| Impact Event
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What is an
Impact Event?
Impact events are
caused by the collision of
large meteoroids, asteroids or comets with Earth and may sometimes
be followed by mass
extinctions of life.
(Artist's impression
of a major
impact event. The collision between Earth and an asteroid a few
kilometers in diameter may release as much energy as several million
nuclear bombs detonating.)
The geology of
Earth impacts
Centuries ago,
the Western vision of the past saw an Earth that had
been created a few thousand years ago, and had been shaped since that
time by a number of global cataclysms . This view gradually gave way to
the consensus that the Earth was several billion years old, and that
its features reflected the slow processes of gradual change.
Since 1970, in a
large part due to the work of Eugene Shoemaker, this
view has gradually expanded to accommodate the fact that the Earth has
in fact gone through periods of abrupt and catastrophic change due to
the impact of large
asteroids and comets on the
planet. A few of these impacts
may have caused massive climate
change and the extermination
of large
numbers of plants and animals.
The fact that
this modified view of the Earth's history did not emerge
until recently seems surprising. Based on crater formation rates
determined from the Earth's closest celestial partner, the Moon,
astronomers have determined that during the last 600 million years, the
Earth has been struck by 60 objects 5 kilometers or more across. The
smallest size of these impactors would release the equivalent of 10
million megatons of TNT and leave a crater 95 kilometers across. For
comparison, the largest nuclear weapon ever detonated, the Tsar Bomba,
had a yield of 50 megatons.
Mass
extinctions and impacts
In the past 600
million years there have been five major mass
extinctions that on average
extinguished half of all species. The largest mass
extinction to have affected
life
on Earth was the Permian-Triassic one that ended the Permian period 250
million years ago and killed off 90% of all species. The last such mass
extinction led to the demise of
the dinosaurs and has been found to have coincided with a large asteroid impact; this is the
Cretaceous-Tertiary (K-T) extinction event. There is no solid evidence
of impacts leading to the four other major mass extinctions, though
many scientists assume that they are at least related to impacts.
In 1980 Luis
Alvarez and his son Walter led a team from the University
of California, Berkeley that discovered unusually high concentrations
of iridium, an element that is rare in the Earth's crust but relatively
abundant in many meteorites. From the amount and distribution of
iridium present in the 65 million year old "iridium layer", the Alvarez
team later estimated that an asteroid of 10-14 kilometers must have
collided with the earth. This iridium layer at the K-T boundary has
been found worldwide at 100 different sites. Multidirectionally shocked
quartz (coesite), which is only known to form as the result of large
impacts or atomic bomb explosions, has also been found in the same
layer at more than 30 sites. Soot and ash at levels tens of thousands
times normal levels were found with the above.
Anomalies in
chromium isotopic ratios found within the K-T boundary
layer strongly support the impact theory. Chromium isotopic ratios are
homogeneous within the earth, therefore this isotopic anomalies exclude
a volcanic origin which was also proposed as a cause for the iridium
enrichment. Furthermore the chromium isotopic ratios determined in the
K-T boundary are similar to the chromium isotopic ratios found in
carbonaceous chondrites. Thus a probable candidate for the impactor is
a carbonaceous asteroid but also a comet is possible because comets are
assumed to consist of material similar to carbonaceous chondrites.
Probably the most
convincing evidence for a worldwide catastrophe was
the discovery of the crater which has since been named Chicxulub
Crater. This so-called smoking gun is centered on the Yucatan Peninsula
of Mexico and was discovered by Tony Camargo and Glen Pentfield while
working as geophysicists for the Mexican oil company PEMEX. What they
reported as a circular feature later turned out to be a crater
estimated to be 180 kilometers in diameter. Other researchers would
later find that the end-Cretaceous extinction event that wiped out the
dinosaurs had lasted for thousands of years instead of millions of
years as had previously been thought. This would be the final piece of
evidence that convinced the vast majority of scientists that this
extinction resulted from a point event that is most probably an
extra-terrestrial impact and not from increased volcanism and climate
change (which would spread its main effect over a much longer time
period).
It was the lack
of high concentrations iridium and shocked quartz which
has prevented the acceptance of the idea that the Permian extinction
(so-called mother of mass extinctions) was also caused by an impact.
However, during the late Permian all the continents were combined into
one supercontinent named Pangaea and all the oceans formed one
superocean, Panthalassa. If an impact occurred in the ocean and not on
land at all, then there would be little shocked quartz released (since
oceanic crust has relatively little silica) and much less material.
Although there is
now general agreement that there was a huge impact at
the end of the Cretaceous that led to the iridium enrichment of the K-T
boundary layer, remnants have been found of other impacts of the same
order of magnitude that did not result in any mass extinctions, and in
fact there is no clear linkage between an impact and any other incident
of mass extinction.
Nonetheless it is
now widely believed, if a little on faith, that mass
extinctions due to impacts are
an
occasional event in the history of the Earth. One such controversial
hypothesis is Tollmann's hypothetical bolide, which claims that the
Holocene was initiated by an impact.
Paleontologists
Michael Raup and Jack Sepkoski have proposed that an
extinction occurs roughly every 26 million years (though many are
relatively minor). This led physicist Richard Muller to suggest that
these extinctions could be due to a hypothetical companion star to the
sun called Nemesis periodically disrupting the orbits of comets in the
Oort cloud, and leading to a large increase in the number of comets
reaching the inner solar system where they might hit Earth.
Indeed, in the
early history of the Earth, about four billion years
ago, bolide impacts were almost certainly common since the skies were
far more full of "junk" than at present. Such impacts could have
included strikes by asteroids hundreds of kilometers in diameter, with
explosions so powerful that they vaporized all the Earth's oceans. It
was not until this "hard rain" began to slacken, so it seems, that life
could have begun to evolve on Earth.
The leading
theory of Moon's origin is the giant impact theory, which
states that Earth was once hit by a planetoid the size of Mars;
possibly the largest impact Earth has ever suffered.
Recent
pre-historic impact events
In addition to
the extremely large impacts that happen every few tens
of millions of years, there are many smaller impacts that occur much
more frequently but which leave correspondingly smaller traces behind.
Due to the strong forces of erosion at work on Earth, only relatively
recent examples of these smaller impacts are known. A few of the more
famous or interesting examples are:
Barringer Crater,
the first crater to be proven the result of an impact
the Rio Cuarto
craters, produced by an asteroid striking Earth at a
very low angle
the Wabar
craters, which apparently formed within the past few hundred
years
the Noerdlinger
Ries, a 24-km crater in Central Europe, formed about 15
million years ago.
Modern impact
events
The most
significant recorded impact in recent times was the Tunguska
event, which occurred at Tunguska in Russia, in 1908. But although the
Tunguska event was both spectacular and unparalleled in any historical
record, it no longer seems as unique and unusual as it once did. We now
know that Earth impacts, fairly big ones, are happening all the time.
The late Eugene
Shoemaker of the US Geological Survey came up with an
estimate of the rate of Earth impacts, and suggested that an event
about the size of the nuclear weapon that destroyed Hiroshima occurs
about once a year. Such events would seem to be spectacularly obvious,
but they generally go unnoticed for a number of reasons: the majority
of the Earth's surface is covered by water; a good portion of the land
surface is uninhabited; and the explosions generally occur at
relatively high altitude, resulting in a huge flash and thunderclap but
no real damage.
Some have been
observed, such as the Revelstoke fireball of 1965, which
occurred over the snows of northern Canada. Another fireball blew up
over the Australian town of Dubbo in April 1993, shaking things up but
causing no harm.
On the dark
morning hours of January 18, 2000, a fireball exploded over
the town of Whitehorse in the Canadian Yukon at an altitude of about 26
kilometers, lighting up the night like day and bringing down a third of
the Yukon's electrical power grid, due to the electromagnetic pulse
created by the blast. The meteor that produced the fireball was
estimated to be about 4.6 meters in diameter and with a weight of 180
tonnes.
A particularly
interesting fireball was observed moving north over the
Rocky Mountains from the US Southwest to Canada on August 10 1972, and
was filmed by a tourist at the Grand Teton National Park in Wyoming
with an 8-millimeter color movie camera. The object was in the range of
size from a car to a house and should have ended its life in a
Hiroshima-sized blast, but there was never any explosion, much less a
crater. Analysis of the trajectory indicated that it never came much
lower than 58 kilometers of the ground, and the conclusion was that it
had grazed Earth's atmosphere for about 100 seconds, then skipped back
out of the atmosphere to return to its orbit around the Sun.
Many impact
events occur without being observed by anyone on the
ground. Between 1975 and 1992, American missile early warning
satellites picked up 136 major explosions in the upper atmosphere.
The Tunguska
event was about a thousand times more powerful than such
events. Shoemaker estimated that one of such magnitude occurs about
once every 300 years. This is not a long interval even by historical
standards, and it is a somewhat nerve-wracking question to consider
when the next "Big One" will be, and more to the point, where.
The 1994 impact
of Comet Shoemaker-Levy 9 with Jupiter also served as a
"wake-up call", and astronomers responded by starting programs such as
Lincoln Near-Earth Asteroid Research (LINEAR), Near-Earth Asteroid
Tracking (NEAT), Lowell Observatory Near-Earth Object Search (LONEOS)
and several others which have drastically increased the rate of
asteroid discovery. Many objects undoubtedly still remain undetected,
however.
http://en.wikipedia.org/wiki/Impact_event
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