Lasers Uncover Craters

1 December 2008 (All day)

Herd et al., Geology

Unmasked. Aircraft LIDAR sweeps found this previously hidden impact crater in central Alberta, Canada.

Researchers have uncovered a pond-sized crater in the woods of central Alberta, Canada, carved out by a meteor that slammed into Earth about 1100 years ago. The technique they used to pinpoint the pit--a laser take on radar--figures to help scientists find evidence of hundreds of similar impacts that have remained hidden until now.

Every 10 years or so, a sizable chunk of asteroid or comet crashes to Earth, leaving a crater about 40 meters wide. The remnants of these objects represent treasure troves for chemists and geologists. With those data, they have been piecing together the history and original composition of the solar system, for example.

Hunting for meteorites isn't easy. Most craters have eroded away, and those that do survive are often concealed by forests or lakes. As a result, researchers have only been able to locate five craters from the past 12,000 years. Presumably hundreds more pockmark Earth's surface, but where are they?

Looking for a better way to dig up these subtle depressions, a team from the University of Alberta in Edmonton went airborne with a device called LIDAR. The technology works by shooting laser pulses at the ground; as these pulses bounce back, they reveal the precise distance from plane to Earth, creating a topographical map of the planet's bare surface. LIDAR differs from radar because it can see through vegetation as it charts surface elevations in great detail.

Near the town of Whitecourt, the team hit pay dirt. In a densely wooded area, the instrument revealed a crater 36 meters wide with sloping walls. Further studies at the site uncovered 74 fragments of the original iron meteorite, the researchers report in this month's issue of Geology. Their analysis of the crater and the scattered meteorite fragments concludes that the original object was about 1 meter in diameter and hit the surface at between 11 and 17 kilometers per second. (A similarly sized object struck central Canada about 2 weeks ago, but researchers have not yet found the crater.)

That's up to 9 kilometers per second slower than the average for bigger objects that have hit Earth over its history, says space scientist and asteroid specialist William Bottke of the Southwest Research Institute in Boulder, Colorado. "That may tell us something about the orbit for this tiny projectile," he says. It's possible the object was traveling in an orbit that required it to journey from the innermost part of the main asteroid belt between Mars and Jupiter. If that's an example of the kind of data available from probing the asteroid fragments in these heretofore hidden small craters, he adds, then they're well worth the search. "LIDAR would appear to be a great tool for finding them," he says.

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