- The Washington Times - Saturday, March 8, 2003

NASA scientists have discovered the strongest evidence yet of the cataclysmic event that wiped out the dinosaurs and most of life on Earth some 65 million years ago.

It is an obscure geologic formation, concealed even from aerial photography within the monotonous limestone plateau on the northwest corner of Mexico's Yucatan Peninsula. It was revealed by new, high-resolution topographic images compiled from data collected by astronauts aboard space shuttle Endeavour back in February 2000.

The geologic formation practically confirms the existence of something proposed by scientists more than 20 years ago. Called Chicxulub, it is an ancient crater, some 112 miles wide and 3,000 feet deep, and it is thought to be the site of the asteroid impact that ended the Cretaceous period and cleared the way for the age of mammals — and humans.

The crater has been exposed in a new, high-resolution terrain map of North America compiled aboard Endeavour using NASA's Shuttle Radar Topography Mission instrument. Space agency scientists said the SRTM has "provided the most telling visible evidence to date" of the most famous catastrophe in the planet's history.

"This new, complete North American data set greatly expands our topographic knowledge of Canada, southern Alaska and its Aleutian Islands, Mexico and Central America," said Michael Kobrick, SRTM project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "There are spectacular features that pop out in these maps as never before, and more subtle features, like Chicxulub, become apparent for the first time."

The Yucatan peninsula is a flat plateau composed mostly of limestone, with differences in elevation varying by less than about 500 feet. In analyzing and enhancing the image by computer, scientists discovered an arcing trough that marks the outer boundary of the crater. The trough is only 10 to 15 feet deep and is about 3 miles wide.

"In fact, much of the surface expression of Chicxulub is so subtle if you walked across it you probably wouldn't notice it," Kobrick said. "That's where the view from space becomes invaluable."

NASA scientists said they think the impact, which was centered off Yucatan's coast in the southwestern Gulf of Mexico, disturbed subsurface rocks severely and made them unstable. Over millions of years, the rocks were buried by limestone sediments, which erode easily. The instability of the crater rim apparently caused the limestone to fracture and collapse, they said, which formed the trough.

Although scientists have been confident of the general area of the crater for some time, they still do not know exactly how the Chicxulub impact caused a mass extinction. There are several theories.

For example, the impact might have ejected huge quantities of dust into the atmosphere. The dust blocked out the sun, thereby plunging the planet into dark and freezing conditions — a scenario supported by the late astronomer, Carl Sagan, who argued that a global thermonuclear war would have a similar effect, which he called "nuclear winter."

Other scientists have speculated chemical reactions caused by the impact led to giant sulfuric acid clouds that also obscured the sun and fell as acid rain, destroying vegetation on a global scale.

Still another possibility is planetary wildfires, triggered both by the magnitude of the impact — which ignited a blast perhaps equivalent to millions of atomic bombs — and by spreading red-hot debris over an enormous area.

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(To view the SRTM image of the Chicxulub crater on the Internet, go to photojournal.jpl.nasa.gov/catalog/PIA03379)


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