From: nasanews
Date: Dec 28 17:17 UTC
Subject: Asteroid Site Holds Clues to Dinosaur Demise
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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- T o d d E. V a n H o o s e a r -
``'''vanhoose@lalaland.cl.msu.edu - vanhoose@msu.edu - vanhoose@lalaland.cl.msu.edu
(._.) Michigan State University - East Lansing, MI USA
(_) Computer Laboratory - Department of Communication
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"I'd love to change the world, but they won't give me the source code!"
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