North America Hit Hard By Asteroid Strike In Yucatan 65 Million...( es the structure of craters on the moon...)

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 pl
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Contact: Scott Turner
Scott_Turner@Brown.edu
401-863-2476
Brown University
30-Oct-1996