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Space

Eon of dust storms traced to asteroid smash

By Maggie Mckee

18 January 2006

A cosmic dust shower that pelted Earth for one-and-a-half million years, more than eight million years ago, has been traced to the break-up of a large asteroid whose remnants still orbit the Sun, a new study reveals. The research may shed light on the effect of extraterrestrial events on the Earth’s climate and life.

Scientists studying the composition of sediments on the ocean floor have noticed two layers – one laid down about 35 million years ago and another 8.2 million years ago – rich in the isotope helium-3. Interplanetary dust contains 100 million times more helium-3 than dust on Earth because it absorbs the isotope from the solar wind, suggesting the layers were produced by cosmic dust showers.

Researchers have not been able to trace the older shower to a specific cosmic event such as a comet or asteroid impact. But now a team led by Kenneth Farley of the California Institute of Technology in Pasadena, US, says a collision in the asteroid belt produced the cosmic dust shower 8.2 million years ago.

They came to their conclusion after analysing the orbits of individual asteroids that travel in loose groups, or families. These families are thought to have formed from a single, large asteroid that broke up after being struck by another space rock in the asteroid belt between Mars and Jupiter. Researchers can date the break up by running models that “rewind” the motion of the remnants until they coalesce into a single body.

When the researchers studied one family of 300 asteroids, called Veritas, they found the break-up occurred 8.3 million years ago – exactly the right time to produce the helium-3 signal in the ocean sediment samples. “The fact you can see these asteroid fragments converging at the same time period tells you that’s the source,” Farley told New Scientist.

Climate change

The collision that spawned the Veritas family produced dust that spread throughout the plane of the solar system in just a few hundred years, says Farley. Much of this fell towards the Sun and in the process rained down on Earth for about one-and-a-half million years. At the peak of the shower, about five times as much interplanetary dust probably fell on Earth than drizzles down today.

The coupling of ocean core data with models of asteroid orbits is “really one of the only ways you can get at the history of the solar system,” says Farley. He says the possibility that the dinosaurs died off after a major impact 65 million years ago highlights the importance of understanding how cosmic events affected Earth and its life.

Colleague Bill Bottke of the Southwest Research Institute in Boulder, Colorado, US, agrees. “It is possible that significant changes in the amount of extraterrestrial dust reaching Earth may have affected Earth’s climate in the past,” he says.

Indeed, the Earth may have cooled 8.27 million years ago by about 0.7°C – a small but significant amount, says climate modeller Thomas Crowley of Duke University in Durham, North Carolina, US. “The climate effect could come through some reduction of solar radiation by dust clouds,” Crowley told New Scientist. “But we won’t know until we look more closely at the data and do some model calculations.”

Bottke hopes to study the helium-3 data covering the last few million years more closely and to search the sedimentary record for traces of dust from smaller break up events in the asteroid belt. He says: “We believe our work has only scratched the surface of this interesting data set.”

Journal reference: Nature (vol 439, p 295)

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