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- Imagine: NASA scientists announce they have detected
a 10-mile-wide asteroid on a collision course with the Earth. They calculate
it will hit Southeast Asia in two weeks. There is no chance of Bruce Willis
being sent on a beefed-up space shuttle to blow up the asteroid. Earthlings
will have to ride out the impact.
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- The Tunguska event in 1908 flattened 800 square miles
of Siberian forest -- and the object didn't even reach the ground.
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- The world economy grinds to a halt as people take to
the hills. Anarchy sets in, civilization breaks down. Accusations fly over
the lack of warning -- where was Spaceguard, the proposed international
search effort for large asteroids?
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- People in Brazil feel less vulnerable than most of the
world's population. They are on the opposite side of the Earth from the
predicted impact point. But one hour after the impact Brazilians notice
some brilliant meteors. Then more meteors. Soon the sky gets brighter and
hotter from the overwhelming number of meteors. Within a few minutes trees
ignite from the fierce radiant heat. Millions of fragments of rock, ejected
into space by the blast, are making a fiery return all over the planet.
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- Only people hiding underground survive the deadly fireworks
display. Within three hours, however, massive shock waves from the impact
travel through the Earth's crust and converge on Brazil at the same time.
The ground shakes so violently that the ground fractures and molten rock
spews from deep underground. Maybe Brazil wasn't the best place to be after
all.
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- The survivors of the firestorms, tsunami and massive
earthquakes emerge to a devastated landscape. Within a few days the Sun
vanishes behind a dark thick cloud -- a combination of soot from the firestorms,
dust thrown up by the impact and a toxic smog from chemical reactions.
Photosynthesis in plants and algae ceases and temperatures plummet. A long,
sunless Arctic winter seems mild compared to the new conditions on most
of the planet.
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- After a year or so the dust settles and sunlight begins
to filter through the clouds. The Earth's surface starts warming up. But
the elevated carbon dioxide levels created by the fires (and, by chance,
vaporization of huge quantities of limestone at the impact site) results
in a runway greenhouse effect. Those creatures that managed to survive
the deep freeze now have to cope with being cooked.
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- Many species of plants and animals vanish. The few hundred
thousand human survivors find themselves reverting to a Stone Age existence.
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- Is it fiction?
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- Computer modeling of asteroid impacts and climatic effects
suggest that this devastating sequence of events could happen. Fortunately
it is extremely unlikely that this scenario will occur in our time. No
known asteroids are on a collision course with Earth. However, every 50
to 100 million years the Earth collides with a comet or asteroid of sufficient
size to cause planet-wide devastation.
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- Evidence of past impacts has only been recognized in
recent decades and is pointing to the conclusion that big impacts have
caused major disruptions to the development of life on Earth.
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- Known impact craters. IMAGE: Canadian Geological Survey
The end of the dinosaurs
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- In 1980 Scientists Luis and Walter Alvarez claimed they
had found evidence of a huge impact event 65 million years ago. This age
corresponded with the demise of the dinosaurs at the end of the Cretaceous
Period. The evidence included a worldwide layer of clay with high levels
of the rare element iridium, usually the signature of an impact.
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- The search was on for a giant crater associated with
this impact. Hopes weren't high because in 65 million years the Earth's
surface has changed dramatically -- nearly all of the present ocean floor
is younger than 50 million years.
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- Some great scientific detective work pointed to an impact
somewhere in Central or North America. Finally, in 1990, the buried remains
of a 150-mile-diameter crater were discovered near the town of Chicxulub
on the Yucatan Peninsula in Mexico. A crater this size would have been
blasted out by a 10-mile-wide comet or asteroid colliding with the Earth
at some 50,000 mph. The "smoking gun" had been found.
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- Impact, volcanoes, or both?
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- The debate continues on whether the Chicxulub impact
caused the mass extinction at the end of the Cretaceous Period or whether
it was one of a sequence of disasters. The Deccan Traps of India are the
remnants of a massive upwelling of molten rock from deep within the Earth
65 million years ago. The toxic fumes and dust from the eruption have been
put forward as a possible alternative cause of climate change that led
to the extinction of the dinosaurs.
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- A possible link between impacts and volcanism became
evident in 1974 when the Mariner 10 spacecraft flew past the innermost
planet Mercury. The planet was found to be covered with impact craters
like the moon. One giant impact crater on Mercury was particularly interesting.
Directly opposite the impact point, on the other side of the planet (called
the "antipodal point") was a region of highly disrupted terrain
with no evidence of an impact. The shock waves from the impact on one side
of Mercury had traveled around the surface and met simultaneously at the
antipodal point to create the chaotic features. Similar features have since
been detected on several moons of the giant planets.
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- Astronomer Duncan Steel has suggested that the same occurred
with the Chicxulub impact and that the shock waves caused the Deccan Traps.
Taking into account millions of years of continental drift, this region
would have been at the antipodal point to Mexico at the time of the impact.
Although the eruption may have contributed to the suffering, it now seems
more likely that the Deccan Traps were just a consequence of the catastrophic
initial event, the Chicxulub impact.
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- More craters are there to be found
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- A meteorite dug the misnamed Meteor Crater in Arizona.
IMAGE: NASA
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- As a tourist destination, impact craters on Earth are
virtually unknown. Americans might know of Meteor Crater in Arizona. Australians
planning an Outback tour may have heard of Wolfe Crater in Western Australia.
American geologist/astronomer Gene Shoemaker (of Comet Shoemaker-Levy 9
fame) spent many "holidays" touring the Australian Outback looking
for impact craters. He and his wife Carolyn helped to identify some of
the twenty or so Australian impact craters that are now known. Tragically,
in 1997, Gene died in a car crash during one of these searches.
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- It is only in the past few decades that scientists have
learnt how to clearly identify impact craters on the surface of the Earth.
In that time they have found more than 150 craters. Most are heavily disguised
by siltation, erosion and vegetation.
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- There are relatively few places on Earth where any geological
features can be expected to survive beyond tens of millions of years. Impact
craters have been found on most of the rare ancient landforms. In several
cases the estimated age of a large crater appears to match that of a mass
extinction event, as told in the fossil record. Although the picture is
still fuzzy -- due to the time scales involved -- massive impacts by comets
and asteroids deserve serious consideration as an explanation for some
of these extinction events.
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