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
National Map Seamless Server
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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.