[meteorite-list] Comet condrules

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu, 18 Sep 2008 23:29:31 -0400
Message-ID: <2276d4lj6edkj5j2nbmr4k6f8i0gdquj00_at_4ax.com>

http://www.sciencedaily.com/releases/2008/09/080918170408.htm

Comet Dust Reveals Unexpected Mixing Of Solar System

ScienceDaily (Sep. 18, 2008) ? Chemical clues from a comet's halo are
challenging common views about the history and evolution of the solar system and
showing it may be more mixed-up than previously thought.

A new analysis of dust from the comet Wild 2, collected in 2004 by NASA's
Stardust mission, has revealed an oxygen isotope signature that suggests an
unexpected mingling of rocky material between the center and edges of the solar
system. Despite the comet's birth in the icy reaches of outer space beyond
Pluto, tiny crystals collected from its halo appear to have been forged in the
hotter interior, much closer to the sun.

The result, reported in the Sept. 19 issue of the journal Science by researchers
from Japan, NASA and the University of Wisconsin-Madison, counters the idea that
the material that formed the solar system billions of years ago has remained
trapped in orbits around the sun. Instead, the new study suggests that cosmic
material from asteroid belts between Mars and Jupiter can migrate outward in the
solar system and mix with the more primitive materials found at the fringes.

"Observations from this sample are changing our previous thinking and
expectations about how the solar system formed," says UW-Madison geologist
Noriko Kita, an author of the paper.

The Stardust mission captured Wild 2 dust in hopes of characterizing the raw
materials from which our solar system coalesced. Since the comet formed more
than 4 billion years ago from the same primitive source materials, its current
orbit between Mars and Jupiter affords a rare opportunity to sample material
from the farthest reaches of the solar system and dating back to the early days
of the universe. These samples, which reached Earth in early 2006, are the first
solid samples returned from space since Apollo.

"They were originally hoping to find the raw material that pre-dated the solar
system," explains Kita. "However, we found many crystalline objects that
resemble flash-heated particles found in meteorites from asteroids."

In the new study, scientists led by Tomoki Nakamura, a professor at Kyushu
University in Japan, analyzed oxygen isotope compositions of three crystals from
the comet's halo to better understand their origins. He and UW-Madison scientist
Takayuki Ushikubo analyzed the tiny grains ? the largest of which is about
one-thousandth of an inch across ? with a unique ion microprobe in the Wisconsin
Secondary Ion Mass Spectrometer (Wisc-SIMS) laboratory, the most advanced
instrument of its kind in the world.

To their surprise, they found oxygen isotope ratios in the comet crystals that
are similar to asteroids and even the sun itself. Since these samples more
closely resemble meteorites than the primitive, low-temperature materials
expected in the outer reaches of the solar system, their analysis suggests that
heat-processed particles may have been transported outward in the young solar
system.

"This really complicates our simple view of the early solar system," says
Michael Zolensky, a NASA cosmic mineralogist at the Johnson Space Center in
Houston.

"Even though the comet itself came from way out past Pluto, there's a much more
complicated history of migration patterns within the solar system and the
material originally may have formed much closer to Earth," says UW-Madison
geology professor John Valley. "These findings are causing a revision of
theories of the history of the solar system."

The research was supported by the Japan Society for the Promotion of Science and
the NASA Stardust Sample Analysis and Cosmochemistry Programs. The Wisc-SIMS
facility is partly supported by the National Science Foundation.
Received on Thu 18 Sep 2008 11:29:31 PM PDT