But what mechanism did the impact trigger to cause mass extinction? The conventional theory is that impact dust obscured the sun, shutting down photosynthesis and snuffing out life. Kevin Pope from Geo Eco Arc Research shows in the February issue of Geology that the assumptions behind this theory are amiss, and therefore damage estimates from future asteroid impacts are also amiss.

This latter point became a recent issue when a large asteroid passed near the Earth on January 7 and news reports exaggerated its potential impact effects.

"Based on the old, inaccurate dust numbers, which erroneously suggested that a medium-sized asteroid (1-2 km in diameter) could cause global climate change and famine, scientists calculated that one's chance of getting killed by an asteroid impact were about the same as dying in a plane crash," Pope said.

"My new impact dust estimates indicate that death by an asteroid is far less likely and that such medium-sized asteroid impacts would not have catastrophic global effects. But of course the regional effects would still be devastating."

To truly understand the influence of impact dust, scientists need to find a way to directly measure the amount of small dust particles in such places as the K-T boundary. In the meantime, Pope studied patterns of coarse dust particles to create a model that showed how the small dust particles were dispersed.

Incorporating these geological observations with new theoretical work, Pope asserts that very few of the particles are of the size that it would take to shut down photosynthesis for any significant length of time and therefore the original K-T impact extinction hypothesis is not valid.

He believes it may have been sulfate aerosols produced from impacted rocks and soot from global fires that could have shut down photosynthesis and caused global cooling.

"The original studies of the clay layer found at the K-T boundary assumed much or all of this layer was derived from fine impact dust," he said.

"More recent studies of this layer have shown this not to be the case. Furthermore, earlier estimates were based on extrapolations of data from surface atomic bomb blasts, which had about 100 million times less energy than the Chicxulub impact. Extrapolation over eight orders of magnitude is risky business."

Pope was involved in the "discovery" of the Chicxulub crater in 1989- 1990 when he worked at the NASA Ames Research Center. (Oil geologists had discovered the crater and reported the finding in 1981, but it was basically ignored.)

He was using satellite images to map water resources in the Yucatan with Adriana Ocampo and Charles Duller when they found the semi-circular ring of sink holes. They thought the crater might be the K-T impact site and published their hypothesis in the May 1991 issue of Nature.

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