[meteorite-list] Mineral Traces in Stardust Samples Upset Long-Held Assumptions About Origins of Comets

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Mar 16 18:08:08 2006
Message-ID: <200603141750.k2EHoJk05724_at_zagami.jpl.nasa.gov>

http://www.chron.com/disp/story.mpl/front/3721363.html

NASA find throws space experts

Mineral traces in Stardust samples upset long-held assumptions about
origins of comets

By MARK CARREAU
Houston Chronicle
March 14, 2006

Tiny pieces of minerals that form at high temperatures have been found
in the comet fragments retrieved by NASA's Stardust mission, scientists
announced Monday. The discovery challenges conventional thinking on how
comets - collections of ice and rock - formed in the early days of the
solar system.

The robotic Stardust spacecraft descended into the Utah desert by
parachute on Jan. 15, ending a seven-year, nearly 3-billion-mile journey
through the solar system to retrieve fragments of the comet Wild 2.

Astronomers have long assumed that comets formed in the most distant
reaches of the solar system, where temperatures barely rise above
absolute zero. But an initial examination of the Wild 2 fragments
revealed tiny pieces of minerals previously extracted from meteorites
that had been born close to the sun at temperatures exceeding 2,000
degrees Fahrenheit.

"This is very exciting. It's a mystery story," said University of
Washington astronomer Don Brownlee, who served as the chief scientist
for the $212 million Stardust mission. Brownlee and others presented
their findings to the 37th annual Lunar and Planetary Conference meeting
in League City during a three-hour session.

Analysis sought

The tiny fragments are being extracted in the same laboratory at
Houston's Johnson Space Center that houses the Apollo moon rocks. They
are being shipped to scientists around the world for additional analysis.

Astronomers believe comets are leftovers from a vast swirling disk of
gas and dust that provided the building blocks for the assembly of the
sun and planets 4.6 billion years ago.

The early studies found microscopic bits of peridot, diopside, anorthite
and other minerals rich in magnesium, calcium, aluminum and titanium in
the comet fragments. Until the Stardust findings, the minerals were
thought to reside no more distant than the asteroid belt between Mars
and Jupiter.

"There's a kind of temperature zoning in the solar system," said Mike
Zolensky, a mineralogist and Stardust co-investigator from Johnson Space
Center.

Meteorites comparison

According to Zolensky, there are two possible explanations for the
Stardust discovery.

A little-understood, con- veyor-belt mechanism may have shuttled
minerals forged close to the sun to the far reaches of the solar system.
Brownlee said the minerals could have been ejected billions of miles
along jet-like currents of matter called X-winds.

The minerals may have formed around other stars before drifting into the
comet-making zone.

Zolensky said experts will study oxygen, carbon and nitrogen atoms in
the mineral pieces to see if they match those found in meteorites. If
not, the minerals probably formed around distant stars.
Received on Tue 14 Mar 2006 12:50:17 PM PST