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Meteor Crater in Arizona: a recent, but not terribly large, impact scar. (National Map Seamless Server)

Crater Face

If we could see all the holes gouged in the Earth by asteroids, we'd run screaming for cover.

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It's a good thing our lives are so short. Stick around longer, a million years or so, and we'd learn just how ugly nature can be. A half-mile-wide asteroid strikes Earth on average every 500,000 years. Objects the size of an aircraft carrier hit ten times as often, and football-field-size rocks come every 10,000 years or so. An asteroid that size, moving at 20 miles a second, can punch out a crater more than a mile wide, slamming into Earth with 80 megatons of energy, more than the largest hydrogen bomb ever exploded.

A three-mile-wide object--still much smaller than the one that most likely killed the dinosaurs--delivers more energy at the moment of impact than all our planet's earthquakes, volcanoes, and tsunamis typically release over hundreds of years. Short of the sun going nova, an asteroid impact is the worst natural disaster that can befall us. Don't take comfort in the frequency estimates, either: They're only statistical averages. Any of these things could happen tomorrow.

Every age needs its conception of the apocalypse, whether it's The Flood, The Plague, or The Bomb. The one that has prevailed since the 1980s, when scientists first linked the demise of the dinosaurs to an impact, has been The Asteroid. The odd thing about this one is that it lies so far outside human experience. We have trouble accepting the reality of impact because it hasn't ever happened to people, or so we think. Yet the evidence of cataclysm is all around, if we take the trouble to look. So I head off to Kentucky, like Doubting Thomas, to touch the scars. 

KEITH MILAM IS ONE OF THOSE young, energetic scientists whose natural curiosity takes him in half a dozen directions at once. Even while getting his Ph.D. from the University of Tennessee's Planetary Geosciences Institute, he's been part of the large scientific team that planned the current NASA rovers' geologic exploration of Mars (see "Next Stop, Gusev Crater," Dec. 2003/Jan. 2004). Today, though, he's made the two-hour drive from Knoxville to show me around a terrestrial crater, the Middlesboro Impact Structure.

Middlesboro lies at one end of the Cumberland Gap, a narrow notch in the Appalachian mountains through which Daniel Boone led settlers from Tennessee into Kentucky at the close of the 18th century. Though they didn't know it at the time, when they descended from the gap, they were walking into the eroded remnant of a 3.4-mile-wide crater caused by a collision with a giant space rock some 300 million years ago.

Milam drives me to several sites at the perimeter of the basin, a couple of miles from ground zero, to see traces of the old crater rim, where the rocks are jumbled and wrenched out of their expected positions. One outcrop is behind a fast food joint, another next to a Baptist church. With his finger Milam traces a thin horizontal seam of coal that turns abruptly upward, testimony to a violent upheaval. "See how it jumps here?" he says. Another site off the highway is on private property. Once Milam was there tapping away with his rock hammer when the landowner, a preacher, came down to see what he was doing. The man listened patiently to the geologist's story, then offered his own Bible-based explanation for the weird rocks.

For many years, that was about as good an explanation as any for geological oddities like Middlesboro. The first systematic mapping of such circular, complex features wasn't made in the United States until the 1920s, when a German-born geologist named Walter Bucher began a survey of formations termed "cryptovolcanic"-the "crypto" implying that some volcano-like trauma had obviously occurred, but that no volcanoes were in sight. Middlesboro didn't catch Bucher's attention, but he noticed circles like Serpent Mound, Ohio, and Wells Creek, Tennessee, both within driving distance of there. Even though other scientists of the time began to suspect that craters like Ries in Germany and the Pretoria Salt Pan (now called Tswaing) in South Africa had been caused by meteorite hits, Bucher stuck with purely Earthbound interpretations, such as gas explosions from rising blobs of magma that had blistered the surface.

By the 1950s, however, the tide was turning toward extraterrestrial explanations. Astronomers had made the connection between lunar craters and meteorite impacts, and in 1963 Eugene Shoemaker of the U.S. Geological Survey finally settled a 50-year argument over the origin of Canyon Diablo in Arizona, a nearly mile-wide bowl carved 50,000 years ago out of the desert floor. Today we know it as Meteor Crater.

A key bit of evidence found at Canyon Diablo is a type of mineral called shocked quartz, which has since become the most accepted proof for identifying impact craters. Under a microscope, the rock grains are arrayed in a distinctive criss-cross pattern, the lattice structure of the quartz having been knocked off kilter by a sudden, intense blast of pressure. In only two places can you find rocks that have been so profoundly crunched. One is a meteor crater. The other is the bottom of a nuclear test pit.

Middlesboro's status as a confirmed, rather than suspected, crater comes largely from the discovery of shocked quartz there in the 1960s. Geologists also have found another telltale sign of impact-"shatter cones," caused when the shock wave from a sudden blast moves through rock at supersonic speeds. Milam shows me one. I'm not sure I would be able to tell its subtle striations from all the other ripples you see in rocks. He's always on the lookout for more, and even his seven-year-old, Zac, who's along on our field trip today, is on the hunt. Every so often he holds up a pebble. "Is this a shatter cone?" No, says Milam, smiling.

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