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Asteroids hit earth more frequently than
thought
Thursday April 26, 2012 09:08:39 AM,
IANS
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Washington: Giant
asteroids, similar or larger than the one believed to have killed
the dinosaurs, hit Earth billions of years ago with more frequency
than previously thought, US space agency NASA said Wednesday.
To cause the dinosaur extinction, the killer asteroid that
impacted Earth 65 million years ago would have been almost 10 km
in diameter.
By studying ancient rocks in Australia and using computer models,
researchers estimate that approximately 70 asteroids the same size
or larger impacted Earth 1.8 to 3.8 billion years ago. During the
same period, approximately four similarly-sized objects hit the
moon, Xinhua reported.
Evidence for these impacts on Earth comes from thin rock layers
that contain debris of nearly spherical, sand-sized droplets
called spherules.
These millimeter-scale clues were formerly molten droplets ejected
into space within the huge plumes created by mega-impacts on
Earth.
The hardened droplets then fell back to Earth, creating thin but
widespread sedimentary layers known as spherule beds.
The findings were published Wednesday in the journal Nature.
"The beds speak to an intense period of bombardment of Earth,"
said William Bottke, principal investigator of the impact study
team at the Southwest Research Institute in Colorado. "Their
source long has been a mystery."
The team's findings support the theory Jupiter, Saturn, Uranus and
Neptune formed in different orbits nearly 4.5 billion years ago,
migrating to their current orbits about four billion years ago
from the interplay of gravitational forces in the solar system.
This event triggered a solar system-wide bombardment of comets and
asteroids called the "Late Heavy Bombardment".
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