Cometary Clues to Solar System Formation
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Materials brought back from a known extraterrestrial source, such
as the Apollo samples from the Moon in the 1970s, provide critical
clues to the history of the Solar System and interpretation of
extraterrestrial samples like meteorites and cosmic dust particles.
Stardust's success depended on two technical achievements, a trajectory
allowing it to pass within 240 km of the comet’s nucleus
at a speed of just 6 km/s and a special low-density material called
aerogel molded into a collector grid. Particles were brought to
a standstill as they penetrated into the aerogel with limited heating
or alteration. Thousands of tiny particles, typically leaving carrot-shaped
tracks, were trapped, most of them smaller than 10 micrometers
in size.
Tracks
left by two comet particles after they struck the Stardust
spacecraft's comet dust collector. The collector is made up of
a low-density glass material called aerogel. Scientists have
begun extracting comet particles from these and other similar
tadpole-shaped tracks.
Image credit: NASA/JPL-Caltech/University
of Washington.
After its launch in 1999, Stardust reached the comet in 2004,
then returned its precious cargo to Earth in a capsule on January
15, 2006. At NASA's Johnson Space Flight Center in Houston, a few
of the captured particles were quickly distributed for inspection
by Preliminary Examination Teams (PETs). At the ALS, measurements
were made at four beamlines. "Keystones" of aerogel,
wedges containing complete tracks and the terminal particles at
their tips, were first removed under the microscope using computer-driven
micromanipulators that sliced the aerogel with glass needles.
X-ray absorption near-edge structure (XANES) yields a distinctive
spectral signature for each chemical constituent in a sample and
is particularly useful for identifying organic compounds. At Beamlines
5.3.2 and 11.0.2, it was possible to combine this technique with
the scanning transmission x-ray microscope (STXM) to image the
spatial distribution of the compounds.
Some particle tracks (top left) revealed
shedding of organic compounds and their diffusion into
the surrounding aerogel. The spectra (top right) show the
intensities of a methylene group peak on and off the track.
Peak distribution is mapped in the false color image. Intensity
is greatest in and near the track, but methylene is present
in the aerogel over 100 micrometers away. Image credit:
S. Bajt, Lawrence Livermore National Laboratory.
Initially it was planned to do infrared (IR0 microspectroscopy at Beamline
1.4.3 only on the terminal
particles, concentrating primarily on the silicates in those particles.
But because the aerogel slowed the particles relatively gently,
team members were also able to capture volatile organics along
most of the length of the track, building up a two-dimensional
image of the different organics at different stages of entry.
Minerals were the main target of studies at Beamline 10.3.2. The
team used a combination of three techniques for mapping the bulk
chemistry and mineralogy of the Wild 2 samples. In x-ray fluorescence,
one obtains an elemental map. By means of XANES and the related
technique of extended x-ray absorption fine structure, or EXAFS,
one can also determine the atomic environment of specific elements.
X-ray diffraction yields the crystalline structure of minerals.
The calcium problem: which spots from x-ray
microfluroescence maps are really from the comet? Left: Large,
bright spots outside the track in this calcium map are contaminants
in the aerogel. Right: In this higher-magnification image, Ti
is mapped in red, Mn in green, and Ca in blue. Thus, the bright
blue spot (particle 1 in the track) contains Ca and little or
no Mn or Ti, so it is aerogel contaminant, whereas the greenish
spot (particle 2 also in the track) contains all three elements,
which is typical of minerals formed at high temperature in the
inner Solar System.
Image credit: Matthew Marcus, ALS.
In all, the Wild 2 samples proved to be highly variable. Some
contained minerals supposedly formed only near a star or in some
other high-temperature environment. One such sample contained aluminum-titanium-calcium-rich
minerals similar to those found in inclusions in the Allende meteorite.
From this tangle, the picture that emerged is of cometary particles
containing primarily silicate materials formed within the Solar System, including some grains born in the high temperatures existing
only close to the Sun. These particles then were carried to the
outer reaches of the Solar System, the Kuiper belt region outside
Neptune’s orbit, where they were incorporated into Comet
Wild 2 along with organic compounds and other volatile materials.
Research conducted by members of the Stardust
Preliminary Examination Team.
Research funding: Research funding: U.S. National Aeronautics
and Space Administration and other institutions supporting the
members of the Stardust Preliminary Examination Team. Operation
of the ALS is supported by the U.S. Department of Energy, Office
of Basic Energy Sciences (BES).
Publications about this research: D. Brownlee et al., “Comet
81P/Wild 2 under a microscope,” Science 314,
1711 (2006); S.A. Sandford et al., “Organics captured from
Comet 81P/Wild 2 by the Stardust spacecraft,” Science 314,
1720 (2006); L.P. Keller et al., “Infrared spectroscopy of
Comet 81P/Wild 2 samples returned by Stardust,” Science 314,
1728 (2006); G.J. Flynn et al., “Elemental compositions of
Comet 81P/Wild 2 samples collected by Stardust,” Science 314,
1731 (2006).
ALSNews Vol. 276, May 30, 2007
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