Fires From Asteroid May Have Spared Some Regions

John Roach
for National Geographic News
September 16, 2002
About 65 million years ago a space rock slammed into Mexico's Yucatan
Peninsula and scattered high-velocity debris around Earth, igniting
wildfires in North America, the Indian subcontinent, and most of the
equatorial part of the world.

However, northern Asia, Europe, Antarctica and possibly much of Australia may have been spared the inferno, according to a new computer simulation of how the wildfires spread around the world.

The wildfires are thought to be a key ingredient in the concoction of environmental changes that killed more than 75 percent of all plant and animal life on Earth, including the dinosaurs.

"Our calculations suggest fires may have been more intense in some parts of the world than in others and that some areas may have been spared fires altogether," said David Kring, a planetary scientist at the University of Arizona in Tucson. "However, other environmental effects would have affected the spared regions."

For example, dust and smoke from the impact and fires would have obscured sunlight causing global temperatures to plummet and acid rain to fall. Then, the increased concentrations of carbon dioxide and other pollutants in the air may have led to global warming.

The impact event, which created a 110 to 180 mile (180 to 300 kilometer) diameter crater in Mexico's Yucatan Peninsula, marks the transition of the Cretaceous Period to the Tertiary Period where mammals replaced dinosaurs as the dominant species on Earth.

Kring along with his colleague Daniel Durda at the Southwest Research Institute in Boulder, Colorado, detail the spread of the wildfires in the September issue of the Journal of Geophysical Research—Planets.

Fire Model

The global wildfires sparked by the impact event that formed the Chicxulub crater were first modeled in 1990 by planetary scientist Jay Melosh at the University of Arizona and colleagues. Their calculation indicated that the fires spread around the world in a single pulse.

The model developed by Kring and Durda, which they say builds on the earlier research, shows that the fires were ignited in multiple pulses.

The impact was 10 billion times more energetic than the nuclear bombs dropped on Hiroshima and Nagasaki, Japan in World War II, the scientists said.

Most of the material from the collision collected around the impact site, but according to the researchers' calculations, some 12 percent of the debris was launched beyond Earth's atmosphere.

"The computer simulation keeps track of the velocity of material being ejected from the crater," said Kring. "A small fraction of the material achieves escape velocities and, thus, escapes Earth."

The debris ejected from the crater and lofted far above Earth's atmosphere rained back down over a period of about four days, said Kring. As the debris rained down, it heated the atmosphere and surface temperatures so intensely that the ground vegetation spontaneously ignited.

This high energy debris concentrated both around the Chicxulub crater and on the opposite side of the Earth around India, the researchers report.

"The pileup of debris on the opposite of the Earth occurs because material is reaching that spot from all directions," said Kring. "Material launched from the crater in an easterly direction runs into material launched from the crater in a westerly direction."

As the Earth rotated, it turned beneath this returning plume of debris, causing the wildfires to migrate to the west, as illustrated by the researchers' computer simulation of the wildfire spread.

Modeling Assumptions

Some asteroid experts, including Melosh, question the pulsing results of the computer model. "The pulsing is probably the result of the assumed ejecta distribution that they choose, but there is no reason to think that what they do is, in fact, correct," he said.

Melosh believes that the proper way to determine the pattern of wildfire spread is to do numerical simulations of the full ejection process and then follow the velocity and direction of the ejected debris to determine the rain back pattern.

Kring and Durda based their computer simulation of the wildfire spread on models of the Chicxulub impact run by Elisabetta Pierazzo, a research scientist at the Planetary Science Institute in Tucson, Arizona, to determine how impact angle affects the results of impact events.

"We had to estimate what the range in those launch conditions could be for a range of plausible Chicxulub impact events," said Durda. Thus, he added, Melosh is correct to say that their results are only as good as the assumptions in the inputs to the model.

Durda and Kring are currently working on a way to get a direct hand-off of the results from Pierazzo's models of the impact itself to their model that follows the debris trajectories around the planet.

"That will allow us to more rigorously follow changes in our global fire distribution as a result of various impact conditions," said Durda.

Nevertheless, Kring and Durda said that they have run a broad range of possible ejecta launch conditions and certain aspects of the wildfire pattern are the same from model to model, such as the pileup of debris on the opposite side of the Earth from the Yucatan Peninsula.

"Different trajectories can modify the distribution of fires in small ways, but not significantly alter the general pattern," said Kring.

Kring and Durda plan to apply their modeling efforts to other impact events, such as the Manicouagan event in Canada some 200 million years ago and the Popigai impact in Russia some 35 million years ago, to determine the extent of wildfires produced.

National Geographic Resources on Comets and Asteroids

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Fossil Leaves Suggest Asteroid Killed Dinosaurs
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U.S. Summons Experts to Draft Asteroid Defense Plan
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