[meteorite-list] Ignition Threshold For Impact-Generated Fires

[Ron Baalke]

Southwest Research Institute (SwRI)
PO Drawer 28510
San Antonio, TX 78228-0510

For more information contact:

Deb Schmid, Communications Department
(210) 522-2254

August 26, 2004

Ignition threshold for impact-generated fires

San Antonio -- Scientists conclude that, 65 million years ago, a 
10-kilometer-wide asteroid or comet slammed into what is now the Yucatan 
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 u
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.

EDITORS: High-resolution images for download are available at
      http://www.swri.org/press/impactfires.htm