Ignition threshold for impact-generated fires

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.

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