[meteorite-list] Stardust Findings Suggest Comets More Complex Than Thought

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 14 Dec 2006 13:42:38 -0800 (PST)
Message-ID: <200612142142.NAA13695_at_zagami.jpl.nasa.gov>

http://stardust.jpl.nasa.gov/news/news110.html

Stardust Findings Suggest Comets More Complex Than Thought
December 14, 2006

Comets may be more than just simple conglomerations of
ice, dust and gases. Some may be important windows on
the early solar system. Others may have contributed
materials necessary to the development of life on our
own planet. Scientists have found a wide range of
compositions and structures for the comet Wild 2
particles that were captured and returned to Earth by
NASA's Stardust spacecraft. Their findings indicate the
formation of at least some comets may have included
materials ejected from the inner solar system to the far
and cold outer edge of the solar nebula.

Minerals formed near the sun or other stars were in the
samples returned to Earth by the Stardust spacecraft in
January 2006. The findings suggest materials from the
inner regions of the solar system could have traveled
to the outer reaches where comets formed. This may alter
the way scientists view the formation and composition of
comets.

"We have found very high-temperature minerals, which
supports a particular model where strong bipolar gas
jets coming out of the early sun propelled material
formed near to the sun outward to the outer reaches of
the solar system," said Michael Zolensky, NASA cosmic
mineralogist and Stardust co-investigator at the
Johnson Space Center (JSC), Houston. "It seems that
comets are not composed entirely of volatile rich
materials but rather are a mixture of materials formed
at all temperature ranges, at places very near the early
sun and at places very remote from it."

Zolensky is lead author of a paper on the mineralogy of
the captured comet particles, one of seven reports about
the mission's initial findings appearing in the Dec. 15
issue of Science Express, the online edition of the
journal Science.

Scientists have long thought of comets as cold, billowing
clouds of ice, dust and gases formed on the edges of the
solar system. But comets may not be so simple or similar.
They may prove to be diverse bodies with complex
histories. Comet Wild 2 certainly is made up of
components with a more complex history than thought.

Scientists found a wide variety in particle composition
and size in the Wild 2 samples. Most of the Wild 2
samples appear to be weakly constructed mixtures of very
small grains with a few larger grains. Also, a wide range
of high- and low-temperature minerals, from olivine to
low- and high-calcium pyroxene compositions, is present
in the Wild 2 samples.

Such a diversity of high- and low-temperature minerals
requires a wide range of formation conditions, probably
reflecting different formation locations. Many particles
did not form in the cold environment and locations where
cometary ices condensed.

Instead, they needed high temperatures to form, as well
as complex and as yet little understood dynamical
processes to end up where comets actually formed. Also,
particles from different environments must have undergone
some process of accretion to end up as aggregates composed
of different minerals.

One of the major discoveries from the analysis of the comet
samples was finding particles rich in organic matter.
"Comets are believed to have brought water and organic
matter to the early Earth, and it is important to
understand the nature of these materials because they
are necessary ingredients for the origin of life," said
Lindsay Keller, NASA scientist at JSC and Stardust
co-investigator. "One of the first analyses we obtained
on the samples showed abundant hydrocarbons in many of the
particles."

Subsequent analyses revealed that some of the organic
matter formed in the cold cloud of dust and gas that was
the precursor to the solar system.

Keller led a team of U.S. and international scientists
using spectroscopic techniques to study the Stardust samples.
Spectroscopy uses different wavelengths of light and other
radiation to discover the chemical makeup of materials.
The team's results also appear in this week's issue of
Science.

In addition to finding these varied compositions in the
Wild 2 samples, Stardust investigators found a wide
diversity of particle densities. The captured Wild 2
samples are predominantly fine-grained, loosely bound
aggregates, most also containing much larger individual
crystals of olivine, pyroxene and iron/nickel sulfides.
All analyses suggest that small and large Wild 2
particles are composed of a similar, if not identical,
suite of minerals.

"Many researchers, but not all, have thought that
cometary solids are similar to interstellar dust, which
is generally fine grained," said Friedrich Horz, NASA
planetary scientist at JSC and lead author of a paper on
impact features on Stardust and comet Wild 2 dust. "These
models entail that such particles existed during the
formation of comets, as 'leftovers' from the initial gas
and solid mixture that were not processed further during
comet formation in the cold environment from the sun."

Scientists found a much wider diversity of particle
densities, including dense minerals, than advocated
earlier by some researchers.

"We see and demonstrate for the first time that there is a
continuum between fine particles and more dense objects,
the latter including pure minerals of, for example,
olivine and iron sulfide," said Horz. "The range in
density and cohesion that we saw and infer thus
disagrees specifically with the popular model that
comets are composed of only fine grains. A minority,
however, did allow for particles of variable density.
We have confirmed the latter viewpoint."

Stardust's pre-mission predictions included a wide range
of sizes. Thus the findings substantiate the view of
highly variable particle sizes. The specific size
distribution found in Wild 2 samples differs from that
of comet Halley dust (Stardust has fewer fine-grained
particles) and from that of comet Grigg-Skjellerup
(Stardust is more fine-grained).

NASA's Jet Propulsion Laboratory, Pasadena, Calif.,
manages the Stardust mission for NASA's Science Mission
Directorate, Washington. Dr. Peter Tsou of JPL is the
deputy principal investigator and is a co-author on
seven reports about the mission's initial findings
appearing in the Dec. 15 issue of Science Express,
the on-line edition of the journal Science.

For more information about Stardust studies and other
mission information, visit:

http://stardust.jpl.nasa.gov/

JPL Stardust Media Contact:
DC Agle
818-393-9011
agle at jpl.nasa.gov
Received on Thu 14 Dec 2006 04:42:38 PM PST