[meteorite-list] Stardust Comet Dust Resembles Asteroid Materials

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 25 Jan 2008 08:33:44 -0800 (PST)
Message-ID: <200801251633.IAA24971_at_zagami.jpl.nasa.gov>

https://publicaffairs.llnl.gov/news/news_releases/2008/NR-08-01-05.html

Lawrence Livermore National Laboratory
News Release

Contact: Anne M. Stark
Phone: (925) 422-9799
E-mail: stark8 at llnl.gov

FOR IMMEDIATE RELEASE
January 24, 2008
NR-08-01-05

Stardust comet dust resembles asteroid materials

LIVERMORE, Calif. - Contrary to expectations for a small icy body, much
of the comet dust returned by the Stardust mission formed very close to
the young sun and was altered from the solar system's early materials.

When the Stardust mission returned to Earth with samples from the comet
Wild 2 in 2006, scientists knew the material would provide new clues
about the formation of our solar system, but they didn't know exactly how.

New research by scientists at Lawrence Livermore National Laboratory and
collaborators reveals that, in addition to containing material that
formed very close to the young sun, the dust from Wild 2 also is missing
ingredients that would be expected in comet dust. Surprisingly, the
Wild 2 comet sample better resembles a meteorite from the asteroid belt
rather than an ancient, unaltered comet.

Comets are expected to contain large amounts of the most primitive
material in the solar system, a treasure trove of stardust from other
stars and other ancient materials. But in the case of Wild 2, that
simply is not the case.

By comparing the Stardust samples to cometary interplanetary dust
particles (CP IDPs), the team found that two silicate materials normally
found in cometary IDPs, together with other primitive materials
including presolar stardust grains from other stars, have not been found
in the abundances that might be expected in a Kuiper Belt comet like
Wild 2. The high-speed capture of the Stardust particles may be partly
responsible; but extra refractory components that formed in the inner
solar nebula within a few astronomical units of the sun, indicate that
the Stardust material resembles chondritic meteorites from the asteroid
belt.

"The material is a lot less primitive and more altered than materials we
have gathered through high altitude capture in our own stratosphere from
a variety of comets," said LLNL's Hope Ishii, lead author of the
research that appears in the Jan. 25 edition of the journal, Science.
"As a whole, the samples look more asteroidal than cometary."

Because of its tail formed by vaporizing ices, Wild 2 is, by definition,
a comet. "It's a reminder that we can't make black and white
distinctions between asteroids and comets," Ishii said. "There is a
continuum between them."

The surprising findings contradict researchers' initial expectations for
a comet that spent most of its life orbiting in the Kuiper Belt, beyond
Neptune. In 1974, Wild 2 had a close encounter with Jupiter that placed
it into its current orbit much closer to Earth.

Comets formed beyond the so-called frost line where water and other
volatiles existed as ices. Because of their setting far from the sun,
they have been viewed as a virtual freezer, preserving the original
preliminary ingredients of the solar system's formation 4.6 billion
years ago. The Stardust spacecraft traveled a total of seven years to
reach Wild 2 and returned to Earth in January 2006 with a cargo of tiny
particles for scientist to analyze.

This is one of the first studies to closely compare Stardust particles
to CP IDPs. This class of IDPs is believed to contain the most primitive
and unaltered fraction of the primordial material from which our planets
and other solar system objects formed. They are highly enriched in
isotopically anomalous organic and inorganic outer solar nebula
materials inherited - through the presolar molecular cloud - from dust
produced around other stars. IDPs are gathered in the stratosphere by
high altitude airplanes (ER-2s and WB-57s) that are typically more than
50 years old.

The Livermore team specifically searched for two silicate materials in
Stardust that are believed to be unique to cometary IDPs: amorphous
silicates known as GEMS (glass with embedded metal and sulfides); and
sliver-like whiskers of the crystalline silicate enstatite (a
rock-forming mineral). Surprisingly, the team found only a single
enstatite whisker in the Stardust samples, and it had the wrong
crystallographic orientation - a form typical of terrestrial and
asteroidal enstatite.

Objects similar to GEMS were found, but Ishii and the team showed they
were actually created during the high speed 6-kilometer per second
impact of Wild 2 comet dust with the Stardust spacecraft's collector by
making similar material in the laboratory.

In analyzing the Stardust material, Ishii's team used Livermore's
SuperSTEM (scanning transmission electron microscope). Ishii said future
analyses should focus on larger-grained materials, so-called
micro-rocks, which suffered less alteration.

"The material found in primitive objects just wasn't there in the
samples," said John Bradley, another LLNL author. "I think this is
science in action. It's really exciting because it's just not what we
expected."

"Wild 2 doesn't look like what we thought all comets should look like,"
Ishii said. "The Stardust mission was a real success because without it,
we would never have learned these things about our solar system. The
sample return was vital for us to continue to unravel how our solar
system formed and evolved."

In addition to Ishii and Bradley, other LLNL researchers include Zu Rong
Dai, Miaofang Chi and Nigel Browning. Other institutions involved
include UC Davis, the Natural History Museum of London, the University
of Kent and the Netherlands Organization for Scientific Research (NWO).

Stardust is a part of NASA's series of Discovery missions and is managed
by the Jet Propulsion Laboratory. Stardust launched in February 1999 and
set off on three giant loops around the sun. It began collecting
interstellar dust in 2000 and met Wild 2 in January 2004, when the
spacecraft was slammed by thousands of comet particles including some
the size of BBs that could have compromised the mission. It is the first
spacecraft to safely make it back to Earth with cometary dust particles
in tow.

Founded in 1952, Lawrence Livermore National Laboratory is a national
security laboratory, with a mission to ensure national security and
apply science and technology to the important issues of our time.
Lawrence Livermore National Laboratory is managed by Lawrence Livermore
National Security, LLC for the U.S. Department of Energy's National
Nuclear Security Administration.
Received on Fri 25 Jan 2008 11:33:44 AM PST


Help support this free mailing list:



StumbleUpon
del.icio.us
reddit
Yahoo MyWeb