[meteorite-list] Dust Found in Earth Sediment Traced to Breakup of Asteroid 8.2 Million Years Ago

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Feb 1 15:56:41 2006
Message-ID: <200602012054.k11Ksun13642_at_zagami.jpl.nasa.gov>


Dust Found in Earth Sediment Traced to Breakup of the Asteroid
Veritas 8.2 Million Years Ago

Caltech News Release
Contact: Robert Tindol (626) 395-3631 tindol_at_caltech.edu
January 18, 2006

PASADENA, Calif.--In a new study that provides a novel way of looking at
our solar system's past, a group of planetary scientists and geochemists
announce that they have found evidence on Earth of an asteroid breakup
or collision that occurred 8.2 million years ago.

Reporting in the January 19 issue of the journal Nature, scientists from
the California Institute of Technology, the Southwest Research Institute
(SwRI), and Charles University in the Czech Republic show that core
samples from oceanic sediment are consistent with computer simulations
of the breakup of a 100-mile-wide body in the asteroid belt between Mars
and Jupiter. The larger fragments of this asteroid are still orbiting
the asteroid belt, and their hypothetical source has been known for
years as the asteroid "Veritas."

Ken Farley of Caltech discovered a spike in a rare isotope known as
helium 3 that began 8.2 million years ago and gradually decreased over
the next 1.5 million years. This information suggests that Earth must
have been dusted with an extraterrestrial source.

"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," says Farley, the Keck Foundation Professor of
Geochemistry at Caltech and chair of the Division of Geological and
Planetary Sciences. "It's one of the biggest dust events of the last 80
million years."

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 level of
asteroid collisions and changes in the number of active comets. By
looking at ancient sediments that include both interplanetary dust and
ordinary terrestrial sediment, the researchers for the first time have
been able to detect major dust-producing solar system events of 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 helium 3, in comparison with terrestrial materials.
Over the last decade, Ken Farley has measured helium 3 concentrations in
sediments formed over the last 80 million years to create a record of
the interplanetary dust flux.

To assure that the peak was not a fluke present at only one site on the
seafloor, Farley studied two different localities: one in the Indian
Ocean and one in the Atlantic. The event is recorded clearly at both sites.

To find the source of these particles, William F. Bottke and David
Nesvorny of the SwRI Space Studies Department in Boulder, Colorado,
along with David Vokrouhlicky of Charles University, studied clusters of
asteroid orbits that are likely the consequence of ancient asteroidal

"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 the interplanetary
seafloor sediments Farley had found.

"The Veritas disruption was extraordinary," says Nesvorny. "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 the 100-mile-wide
Veritas breakup event. Their work shows that the Veritas event could
produce the spike in extraterrestrial dust raining on the Earth 8.2
million years ago as well as a gradual decline in the dust flux.

"The match between our model results and the helium 3 deposits is very
compelling," Vokrouhlicky 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 & Geophysics
program and received additional financial support from Czech Republic
grant agency and the National Science Foundation's COBASE program. The
Nature paper is titled "A late Miocene dust shower from the breakup of
an asteroid in the main belt."
Received on Wed 01 Feb 2006 03:54:55 PM PST

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