[degrees]F). That's hot enough to melt rock and form magma, the semi-melted ooze that churns beneath Earth's surface. In addition, the enormous weight of rocks pressing down creates pressure in the mantle that's tens of thousands of times Earth's atmospheric pressure at sea level.

The intense heat and pressure "cook" and squeeze atoms of carbon in certain rocks and minerals (like limestone and graphite) within the magma. The heat and pressure actually rearrange the carbon atoms so that they form repeating patterns with flat sides, or faces--crystals. Voila, a diamond is born! Explosions of gas within Earth's mantle sometimes force these diamonds and surrounding rocks closer to the surface.

FAST TRACK

Diamonds can take millions of years to form underground, Sharpton says. But a cosmic collision can create diamonds in just a fraction of a second.

The pressure created by a comet colliding is more than a million times the atmospheric pressure at sea level. The impact forces a huge cloud of dust, rock, and vapor into the air. "You can compare this cloud to a mushroom cloud that forms after a nuclear explosion," Grieve says. (Scientists suspect that the huge amount of debris from one such collision 65 million years ago may have cast a haze around the globe and wiped out the dinosaurs.)

Based on computer models, scientists think that temperatures in the cloud may reach 4,000[degrees]C (7,232[degrees]F. As the cloud cools, carbon blasted from either graphite or limestone on the ground can crystallize to form diamonds.