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Ignition threshold for impact-generated fires

San Antonio -- August 26, 2004 -- Scientists conclude that, 65 million years ago, a 10-kilometer-wide asteroid or comet slammed into what is now the Yucatán peninsula, excavating the Chicxulub impact crater and setting into motion a chain of catastrophic events thought to precipitate the extinction of the dinosaurs and 75 percent of animal and plant life that existed in the late Cretaceous period.

"The impact of an asteroid or comet several kilometers across heaps environmental insult after insult on the world," said Dr. Daniel Durda, a senior research scientist at Southwest Research Institute® (SwRI®). "One aspect of the devastation wrought by large impacts is the potential for global wildfires ignited by material ejected from the crater reentering the atmosphere in the hours after the impact."

Large impacts can blast thousands of cubic kilometers of vaporized impactor and target sediments into the atmosphere and above, expanding into space and enveloping the entire planet. These high-energy, vapor-rich materials reenter the atmosphere and heat up air temperatures to the point that vegetation on the ground below can spontaneously burst into flame.

"In 2002, we investigated the Chicxulub impact event to examine the extent and distribution of fires it caused," said Durda. This cosmic collision carved out a crater some 40 kilometers (25 miles) deep and 180 kilometers (112 miles) across at the boundary between two geologic periods, the Cretaceous, when the dinosaurs ruled the planet, and the Tertiary, when mammals took supremacy.

"We noted that fires appeared to be global, covering multiple continents, but did not cover the entire Earth," Durda continued. "That suggested to us that the Chicxulub impact was probably near the threshold size event necessary for igniting global fires, and prompted us to ask 'What scale of impact is necessary for igniting widespread fires?'"

In a new study, Durda and Dr. David Kring, an associate professor at the University of Arizona Lunar and Planetary Laboratory, published a theory for the ignition threshold for impact-generated fires in the August 20, 2004, issue of the Journal of Geophysical Research. Their research indicates that impacts resulting in craters at least 85 kilometers wide can produce continental-scale fires, while impact craters more than 135 kilometers wide are needed to cause global-scale fires.

To calculate the threshold size impact required for global ignition of various types of vegetation, Durda and Kring used two separate, but linked, numerical codes to calculate the global distribution of debris reentering the atmosphere and the kinetic energy deposited in the atmosphere by the material. The distribution of fires depends on projectile trajectories, the position of the impact relative to the geographic distribution of forested continents and the mass of crater and projectile debris ejected into the atmosphere.

They also examined the threshold temperatures and durations required to spontaneously ignite green wood, to ignite wood in the presence of an ignition source (such as lightning, which would be prevalent in the dust-laden energetic skies following an impact event) and to ignite rotting wood, leaves and other common forest litter.

"The Chicxulub impact event may have been the only known impact event to have caused wildfires around the globe," Kring noted. "The Manicouagan (Canada) and Popigai (Russia) impact events, however, may have caused continental-scale fires. The Manicouagan impact occurred in the late Triassic, and the Popigai impact event occurred in the late Eocene, but neither has been firmly linked yet to the mass extinction events that occurred at those times."

Kring is currently at the International Geological Congress in Florence, Italy, giving a keynote address on the Chicxulub impact event and its relationship to the mass extinctions at the Cretaceous-Tertiary boundary period. Durda is available for comment at the SwRI offices in Boulder, Colo.

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