Sizing Up a Deadly Crater

The massive meteorite impact that may have doomed the dinosaurs apparently blasted out a crater much smaller than some scientists had thought. New measurements of the buried crater, reported in tomorrow's issue of Nature, suggest that smaller, less energetic collisions can still wreak an enormous amount of havoc. Indeed, the researchers also unearthed surprising evidence that the impact's scars extend far deeper than expected, penetrating into Earth's upper mantle.

Accurately measuring the size of the Chicxulub crater--the best-preserved large crater on Earth--has been a challenge. A kilometer of sediment shrouds the crater, which sits partly on land and partly under water at the tip of the Mexico's Yucatan Peninsula. Several teams have analyzed subtle variations in Earth's gravitational field above the crater to infer the locations of its buried walls. However, that technique is fraught with ambiguity: Estimates of the crater's initial diameter, blasted within moments of impact, ranged from 90 to 170 kilometers. This yielded different calculations of the amount of energy released by the impact at the close of the Cretaceous period some 65 million years ago.

To resolve those questions, an international team led by geologists Jo Morgan and Mike Warner of Imperial College in London turned to a powerful technique called seismic-reflection profiling. By firing powerful air guns beneath a research ship, the researchers sent sound waves into the sediment, then detected the echoes bouncing off rock structures with sensitive seismometers. Four such vertical profiles through the part of the crater that lies offshore clearly reveal the remnants of a 100-kilometer-wide "transient crater," the meteorite's earliest excavation--confirming some estimates of the crater's size that other scientists had dismissed as too small. Within minutes of the impact, this enormous hole in the crust apparently caved inward to create a striking multiringed basin of walls of collapsed rock. This feature, the first seen on Earth, is similar to the nested rings that encircle large craters on the moon and Venus.

The seismic data also exposed a fault beneath one of the crater walls, knifing down some 35 kilometers under the sea floor. Researchers suspect the fault formed as huge slabs of earth collapsed into the initial crater. "This was unanticipated and quite exciting," says team member Alan Hildebrand, a planetary scientist with the Geological Survey of Canada. Previously, researchers thought the lower crust was too weak and pliable to fracture in such a way.

The new results should settle the debate over the crater's initial size, says planetary scientist H. Jay Melosh of the University of Arizona, Tucson, an expert on impact craters. "Many paleontologists had trouble believing that a 100-kilometer crater could eject enough material to cause the Cretaceous extinctions," he says. "It now appears that the original estimate was enough to have caused all the devastation we see."

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