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Asteroid vs. Dinosaurs [and all other life]]



Don Savage
Headquarters, Washington, DC (Phone:  202/358-1547)

Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA 
(Phone:  818/354-5011)

RELEASE:  94-219

SULFUR-RICH ASTEROID SITE HOLDS CLUES TO DEMISE OF DINOSAURS

     NASA scientists now believe it was the sulfur-rich atmosphere created 
in the aftermath of an immense asteroid collision with Earth 65 million 
years ago that brought about a global freeze and the demise of the 
dinosaurs.

     The impact of this large asteroid hit a geologically unique, sulfur-rich 
region of the Yucatan Peninsula in Mexico, according to planetary 
geologist Adriana C. Ocampo and atmospheric scientist Dr. Kevin H. 
Baines, both of the Jet Propulsion Laboratory's Earth and Space 
Sciences Division, Pasadena, CA.  They estimate the impact kicked up 
billions of tons of sulfur and other materials and was between 10,000 to 
50,000 times more powerful than the comet Shoemaker-Levy 9 impact 
on Jupiter last July.

     The researchers and colleagues, Dr. Kevin O. Pope of Geo Eco Arc 
Research in La Canada, CA, and Dr. Boris A. Ivanov of the Russian 
Academy of Sciences in Moscow, have co-authored a paper detailing the 
global atmospheric impact of this asteroid collision at Chicxulub, Mexico, 
in the latest issue of Earth and Planetary Science Letters.

     "We estimate that this asteroid was between 10 to 20 kilometers (6 to 
12 miles) in diameter and its collision on Earth brought about total 
darkness around the world for about half a year," Ocampo said.  "But 
more importantly, persistent clouds generated by the impact on this 
geologically distinct region of sulfur-rich materials caused temperatures 
to plunge globally to near freezing."

     "These environmental changes lasted for a decade and subjected 
organisms all over the world to long-term stresses to which they could 
not adapt in such a brief time span," Pope added.  "Half of the species on 
Earth became extinct as a result."

     The researchers based their work on computer models of the impact 
and atmospheric effects, studies of the crater geology and extensive 
fieldwork at a rock quarry located 360 kilometers (223 miles) south of 
Chicxulub at Albion Island in Belize.  Fragments bearing the unique 
characteristics of the impact were found in this area.

     In studying the sites and modeling the resulting changes in the 
biosphere, the scientists discovered that it was the specific geological 
location of the impact in a region that is rich in sulfur materials that 
created catastrophic climate changes and led to the downfall of the 
dinosaurs.

     "If this asteroid had struck almost any other place on Earth, it 
wouldn't have generated the tremendous amount of sulfur that was 
spewed into the atmosphere to create such a devastating, worldwide 
climate change," Baines said. 

     On impact, the asteroid hurled some 35 billion to 770 billion tons of 
sulfur high into the atmosphere, along with other materials.  The NASA 
team, in collaboration with Dr. Alfred Fischer of the University of 
Southern California, recently discovered rocks in Belize -- some the size 
of a small car -- that were blown out of the crater and landed south of 
the Chicxulub site.

     The boulder deposit in Belize also contained fragments of glass that 

were created by the melting of rock when the asteroid crashed into 
Earth, Ocampo said.  Spherical fragments, known as "tektites," were 
scattered and formed as the molten glass flew through the air and 
cooled.  The tektites have been found in other regions near the crater, 
such as Haiti, Mexico, Texas and Alabama, but never in association with 
large boulders.

     Another important find at the Belize rock quarry was limestone with 
fossils dating to the early part of the Cretaceous.

     "Fossils of this age don't belong in northern Belize," Ocampo said.  
"Early Cretaceous fossils have been found deep below the surface near 
the crater during drilling by the Mexican Petroleum Company.  We think 
the limestone found in Belize was excavated by the impact, which 
probably blew a hole more than 15 kilometers (nine miles) deep in the 
Yucatan Peninsula."

     Since 1980, when University of California-Berkeley geology professor 
Walter Alvarez and his colleagues first proposed the theory, researchers 
have been searching for impact sites that would explain the sudden 
disappearance of the dinosaurs.  The main evidence to support the 
theory came from finding a substance called iridium in a layer of clay in 
Italy.  The concentration of iridium, an element found on Earth in very 
small quantities, was quite large.  High concentrations of iridium are 
found in asteroids and comets.

     In 1989, Pope and Charles Duller of NASA's Ames Research Center, 
Mountain View, CA, discovered a semi-circle of sinkholes at Chicxulub.  
Ocampo studied gravity and magnetic data from the crater and 
correlated them with the sinkholes. She concluded that the area had the 
classic characteristics of an impact crater, indicating that Chicxulub 
was, in fact, the place where a colossal asteroid had smashed into Earth 
millions of years ago.  Current estimates of the crater size range from 
180 kilometers to 300 kilometers (112 miles to 186 miles) in diameter, 
making it one of the largest craters known on Earth.

     The researchers used sophisticated atmospheric models of the sulfur-
rich atmosphere of Venus to model their impact scenario.

     "Initially, thick sulfur clouds, combined with soot and dust generated 
by this impact, would have spread worldwide and blocked out the Sun," 
Baines and Pope said.  "Night-like conditions probably existed all over 
Earth for at least six months essentially bringing photosynthesis to a 
halt.  Unlike the aftermath of typical impacts, the skies remained murky 
for at least a decade, due to chemically generated clouds of sulfuric acid 
high in the stratosphere."

     The reflection of sunlight back into space from these high-altitude 
clouds caused surface temperatures to drop to nearly freezing for many 
years all over the planet.

     These atmospheric conditions occur in Venus' perpetually cloudy 
atmosphere, Baines said, where ultraviolet sunlight and water in the 
high atmosphere can convert sulfur dioxide into sulfuric acid clouds.  
Sulfuric acid clouds like those that cover Venus may have continued to 
blanket the Earth for more than a decade after the initial impact of the 
asteroid, causing a secondary and more long-lasting effect which killed 
much of life on Earth.

     "The entire ecosystem of Earth, including plants and animals, was 
subjected to extreme environmental conditions, which a large number of 
well-established species, such as the dinosaurs, simply could not cope 
with," Baines said.  Six months of total darkness and 10 years of global 
freezing ultimately destroyed the dinosaurs and many other organisms, 
Pope added. 


     This research was sponsored by the Exobiology Program in NASA's 
Solar System Exploration Division, Washington, DC.  Fieldwork in Belize 
was supported in part by The Planetary Society in Pasadena, CA.

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