August 16, 1996. Copyright, 1996, Graphic News. All rights reserved

DID METEOR WIPE OUT DINOSAURS?
By Laura Spinney, Science Editor

LONDON, August 16, Graphic News - A vast meteor that landed in Mexico when dinosaurs still roamed the Earth could have caused climatic changes that led to their extinction 65 million years ago. Scientists disagree as to whether the meteor was big enough to have resulted in such dramatic changes. So next month, using the latest remote sensing techniques, an international team of geologists will measure the diameter of the crater left behind.

The Chicxulub Crater in Mexico's Yucatan Peninsula is the largest crater on Earth. But because it is half on land and half under the sea, buried beneath sediment one kilometre deep, measuring its exact diameter has proved a difficult task. Estimates vary between 180 km and 280 km, and that discrepancy makes a crucial difference. Atmospheric models suggest that the impact of a meteor whose crater is less than 180 km across would have been too small to be significant, but that the dust injected into the atmosphere from a crater 280 km across could have blocked out sunlight and reacted with water in the air to create acid rain. Changes on that scale could have led to the demise of the dinosaurs. Now Dr Dave Snyder, a seismologist at the University of Cambridge, and British, American and Mexican colleagues are planning to measure the diameter using a technique called reflection profiling, in which sound waves are reflected off the crater at different depths to build a profile of its rock layers and faults.

'We've got great confidence that we'll have enough resolution in our undersea images of the crust to convince everybody which size of the crater is actually the significant one,' says Snyder, who expects to go public with his findings before the end of the year. But, he points out, not only are the models relating the size of the crater to climatic changes controversial, scientists are still arguing about the way in which those changes might have affected living organisms. 'It looks like some plankton species in the ocean died within a few tens of thousands of years, but the dinosaurs may not have died out for maybe one or two million years,' says Snyder. 'So it would be a catastrophic effect that would slowly work its way up the food chain.'

Sources: British Institutions Reflection Profiling Syndicate (BIRPS) at University of Cambridge