If we could see all the holes gouged in the Earth by asteroids, we'd run screaming for cover.
- By Tony Reichhardt
- Air & Space magazine, May 2004
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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.
He's still puzzling out some of the complex evidence himself, trying to reconstruct the details of the impact, which partly depend on what the impactor was made of (there's a big difference between the wallop of a loose iceball and that of a chunk of rock or iron), and how big it might have been (100 to 500 yards across, he guesses). He's starting to think the crater might not be as neatly circular as it appears, that it may be more elongated.
The Middlesboro impact was big enough to form what's called a complex crater. The initial excavation was followed by a rebound of material from the center, creating a central uplift feature, the way a pebble dropped in water splashes up a jet of water. Fractured material then slumped back into the crater, and the outer rim became terraced, unlike the neat bowl you find at the smaller (and fresher) Meteor Crater.
In this case the central uplift feature-ground zero-is on the grounds of the Middlesboro Golf Course, which dates back to 1889 and bills itself as the oldest golf club in the country. On this unseasonably warm Tuesday afternoon in November, the course is mostly empty. I watch a twosome approach the second green, right behind me. A guy who looks like he just stepped from the Land's End catalog whacks stiffly at a golf ball, which rolls about 10 feet. His next shot dribbles onto the green, and two undistinguished putts later, he picks up his ball, fuming. He looks to be having a bad day.
Of course, bad days are relative. Right next to the first tee (a 273-yard par 4) is an old, weathered block of sandstone that shouldn't be there. By rights this particular rock should be buried 1,300 feet below, with the rest of the Lee Sandstone formation. The asteroid that hit some 300 million years ago yanked it up in a horrifying instant, the central peak uplifted from underneath the now empty bowl.
Out at the perimeter of the original crater, Milam shows me rock beds turned completely upside down, a disorder that can't easily be explained by trivial events like earthquakes. "When I bring professional geologists here who aren't familiar with impacts, they just scratch their heads," he says. In the normal course of geological research, they would never encounter forces like the ones on display here.
Driving north from Middlesboro on Highway 25 late that afternoon, I see that the exposed rock on either side of the road returns to its normal pattern. Stacks of beds, the once-muddy floors of ancient seas, are as flat and regular as the layers of a cake. Order is restored. Leaving this place of past violence, I see a hand-drawn sign off the highway that says "Prepare to Meet God."
My next stops are Versailles (locally pronounced "ver-SALES") and Jeptha Knob, two other suspected impact structures in Kentucky. They lack shocked quartz or other proof and so are unconfirmed as craters but very likely are anyway.
There's nothing particularly asteroid-attracting about Kentucky. I could easily have chosen neighboring Tennessee or Missouri, each of which has two confirmed craters, or stayed home in Virginia and visited the Chesapeake Bay, which overlies a 35-million-year-old crater. At least 168 impact scars have been identified on Earth, with new ones added to the list each year, and hundreds more suspected. The confirmed ones range from the 200-mile-wide Vredefort crater in South Africa to a piddly little car-size dent in a field near Haviland, Kansas. We only know about that one because it happened so recently, about 1,000 years ago. Most craters smaller than 12 miles in diameter are long gone, eroded flat over geologic time, covered over with sediments, or subducted back into Earth's mantle.
The official list of confirmed craters, called the Earth Impact Database, is kept by geologists at the University of New Brunswick in Canada. Why them? In part because Canada, having large areas of old, exposed crust, has lots of old craters. And in part because Canadian scientists have taken an interest in the subject. Crater hunting is like that, says Richard Grieve, a geoscientist and impact expert with Canada's Natural Resources Department, which used to maintain the database. A dozen craters turned up recently in Scandinavia and Finland, he says, "quite simply because there's been a group of people who've made it their business to go out and find them."
Mark F. Thompson, a geologist and geospatial data analyst with the Kentucky Geological Survey, would like very much to add a new crater to The List. Actually, two craters: Versailles and Jeptha Knob. Today Thompson and I are driving in a soft rain through the rolling bluegrass country outside Lexington, past thoroughbred horse farms and bourbon distilleries, not far from the small town where Colonel Sanders opened his first chicken joint. I feel like I'm driving over a Howard Johnsons placemat showing the landmarks of Kentucky.
I never would have recognized the mile-wide Versailles structure had Thompson not pointed it out. For one thing, there's no obvious depression. The circular outline of the crater is marked only intermittently by small, deep sinkholes in the limestone terrain. The impact, if one occurred, happened as long as 440 million years ago. Versailles was discovered in 1962 and was originally thought to be a giant sinkhole itself. But later geologic mapping showed faults around the perimeter similar to those seen around impact craters. And the surrounding fields yielded a surprising number of breccias, lumpy conglomerate rocks made of sharp rock fragments fused together. Geologists often find breccias near volcanoes, but there aren't any volcanoes around here.
By the roadside, across a barbed-wire fence, Thompson spies what he thinks might be a breccia. "I wouldn't mind having a piece of that for myself," he says, then decides against it. We haven't gotten permission from the landowner, and he can get a piece when he comes back. He'll need to do more extensive fieldwork if he wants to prove Versailles a crater.
Kentucky's other candidate, Jeptha Knob, is 24 miles away. A three-mile-wide circular formation with a clump of hills sticking up from its center, it was one of the original "cryptovolcanic" structures that puzzled Walter Bucher back in the 1920s. Geologic maps reveal a neatly circular pattern of faults at the perimeter, where Thompson shows me contorted rock beds, evidence of the impact that occurred some 440 million years ago, back when Kentucky was under a warm, Caribbean-like sea. This was long before the dinosaurs, so there were no large animals to look up at the screaming fireball. Just a bunch of shelled creatures whose fossilized corpses I can easily pick out of the crumbly Ordovician-era breccia with my finger.
We head toward the center of the crater, which is on land owned by Cal Schmidt, a genial, soft-voiced man who looks to be in his early 70s. Schmidt greets us at his home on the edge of a small private lake. He's obviously proud of owning the central uplift feature of a suspected impact crater. When I tell him I spent the day before at Middlesboro, he quickly turns to Thompson and asks, "Is Middlesboro bigger than me?" A joke, but with a hint of concern.
Over lunch, he gives us the recent-at least in geologic terms-history of Jeptha Knob, how his dad bought it for $26 an acre in 1926, how the bandit Frank James once spent a winter hiding there, how the name comes from an Old Testament warrior. Later, the three of us walk up onto the wooded hill behind the house, where Thompson is on the lookout for breccia deposits that another geologist mapped here many years ago. Shocked quartz is unlikely to turn up at this particular site because it was carbonate rocks, not quartz sandstone, that got smashed in the long-ago impact. Bad luck. But Thompson is studying a core sample, a small amount of material only two inches in diameter but drawn from as deep as 2,000 feet, to find out more about the geology below the surface. He plans to compare the characteristics of the sample with those of core samples from confirmed impact structures. In doing so, he may discover evidence that will place Jeptha Knob on The List, right there between Jänisjärvi in Russia and Kaalijärv in Estonia.
Thompson has had to dig a little deeper than geologist Kevin Evans of Southwest Missouri State University in Springfield. Evans recently discovered a 12-mile-wide crater, the Weaubleau-Osceola structure in Missouri, that, if confirmed, would be one of the biggest impact craters in the United States. It's not on The List yet, but that's just a matter of publishing the scientific paper, thinks Evans. He's already found shocked quartz, breccias, the whole bit.
Not to mention a huge bull's eye that popped up on his computer one day.
Weaubleau had been suspected as an impact site because of the tortured rocks in the region, but no crater was obvious. Evans was using a computer graphics program to stitch together four U.S Geological Survey digital images of the area when a thumbnail composite image came up on his screen. "Boom, there was a big circle," he recalls. Nobody had noticed it before because the ring appears broken and happens to straddle the boundaries of the four USGS quadrangles.
Since then Evans has used what promises to become another powerful tool for crater hunting, digital images from the Shuttle Radar Topography Mission, a NASA-Pentagon collaboration that four years ago mapped elevations over the entire globe to an accuracy of about 30 yards, using a radar imager mounted on the space shuttle. When Evans looked at the mission's pictures of Weaubleau-Osceola, another ring only four and a half miles wide appeared inside the circular drainage basin he'd identified from the USGS composite. That, he believes, is the true ground zero.
Remote sensing tools like these might speed up the search for previously unknown craters, particularly ones that are buried or whose surface "expression" is subtle. Other methods have already been applied to that task, including gravity surveys that sometimes show the comparatively loose rubble excavated from a crater as a gravity "low" against a background of solid rock. Using gravity and magnetic maps, Pradeep Talwani of the University of South Carolina last year reported a suspected crater 500 yards under Johnsonville, South Carolina, not far from Interstate 95. If confirmed, it would be the first crater found along the southeast Atlantic coast. There may be other coastal impact scars, similarly buried by sediments.
Of course, any meteorite that slams into Earth has a 75 percent chance of hitting water. Since the ocean floor is fairly young, geologically speaking, and is continually recycled in deep ocean trenches, carrying the evidence of impact with it, scientists will never know about a lot of the hits Earth has taken. Richard Grieve, the pioneering Canadian impact expert, would love to see the detailed maps of the sea floor that the U.S. and Soviet navies made during the cold war, which undoubtedly show lots of intriguing holes. Despite talk in the 1990s of releasing the maps, he's not holding his breath. There's still "a fair level of paranoia" about declassifying the information, he says.
At least two suspected craters have turned up on the ocean floor. Two years ago, British oil geologists looking at seismic reflection data reported finding a 12-mile-wide multi-ringed impact structure in the North Sea. Much earlier-in the 1960s-a Navy oceanographic research vessel called the Eltanin found unusually high levels of the element iridium-which is abundant in meteorites-three miles beneath the Bellingshausen Sea, off Antarctica. Later expeditions to the site found meteorite fragments, and scientists now believe the Eltanin site was hit by an object up to a half-mile in diameter a mere 2.1 million years ago.
Dallas Abbott, a geoscientist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, claimed a couple of years ago to have found a whopping 80-mile-wide crater at the Eltanin site. Last year, though, a group of researchers in Germany and the United States disputed her claim in an unusually blunt paper delivered at a meteorite conference in Houston. They argued, based on their extensive research in the area, that Abbott "cannot possibly have found the Eltanin impact site."
Undaunted, Abbott made a truly astonishing claim at a geological meeting in Seattle last November. It wasn't so much the size (12 miles) of the crater-like ring she reported seeing in crude satellite-derived maps of the continental shelf off the coast of New Zealand. It was the age-only 500 years. Abbott offered a tantalizing grab bag of supporting evidence. She reported finding tektites in dredges taken near the crater-small beads of glass formed from cooled droplets of liquefied rock, which are also associated with impacts. She noted that other researchers have found tsunami deposits in Australia and New Zealand that some say are too high to have been dumped there by mere earthquake-generated waves. She pointed to curious Maori and aboriginal legends of a mysterious fire in the sky, images that, if based on an actual event, represent the only known account of a large impact.
So far, though, the critics remain unconvinced. An asteroid hit of that magnitude would have regional and perhaps global effects (a rough rule of thumb is that impactors are 1/20th the size of the crater, so this one would have been more than a half-mile wide). There's no clear evidence of such a devastating event circa A.D. 1500.
Other claims of young craters have been less controversial, but the scars are much smaller. The Wabar craters in Saudi Arabia, some of which are buried beneath the sand, were recently dated to 290 years ago, give or take a few decades. The largest is only about 380 feet in diameter. Two Arabic poems, which can be interpreted as referring to a meteor sighting from neighboring Yemen, may even give the exact date: September 1, 1704. And material collected from the proposed (but not yet added to The List) Sirente craters in central Italy dates from about A.D. 412. If those craters turn out to have been made by meteorites, the residents of late imperial Rome would have seen explosions more powerful than most humans have ever witnessed.
These signs of recent catastrophe-in fact, the entire catalog of 168 craters-do not help in the least in determining when Earth will be struck again. Planetologist Clark Chapman of the Southwest Research Institute in Boulder, Colorado, has estimated the frequency of large and small impacts based on other kinds of data, including ongoing telescope searches for "Earth-crossing" asteroids and military cameras that track fireballs in the sky and let scientists count smaller objects that burn up and never hit the ground. "Earth's geological record was important a couple of decades ago when we didn't have much data from a telescopic survey," says Chapman. "But now, instead of a handful of objects [that orbit in Earth's neighborhood], we know of literally thousands of them."
So even though finding new craters is fun, at present it's more of a scientific trophy hunt than a significant research program. Still, the pace of discovery is picking up as new tools, and new searchers, enter the game. Nearly a third of the craters known today were discovered in the last 15 years.
Every few months, a new claim crops up. Seneca County, Ohio. Nagano Prefecture, Japan. Grieve often has would-be crater discoverers bring him samples of rock they're certain contain shocked quartz. Identifying the deformation features can be tricky, but Grieve has seen them so many times it takes him only a few minutes with a microscope to give the visitor a yay or nay. He recalls the time a Chinese scientist brought him a sample. Grieve put it under the microscope. Nope. He handed it back to the researcher, who looked down at the rock, then back up at Grieve. "Chinese craters different," he said.
Kevin Evans, who expects Weaubleau-Osceola to be added to The List someday, doesn't claim to be an impact expert. He says, "I'm just kind of an interloper," one who normally works on plain old sediment geology. Conducting research in Antarctica had been one of the thrills of Evans' career, but of the Weaubleau-Osceola discovery he says, "This is the most exciting project I've ever been on."
The tiny town of Vista, Missouri, population "about 80," is excited about nearby Weaubleau-Osceola too, and is thinking of making it a tourist attraction. Evans enjoys talking to schoolkids about the site; he likes opening their eyes to the reality of asteroid impacts by pointing to the giant hole in their own backyard.
The signs are all around if we take the trouble to look.