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(meteorobs) Asteroid hits at deadly oblique angle...65 mil years ago(long article)



A friend sent me the following ...thought to share it with you all.
George Z.
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 ASTEROID HIT AT DEADLY OBLIQUE ANGLE 65 MILLION YEARS AGO 
 From The Brown University News Bureau
 
 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.
 
 "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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