North America Hit Hard By Asteroid Strike In Yucatan 65 Million Years Ago

October 30, 1996

PROVIDENCE, R.I. -- All asteroid-based extinctions great and small are not alike.

A new study says the asteroid that struck Earth 65 million years ago and wiped out the dinosaurs was particularly deadly to North America because it hit the Yucatan peninsula from the southeast at a 20- to 30-degree angle, spreading the devastating impact of its energy northwest.

The oblique angle of the asteroid's contact with Earth coupled its impact energy with that of the atmosphere and planetary surface to send waves of ground-hugging, vaporous fireballs onward, the study says. This resulted in an extinction intensity most severe downrange of the impact in North America.

The study suggests one rationale for the dire consequences of such an impact: The severity of extinctions that result from an object's impact on Earth may reflect the incoming object's angle.

Editors: Color transparencies are available through the News Bureau.

"This finding may help us determine what other impacts did to Earth in the past and what they may do in the future," said Peter Schultz, professor of geological sciences at Brown University. Schultz and Steven D'Hondt, professor of oceanography at the University of Rhode Island, are co-authors of the study in the November issue of the journal Geology.

The researchers suggest that the relatively low angle of the Yucatan impact propelled a ballistic fireball downrange into North America. The fireball carried a two-mile-deep layer of vaporized rock and other material sheared off the Yucatan. The killing zone of matter cascaded through the atmosphere at near orbital speed, across North America and eventually around the globe.

"It was like a nuclear explosion taken north on a jet-powered sleigh ride," Schultz said. "This was indeed the day the Earth shook."

As evidence, the researchers show that the horseshoe-shaped Yucatan crater matches the structure of craters on the moon and Venus that were created when objects struck those heavenly bodies at oblique angles.

Venus's thick atmosphere holds in place gases emitted from a crater after an impact. The researchers studied images of these corked-in Venusian vapors, which show that gaseous material is propelled in waves downrange after an object strikes a planetary surface at an oblique angle.

Schultz used a high-powered gun to recreate the dynamics of an object striking Earth's surface at a 20- to 30-degree angle. The experiment produced horseshoe-shaped craters, while high-speed film captured gas and materials jettisoned downrange.

The researchers said that biological evidence appears to support their oblique-impact hypothesis. North America, the first region to experience the fireball, had the most severe extinctions of plants.

After the devastation, ferns dominated the flora of central North America. Ferns accounted for 70 to 100 percent of the spore- or pollen-producing plants in the region after the impact, compared with only 10 to 40 percent before it. At the base of the food chain, plants are considered sensitive indicators of environmental devastation. Because ferns reproduce through the use of hardy spores, the plants are regarded as key flora in colonizing the site of a natural disaster.

Plants in parts of the world not downrange from the impact took a lesser hit from the corridor of incineration. For example, several ancient evergreen trees found in North America before the impact, but not after, still grow in parts of Australia and South America. Modern relatives of these trees, often called "primitive conifers," include the Norfolk Island pine, Chilean monkey puzzle and Wollemi pine.

"The basic point of the study is that we can determine the impact angle of this object and that the angle matters," D'Hondt said. Most scientists study the aftermath of collisions that caused Earth's craters as if objects struck the planet at 90-degree angles, or from directly overhead. But such vertical impacts are very rare.

An oblique angle of impact may have more deadly global consequences than a vertical impact, because an oblique impact should release a greater fraction of impact energy to the atmosphere and surface target, said Schultz and D'Hondt.

"The study also underscores the point that regional repercussions can be expected from an Earth-object impact, something scientists have rarely considered in previous studies of this 65-million-year-old event," D'Hondt said.
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Brown University

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