[meteorite-list] Comet Chasers Get Mineral Shock (Stardust)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Mar 16 18:08:03 2006
Message-ID: <200603141746.k2EHkag04695_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2006/060313/full/440260a.html

Comet chasers get mineral shock
Stardust mission yields white hot results.
Mark Peplow
nature.com
March 14,2 006

The first results from a mission to catch dust from a comet's tail have
revealed a surprise: these balls of dirty snow are born of fire as well
as ice. Scientists were stunned to find a huge range of minerals in the
particles captured by NASA's Stardust probe as it swooped past the comet
Wild 2 on 2 January 2004. Many of the compounds could only have formed
close to a star - far from the chilly outskirts of the Solar System
where the comet first coalesced.

The Stardust team presented its preliminary results to a packed room of
more than 600 scientists at the Lunar and Planetary Science Conference
in League City, Texas, on 13 March. The grains are the first material
ever brought back from a comet. Indeed, they are the first geological
samples collected from space since the Soviet Luna 24 mission brought
back Moon rocks two decades ago.

Of the 30 or so microscopic grains studied so far, almost a quarter are
of a type known as calcium-aluminium inclusions (CAIs). These solidify
at several thousand degrees and have never been detected in remote
observations of comets. "When these grains first formed they were white
hot," says Don Brownlee of the University of Washington, Seattle, who is
the mission's chief scientist.

One explanation for the grains' presence is that they were created very
close to the young Sun, and then flung to the outer reaches of the dusty
cloud that spawned the planets. If that is the case, planetary
scientists have a lot of rethinking to do - it would mean that the
primitive Solar System saw a lot more mixing than previously believed.

The presence of CAIs was predicted by one controversial theory, however.
The 'X-wind' model sees strong magnetic fields around the young Sun
channelling heated material to the far reaches of the protoplanetary
cloud. The Stardust results do not prove anything, but they certainly
fit with the idea, says Mike Zolensky of NASA's Johnson Space Center in
Houston, Texas, who is leading the mineral analyses.

The other possible source of the minerals is another star completely.
Once formed, the grains may have drifted through interstellar space for
eons, before reaching our own Solar System and being incorporated into
Wild 2. Zolensky says it is too early to know which theory is right. But
Brownlee adds that measurements of isotopes in the grains could settle
the question within months - such materials often bear a distinctive
isotopic signature determined by their star of origin.

Scientists involved in the work are jubilant about the progress so far.
Last month, Nature spent a day with Phil Bland, a planetary scientist at
Imperial College London, as he and his team analysed part of a grain
(see 'A comet's tale
<http://www.nature.com/news/2006/060213/full/060213-2.html>').

Nature was asked by NASA not to report the results at the time, but
Bland was excited to find large amounts of calcium. He speculated that
it might be present in the form of calcium carbonate - even making a bet
with his colleague Matt Genge. This mineral almost always forms in
water, and would imply that the comet had been heated enough for its ice
to melt, perhaps by collisions with meteorites.

Bland's sample has now been shown to be a CAI, and NASA scientists have
not yet found any carbonates in their grains. So Bland owes Genge
dinner, but he is still overjoyed to have found hints of the comet's
fiery past.

And he is not the only one. "Stardust has exceeded all my expectations,"
says Peter Tsou of NASA's Jet Propulsion Laboratory in Pasadena,
California. Tsou developed the technique of catching comet grains in an
aerogel, an extremely lightweight, glass-like substance, and he has been
overseeing efforts to extract dust from the carrot-shaped impact tracks.
"This has been my dream for 25 years," he says.

Tsou adds that the success of Stardust vindicates the sometimes maligned
'faster, better, cheaper' approach to NASA missions in the 90s, which
was designed to focus on low-budget missions over short timescales. Some
scientists fear that such missions are now in danger of being overlooked
in favour of giant projects such as the James Webb Space Telescope (see
' US astronomy: Is the next big thing too big?
<http://www.nature.com/news/2006/060306/full/440140a.html>'), especially
in a NASA budget that is feeling the strain of the shuttle and
International Space Station programmes.

In the meantime, work on the grains continues. Thousands of particles
have yet to be analysed, says Brownlee, and "we could easily work for a
year on a single particle".
Received on Tue 14 Mar 2006 12:46:36 PM PST