posted July 21, 2005 11:50 AM         

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A high-resolution map from NASA's Shuttle Radar Topography Mission (SRTM), released yesterday, has provided the most telling visible evidence to date of a 112-mile (180-kilometer) wide, 3,000-foot (900-meter) deep impact crater, the result of a collision with a giant comet or asteroid on one of Earth's all-time worst days.

The existence of the impact crater, Chicxulub, was first proposed in 1980. In the 1990s, satellite data and ground studies allowed it to gain prominence among most scientists as the long sought-after "smoking gun" responsible for the demise of the dinosaurs and more than 70 percent of Earth's living species 65 million years ago.

The relatively obscure feature is all but hidden in the flat limestone plateau of Mexico's Yucatán Peninsula. But using 3-D measurements of the Earth made with instruments on board an orbiting Space Shuttle, the remnants of the crater can now be seen clearly.

"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," said Michael Kobrick, SRTM project scientist at NASA's Jet Propulsion Laboratory (JPL), Pasadena, California. "In fact, much of the surface expression of Chicxulub is so subtle; if you walked across it you probably wouldn't notice it. That's where the view from space becomes invaluable, " Kobrick said.

The Chicxulub data show a subtle, yet unmistakable, topographic indication of the impact crater's outer boundary: a semicircular trough 10 to 15 feet (3 to 5 meters) deep and 3 miles (5 kilometers) wide. Scientists believe the impact, centered off Yucatán's coast in the Caribbean, disturbed the subsurface rocks, making them unstable. The rocks were subsequently buried by limestone sediments, which erode easily. The crater rim's instability caused the limestone to fracture along the rim, forming the trough.

Exactly how the Chicxulub impact caused Earth's mass extinctions is not known (see links below). Scientists imagine three possible scenarios: Some think the impact threw massive quantities of dust into the atmosphere which blocked the sun and arrested plant growth. Others believe sulfur released by the impact lead to global sulfuric acid clouds that blocked the sun and also fell as acid rain. Another possibility is that red-hot debris from the falling asteroid or comet triggered global wildfires.

In February, NASA finished processing the SRTM mission's North America data set and delivered it to the National Imagery and Mapping Agency (NIMA). More than eight terabytes of data recorded aboard the Space Shuttle Endeavour were refined into 200 billion research-quality measurements of Earth's landforms. NIMA will perform additional data finishing and send it to the U.S. Geological Survey's Earth Resources Observation Systems Data Center, Sioux Falls, South Dakota, for final archiving and distribution.

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