[meteorite-list] Researchers Find Surprise in Makeup of a Comet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 15 Dec 2006 09:34:46 -0800 (PST)
Message-ID: <200612151734.JAA22382_at_zagami.jpl.nasa.gov>

http://www.nytimes.com/2006/12/15/science/space/15comet.html

Researchers Find Surprise in Makeup of a Comet
By WARREN E. LEARY
New York Times
December 15, 2006

WASHINGTON, - Comets are not all made of interstellar dust and
ice, but instead may contain material shot from the heart of the solar
system during its tumultuous birth, scientists reported Thursday after
examining pristine particles of a comet that were brought back by the
Stardust spacecraft.

The evidence suggests that comets did not form in isolation in the outer
parts of the solar system as it coalesced from a swirling mass of primal
material, the researchers said. Instead, they said, some of the hot
material that formed planets around the Sun seems to have spewed off
into distant areas and become a component of distant comets.

"Many people imagined that comets formed in total isolation from the
rest of the solar system; we have shown that's not true," said Donald
Brownlee, a University of Washington astronomer who is the lead scientist
for the Stardust mission.

"As the solar system formed 4.6 billion years ago," Dr. Brownlee said,
"material moved from the innermost part to the outermost part. I think
of it as the solar system partially turning itself inside out."

The first results of Stardust, appearing in seven reports published in
the Dec. 15 issue of the journal Science, were reported in San Francisco
on Thursday at the fall meeting of the American Geophysical Union.

NASA launched Stardust in 1999, and the robot spacecraft met comet Wild 2
beyond the orbit of Mars in January 2004. The craft flew within 150
miles of the comet's nucleus and trapped particles spewing from the body
in a light, porous foam called aerogel. After a 2.88-billion-mile
journey, Stardust returned to Earth last January with a payload of
thousands of tiny particles from Wild 2.

The comet formed more than 4.5 billion years ago and had remained
preserved in the frozen reaches of the outer solar system until 1974,
when a close encounter with Jupiter
shifted its orbit to a path between Jupiter and Mars.

More than 180 scientists from around the world examined some of the
samples with specialized equipment to determine what makes up the icy,
dusty comets that largely populate a vast area beyond the orbits of
Pluto and Neptune.

"Comet dust seems to be a real zoo of things; we see all kinds of
particles that are clearly formed at different places, possibly at
different times and certainly under different conditions," said Scott
Sandford of the NASA Ames Research Center in California, who was the
lead author of one of the papers.

Dr. Sandford said results of Stardust studies so far "all indicate that
when the solar system was forming, there was a whole lot of mixing going
on."

Dr. Brownlee estimates that as much as 10 percent of the material in
comets came from the inner solar system. "That's a real surprise,' he
said, "because the common expectation was that comets would be made of
interstellar dust and ice."

"It's not just dust and particles," he said. "We are working on rocks -
some of them igneous rocks, formed by heating and melting. We want to
know how these rocks were formed and how they became parts of comets
that were formed far out on the edge of the solar system."

Material from comet Wild 2 has mineral characteristics that appear to be
different from those observed in comet Tempel 1. In that case, involving
a spacecraft called Deep Impact, a probe crashed into Tempel 1's surface
in July 2005, and the properties of the resulting dust were analyzed by
the spacecraft and distant telescope observations. Dr. Brownlee noted
that whereas Tempel 1 had been examined from a distance, Stardust had
returned actual samples for scientists to study.

"The comets may be different from each other," he said, 'or different
observations could simply be a result of the different techniques used
to examine them. It is a challenge for us to understand how they are
different, and why."

Michael F. A'Hearn, chief scientist for the Deep Impact mission, said it
was too early to say whether there were significantly different classes
of comets or whether Wild 2 and Tempel 1 reflected different stages of
cometary evolution. Tempel 1 has traversed the inner solar system for
hundreds of years, while Wild 2 is a new arrival, Dr. A'Hearn said, and
Stardust gathered surface material while Deep Impact blasted out part of
the interior.

"We need more analysis of the data we already have," he said, "and we
certainly need more comet sample missions to fully understand these bodies."
Received on Fri 15 Dec 2006 12:34:46 PM PST