Asteroid Dust May Have
Helped Kill Dinosaurs
By Maggie Fox
Health and Science Correspondent

WASHINGTON (Reuters) - Space dust from broken-up asteroids may have started the dinosaurs down the road to gradual extinction, astronomers said Thursday. The final blow probably came when larger pieces of the asteroids actually collided with the Earth, the researchers at the University of Florida and the Carnegie Institution of Washington said. The good news, according to the report in the journal Science, is that Earth would have about a million years warning before another such asteroid impact.
Stephen Kortenkamp, a planetary scientist at the Carnegie Institution who co-wrote the report, said astronomers have known about this space dust for some time. He and Stanley Dermott at the University of Florida ran computer models based on a collection of observations on Earth and in space. ``You would have an asteroid that would have been in a collision. The type of asteroid is called a rubble-pile asteroid,'' Kortenkamp said in a telephone interview.
``It's just a collection of debris that ranges in size from dust particles to two-km (one-mile) sized bodies.'' The dust strings out, and such scatterings of dust have been seen strung between the asteroid belt and the Earth, Kortenkamp said.
It comes from three ``families'' of asteroids -- agglomerations of rocks and dust that were once a single asteroid. The asteroid belt lies between Mars and Jupiter. The dust, being lighter than the larger bits, is more easily influenced by the Sun's rays. It ``falls'' in toward the Sun much faster than the larger pieces, gradually spiraling in as it orbits.
The Earth passes through this dust cloud as it orbits, too. ``The dust gets here first and any impacters that come, come later,'' Kortenkamp said. Kortenkamp and Dermott, who looked at other researchers' studies of space dust taken from sea floor sediments, said they found the dust accumulates on a 100,000-year cycle. ``There are two different mechanisms -- one delivers the dust and another delivers the asteroids, dust first, and the fragments a million years later,'' Kortenkamp said. There was evidence for this in the geological record, Kortenkamp added.
This could explain why the dinosaurs appear to have been gradually becoming extinct before the asteroid collision that is believed to have caused the big extinction 65 million years ago. Currently, about 30,000 metric tons of dust from these asteroids falls to the Earth every year, but in the past the rate has been as much as 10 million tons a year. ``That is comparable to what we know is injected into the atmosphere by a large volcano, and we know volcanoes have affected the climate,'' Kortenkamp said.
That much matter could cause a ``nuclear winter'', Kortenkamp said -- a pall of dust that would block out so much sun that temperatures would fall, the ice caps would grow and much plant and animal life would die off. None of this has immediate implications for humanity, but is of great geological interest, Kortenkamp said. ``It's not like you find one and a week later it's got you,'' he said. ``It's more looking back in time.'' He said this accretion of dust probably did not cause the most recent Ice Ages because the timescales are too long, although it may have caused ice ages millions of years ago.

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