Deep Impact in the Desert

Comparing craters, new and old

Millions of years ago, a meteorite three-fourths of a mile (1.2 km) wide slammed into what's now the Sahara Desert. This week, scientists announced they've found the crater it left behind--a 19-mile-wide (31-km-wide) depression in Egypt.

The crater is by far the largest Saharan impact crater ever found--and the researchers didn't even have to brave the desert heat to find it. They found it by searching satellite images. "The search for craters typically concentrates on small features," one researcher noted, but "the advantage of a view from space is that it allows us to see regional patterns and the big picture." Plus, you get less sand in your shoes.

There's Large, and Then There's Large

The researchers named their new crater Kebira, Arabic for "large." Yet as large as Kebira is, it's nowhere near the size of other famous impact craters. The Chicxulub crater along Mexico's Yucatan Peninsula is more than 100 miles wide. A careening comet or asteroid smashed it into Earth's crust some 65 million years ago, triggering environmental changes that many scientists say wiped out the dinosaurs.

The Sudbury crater in Ontario, Canada, is more than 150 miles wide and marks the spot of a deep impact 1.85 billion years ago. And Earth's oldest and largest known impact crater, South Africa's Vredefort crater, spans more than 180 miles and dates back more than 2 billion years. The impact packed more punch than the world's entire nuclear arsenal. But back then, there weren't any animals to go extinct.

And Then There's Extraterrestrial

Earth isn't the only celestial body to take a pounding. In fact, impact craters are much easier to spot on the surfaces of the Moon, Mars, and Mercury than they are here at home. On Earth, erosion, volcanic resurfacing, and tectonic shifting continually erase old craters. Elsewhere in the solar system, these geologic processes have ceased, or never started.

The largest impact crater on Mercury, called Caloris, spans nearly 840 miles (1,350 km). That's longer than California. The Los Angeles-sized object that made Caloris probably smashed right through Mercury's crust and into its mantle. No one knows if an object that big has ever hit the Earth, but some think so. According to the "giant impact hypothesis," a body the size of Mars smashed into Earth 4 billion years ago and broke off the rock that became our Moon.

Steve Sampson
March 10, 2006

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From: KnowledgeNews