One of the biggest cosmic dust storms of the past 80 million years left a blanket of material on Earth after an asteroid in space broke apart, researchers said today.

The conclusion is based on evidence in ocean sediments, which computer models have tied to an observed bevy of asteroid siblings still roaming the solar system.

The thinking is that the space rocks were once part of a larger asteroid, some 100 miles (160 kilometers) wide, that broke up - possibly in a collision -  out in the asteroid belt between Mars and Jupiter. The drama took place 8.2 million years ago. That much has been reasoned before. The event would have created vast amounts of dust, some of which would have been scooped up by our planet.

In the Jan. 19 issue of the journal Nature, scientists report a spike in helium 3, a type of helium that's rare on Earth and typically of extraterrestrial origin, in a layer of sediment dated to that time frame.

The dust rich in helium 3 spiked about 8.2 million years ago and gradually decreased over the course of 1.5 million years, the data shows.

"The helium 3 spike found in these sediments is the smoking gun that something quite dramatic happened to the interplanetary dust population 8.2 million years ago," said Caltech geochemist Ken Farley.  "It's one of the biggest dust events of the last 80 million years."

Interplanetary dust is nothing new to scientists. About 20,000 tons of it lands on Earth every year. While in space, it picks up helium 3 from a wind of material flowing out from the Sun.

When asteroids collide, more debris wafts through the solar system. Much of it is drawn toward the Sun, and on its way in some is captured by Earth.

The particles are small and rare, making up much less than less than one part per million of terrestrial sediments, so the new discovery was no easy task. Farley and his colleagues studied sediment from beneath the Indian Ocean and the Atlantic, showing that the same event was recorded at two widely separated locations.

The long-gone large asteroid has been previously dubbed Veritas. It is survived by fragments that roam in orbits researchers have traced back to a common origin.

"While asteroids are constantly crashing into one another in the main asteroid belt," said study member William Bottke of the Southwest Research Institute, "only once in a great while does an extremely large one shatter."

Computer simulations show the Veritas event could have produced the sudden spike in dust and the gradual decline in accumulation.

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