Anicent impact debris sparked global wildfires
Posted: September 4, 2002

Simulated impact fire map. Credit: Daniel D. Durda - Southwest Research Institute
Global wildfires ignited by high-velocity debris from the catastrophic impact of an asteroid or comet with Earth 65 million years ago spread over southern North America, the Indian subcontinent and most of the equatorial part of the world one to three days after impact, according to a new study.

But northern Asia, Europe, Antarctica and possibly much of Australia may have been spared, David A. Kring of the University of Arizona and Daniel D. Durda of the Southwest Research Institute report in the Journal of Geophysical Research - Planets.

UA planetary scientist H. Jay Melosh in 1990 and others modeled global wildfire scenarios from the horrific impact that is thought to have led to one of the greatest mass extinctions in Earth history, including dinosaur extinction. The impact that blasted the immense Chicxulub crater near Yucatan, Mexico, marked the end of the Age of Reptiles, the Mesozoic, and heralded the Age of Mammals, the Cenozoic.

"We've added more detail in re-evaluating the extent of the wildfires," Kring said. "Our new calculations show that the fires were not ignited in a single pulse, but in multiple pulses at different times around the world. We also explored how the trajectory of the impacting object, which is still unknown, may affect the distribution of these fires."

Their more detailed modeling suggests pulses of misery for life on Earth during days after impact. More than 75 percent of the planet's plant and animal species did not survive to see the Cenozoic.

"The fires were generated after debris ejected from the crater was lofted far above the Earth's atmosphere and then rained back down over a period of about four days. Like countless trillions of meteors, the debris heated the atmosphere and surface temperatures so intensely that ground vegetation spontaneously ignited."

The collision was so energetic -- 10 billion times more energetic than the nuclear bombs that flattened Hiroshima and Nagasaki in 1945 -- that 12 percent of the impact debris was launched beyond Earth into the solar system, Kring said.

About 25 percent of the debris rained back through the atmosphere within two hours of impact. Fifty-five percent fell back to Earth within 8 hours of impact, and 85 percent showered down within 72 hours of impact, according to Kring's and Durda's calculations.

Both physics and Earth's rotation determined the global wildfire pattern. High-energy debris would have concentrated both around the Chicxulub crater in Mexico and its global antipode -- which corresponded to India and the Indian Ocean 65 million years ago. "The way to think of this is, the material was launched around Earth and headed on a return trajectory to its launch point," he explained.

"Then, because the Earth rotates, it turned beneath this returning plume of debris, and the fires migrated to the west. That's what causes the wildfire pattern."

Kring and Durda noted not in this paper, but in an unrefereed abstract, that post-impact wildfires generated as much carbon dioxide, and perhaps more carbon dioxide, than limestone vaporized at the impact site. Wildfires played at least as big a role as the limestone target site in disrupting the carbon cycle and in greenhouse warming.

The team proposes to model other impact events using the code they developed for these simulations.

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