By Sid Perkins
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.
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.