[meteorite-list] Asteroid Breakup Covered The Earth In Extraterrestrial Dust

[Ron Baalke]

http://www.swri.org/9what/releases/2006/Asteroid.htm

Asteroid breakup event covered the Earth in extraterrestrial dust
Southwest Research Institute (SwRI) News

Boulder, Colorado -- January 18, 2006 -- 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 Nesvornyof the SwRI Space Studies Department 
in Boulder, Colo., along with Prof. David Vokrouhlicky 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 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 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," 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 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, Vokrouhlicky 
Bottke and Nesvorny, aappears in the Jan. 19 issue of Nature.

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

For more information contact Deb Schmid at (210) 522-2254, Communications 
Department, Southwest Research Institute, PO Drawer 28510, San 
Antonio, TX 78228-0510.

For more information, contact Deb Schmid, Communications Department, 
(210) 522-2046, Southwest Research Institute, PO Drawer 28510, 
San Antonio, TX 78228-0510.