Boulder, Colo. – Scientists from the California Institute of Technology (Caltech), Southwest Research Institute® (SwRI®), and Charles University in the Czech Republic have made the first positive link between a breakup event in the main asteroid belt and a large quantity of interplanetary dust particles deposited on Earth.

Sediments found in oceanic core samples indicate that millions of years ago, the Earth was blanketed by extraterrestrial dust. Computer simulations indicate these particles are fallout from the breakup of a large asteroid in the main asteroid belt, a population of interplanetary bodies ranging from tiny pebbles to Texas-sized rocks located between the orbits of Mars and Jupiter. These findings appear in the Jan. 19 issue of the journal Nature.

Interplanetary dust is composed of bits of rock – from a few to several hundred microns in diameter – produced by asteroid collisions or ejected from comets. Interplanetary dust migrates toward the Sun, and en route some of this dust is captured by the Earth's gravitational field and deposited on its surface. Presently, more than 20,000 tons of this material accumulates on Earth each year, but the accretion rate should fluctuate with the number of asteroid collisions and active comets. By looking at ancient sediments that include both interplanetary dust and ordinary terrestrial sediment, it should be possible to detect major dust-producing solar system events in the past.

Because interplanetary dust particles are so small and rare in sediment – significantly less than a part per million – they are difficult to detect using direct measurements. However these particles are extremely rich in a rare isotope of helium – helium 3 – compared with terrestrial materials. Over the past decade, Professor Ken Farley of Caltech has measured helium 3 concentrations in sediments formed over the last 75 million years to create a record of the interplanetary dust flux.

Recently, Farley found a large excess of helium 3 in some 8.2 million-year-old sediments, indicating that the accretion rate of interplanetary dust suddenly increased by a factor of about 4 and then decreased over about 1.5 million years to pre-event levels. To assure that the peak was not a fluke present at only one site on the seafloor, two different localities were studied: one in the Indian Ocean and one in the Atlantic. The event is recorded clearly at both sites.

"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. It's one of the biggest dust events of the last 80 million years," says Farley.

To find the source of these particles, Dr. William F. Bottke and Dr. David Nesvorný of the SwRI Space Studies Department in Boulder, Colo., along with Prof. David Vokrouhlický, a visiting scientist at SwRI from Charles University in Prague, studied clusters of asteroid orbits that are likely the consequence of ancient asteroidal impacts.

"While asteroids are constantly crashing into one another in the main asteroid belt," says Bottke, "only once in a great while does an extremely large one shatter."

The scientists identified one cluster of asteroid fragments whose size, age and remarkably similar orbits made it a likely candidate for the Earth-dusting event. Tracking the orbits of the cluster backwards in time using computer models, they found that, 8.2 million years ago, all of its fragments shared the same orbital orientation in space. This event defines when the 100-mile-wide asteroid called Veritas was blown apart by impact and coincides with the spike in interplanetary seafloor sediments described above.

"The Veritas disruption was extraordinary," says Nesvorný. "It was the largest asteroid collision to take place in the last 100 million years."

As a final check, the SwRI-Czech team used computer simulations to follow the evolution of dust particles produced by Veritas breakup. Their work shows that the Veritas event could produce the spike in extraterrestrial dust raining on the Earth as well as a gradual decline in the dust flux.

"The match between our model results and the helium 3 deposits is very compelling," Vokrouhlický says. "It makes us wonder whether other helium 3 peaks in oceanic cores can also be traced back to asteroid breakups."

This research was funded by NASA's Planetary Geology and Geophysics program and received additional financial support from the Czech Republic grant agency and the National Science Foundation's COBASE program. The paper, "A Late Miocene Dust Shower from the Breakup of an Asteroid in the Main Belt" by Farley, Vokrouhlický, Bottke and Nesvorný, appears in the Jan. 19 issue of Nature.

EDITORS: High-resolution image for download is available at http://www.swri.org/press/2006/asteroidbreakup.htm

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