- Fragments from a colossal collision that took place in
the inner asteroid belt 500 million years ago reached Earth in just 100,000
years - the quickest travel time on record, according to a new study.
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- The research supports models showing that asteroid smash-ups
at certain places in the Solar System can shoot debris on a fast track
towards Earth.
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- Many meteorites found over the past few decades show
signs of suffering a massive shock - as if from a single impact - 500 million
years ago. Astronomers believe a titanic crash between two asteroids occurred
then, about 300 million kilometres from the Sun in the Solar System's inner
asteroid belt.
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- Most debris from this smash is expected to take about
30 million years to reach Earth. But meteorites freshly unearthed from
ancient rocks in a Swedish quarry show that the first detritus from the
collision arrived on Earth in only 100,000 years.
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- Such speedy deliveries are predicted by computer models
if meteorite-producing collisions occur in particular zones around the
Sun, called resonant orbits. Objects in these zones are thought to be boosted
by gravitational kicks from the planets - mainly Jupiter and Saturn.
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- The models suggest that, if an impact occurs close to
the edge of such a zone, some debris may get injected into a resonant orbit,
causing it to speed towards Earth. But fragments that initially avoid the
zone, and take a more leisurely route through space, arrive on Earth many
millions of years later.
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- Trapped gases
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- Philipp Heck, a graduate student at the Swiss Federal
Institute of Technology in Zurich, Switzerland, and colleagues studied
nine meteorites from different layers in the quarry. To determine the time
it took them to travel to Earth, the researchers measured neon and helium
trapped in the space rocks.
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- These gases are produced when energetic particles called
cosmic rays hit silicon atoms in the meteorites. But cosmic rays can penetrate
only the top two metres of rock. So before the collision, most of the asteroid
would have been shielded from the particles.
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- After the break-up, however, cosmic rays were able to
zap the smaller fragments. This left behind different amounts of neon and
helium depending on how long the rocks spent travelling to Earth.
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- Meteorites at the greatest depths within the quarry showed
transit times of just 100,000 years. This is 100 times faster than the
leisurely 10 million years taken by meteorites on average.
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- Resonant orbits
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- "Near these places, the shape of the meteorites'
orbits gets changed rather quickly, and these fragments can quickly hit
Earth," Heck told New Scientist.
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- William Bottke, a planetary scientist at the Southwest
Research Institute in Boulder, Colorado, says the new data are "dead
on" with predictions. "For the first time, over timescales of
hundreds of millions of years, we really have some positive evidence our
models are working the way they should," he told New Scientist.
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- Nearly all of the asteroids that pass by Earth have been
redirected by these resonant orbits, but only about one per cent of those
objects will ever hit Earth, says Bottke. The chance of an asteroid collision
happening near a zone - similar to the violent crash 500 million years
ago - is also quite slim.
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- "But it would be quite spectacular," Heck told
New Scientist. "If humans were still here, they would probably see
100 times more bright fireballs, some visible in daylight. The meteorite
rain would last a million years."
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- Journal reference: Nature (vol 430, p 323)
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