Unveiling hidden craters

Laser radar technique discerns evidence of a relatively young impact

Using laser-radar equipment during aerial surveys of forested land can reveal small craters that are otherwise not easy to detect, a new study shows.

VOILA! | Typical laser-altimeter data of the area surrounding the Whitecourt crater resembles a normal aerial photograph of a forest. Click the link to see how the region would look without vegetation. Herd et al.
AFTER THE FALL | During summer months, vegetation completely hides the 36-meter–wide Whitecourt crater. Viewing the animation will show the crater being revealed thru the change in vegetation from summer to fall. Herd et al.

Extraterrestrial objects collide with Earth on a regular basis. At the rate at which objects are known to strike Earth, meteorites should have blasted about 20 craters measuring less than 100 meters across in the last 10,000 years or so, says Duane Froese, an earth scientist at the University of Alberta in Edmonton, Canada.

However, only five of the known craters formed during that time fall into that size category — a testament in large part to the weathering processes that can quickly resculpt Earth’s surface. Worldwide, scientists have identified only 175 or so scars from extraterrestrial impacts, Froese says. Many of those are large: About 65 million years ago, a 10-kilometer-wide asteroid slammed into what is now the Yucatán Peninsula, triggering a mass extinction that wiped out the dinosaurs. As recently as September 2007, a meteorite blasted a 13.5-meter–wide crater in eastern Peru.

Now, in the December Geology, Froese and his colleagues describe a small, newly recognized crater long hidden in the forests of west-central Alberta.

About 20 years ago, hunters discovered a bowl-shaped depression in the forest about 17 kilometers south-southeast of Whitecourt, Alberta. Then, about two years ago, one of the hunters returned to the site and discovered meteorite fragments nearby. Froese and his colleagues visited the site and confirmed it was an impact crater, but they also noted the scar was well hidden; it didn’t even show up on images of the thickly vegetated area taken from low-altitude aircraft. “You can’t even spot this on Google Earth,” says Froese.

So, he and his colleagues checked to see if high-tech laser altimeters — equipment that, like radar, uses electromagnetic radiation to measure distance — could discern the crater. By ignoring the laser reflections that denote the uppermost leaves and branches of trees and other vegetation, scientists can, in essence, create a three-dimensional map of what a region’s bare terrain would look like, Froese explains. When he and his team reanalyzed laser altimeter data previously gathered by a logging company, the crater stood out.

Today, the crater is about 36 meters across and about 6 meters deep. In all, the blast from the meteorite impact excavated about 2,900 cubic meters of soil, flinging it dozens of meters in all directions. In some places, the ejecta blanket is 20 centimeters or more thick, says Froese. Carbon-dated samples of undisturbed, charcoal-rich soil under this debris layer suggest that the impact occurred in the past 1,000 years.

Impact craters, especially small ones that weren’t created recently, often don’t stand out very well, says Philip Bland, a planetary scientist at Imperial College London. “We lose them pretty quickly” due to erosion or the masking growth of vegetation, he notes. “It’s really nice to see a technique that allows us to see such craters,” he adds.

Discovery of the recently formed Canadian crater is “a reminder that impacts are still shaping Earth’s surface,” says John Spray, a planetary scientist at the University of New Brunswick in Fredericton. “We do get hit regularly,” he notes.

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