IMAGINE traveling halfway to Jupiter—3.2
billion kilometers—for a small handful of comet dust. That’s
the mission for the National Aeronautics and Space Administration’s
(NASA’s) Stardust spacecraft launched on February 7, 1999. This past
January, Stardust flew by Comet Wild 2’s nucleus and through a halo
of gases and dust at the comet’s head, collecting cometary dust particles
released from the surface just hours before. In 2006, the spacecraft
will deliver the less than 1 milligram of particles to Earth. A
team is perfecting ways to extract and analyze the tiny particles
using its new focused-ion-beam instrument and SuperSTEM, a scanning
transmission electron microscope.
the first NASA space mission dedicated solely to collecting comet
dust and will be the first to return material from a comet to Earth.
(See Stardust: NASA’s Comet
Sample Return Mission Web page
for current news and live views). Comets are the oldest and most
primitive bodies in the solar system. They are formed from frozen
gas, water, and interstellar dust and may have brought water to
Earth, making life possible. Wild 2—pronounced “Vilt
2” after the name of its Swiss discoverer—was formed with the
Sun and the rest of the solar system 4.5 billion years ago. For
billions of years, it has circled the Sun in the Kuiper Belt, a
region beyond the
orbit of Neptune. Scientists think comets from this region have
escaped the warming, vaporization, and collisions that have altered
matter in the
inner solar system. Unlike Halley’s Comet, which has been altered
as a result of orbiting the Sun for a long time, Wild 2’s pristine
composition is expected to offer a rich source of information about
the solar system ’s
potential building blocks.
The Stardust spacecraft’s collector
grid, which is filled with aerogel, was designed to capture
particles from Comet Wild 2 as the spacecraft flew through
the comet’s dust and gas cloud in January 2004. Because
the aerogel is composed of 99 percent air, it can collect
and store fast-moving dust particles without damaging them.
As the 5-meter-long
Stardust spacecraft traveled through Wild 2’s
dust and gas cloud, to within about 100 kilometers of the comet’s
nucleus, particles were captured in the spacecraft’s collector grid.
The 1,000-square-centimeter grid is filled with the silica-based material
aerogel, whose lightness minimizes damage to the grains as they encounter
the spacecraft at a speed of about 21,000 kilometers per hour—or
six times faster than a bullet. In the late 1980s, Livermore scientists
developed an aerogel made up of 99 percent air, making it ideal for NASA
projects. Mission planners expect to have collected more than 1,000 grains
between 2 to 5 nanometers in diameter. Most of the grains will be heterogeneous
aggregates of carbonaceous matter, glass, and crystals.
part of the Bay Area Particle Analysis Consortium (BayPac) formed to develop
regional expertise on interplanetary dust particles.
BayPac’s members include University of California (UC) at Berkeley,
UC Davis, Lawrence Berkeley National Laboratory, and Stanford University.
Funding for SuperSTEM—the first of its kind in the world—comes
from NASA and Livermore’s Laboratory Directed Research and Development
Program. John Bradley, director of the Laboratory’s Institute of Geophysics
and Planetary Physics, says, “This consortium provides a unique opportunity
for a collaboration between universities and national laboratories in the
San Francisco Bay Area to work together on a NASA mission.”
This image shows the nucleus of Comet Wild
2, taken by the Stardust spacecraft in January 2004.
Perfecting Extraction and Analysis Techniques
collected by Stardust will be extremely small, so analytical instruments
with a spatial resolution of approximately 2 nanometers
or less will be needed to focus on the individual grains. Each
member of BayPac works on particle manipulation and analysis using
a variety of methods
and instruments. To study the isotopic compositions of the dust
particles, the Livermore team uses the 200-kiloelectronvolt SuperSTEM,
which has a
10- to 100-fold resolution increase over other instruments, and
its NanoSIMS (nano secondary-ion mass spectrometry), which is one
of only two ion microprobes
in the U.S. The team also uses a nuclear microprobe that radiates
a sample with 3 megavolts of protons to measure its density.
the consortium are studying interplanetary dust particles from
Russia’s Mir Space Station and the International Space Station
to refine the extraction techniques they will use on Wild 2 dust. “These
particles are perfect analogs to study,” Bradley says, “because
they were also collected in aerogel, although at a significantly higher
speed (11 kilometers per second) than the Stardust collection speed.”
Interstellar dust tracks are shown trapped in a sample of aerogel
from a collector grid. (b) The keystone technique allows scientists
to extract a fragmented particle from a sample of aerogel.
The finer-grain material distributed along the impact tracks
must be recovered for comprehensive analysis of comet dust.
The technique was developed at the University of California
(UC) at Berkeley. (Image courtesy Space Sciences Laboratory,
years ago, researchers from UC Berkeley’s Space Sciences
Laboratory developed the “keystone” technique to remove a particle
from a sample of aerogel. The term keystone is derived from the tiny wedge
that contains the particle track and that is cut out of the aerogel. Detailed
optical images of these impact tracks show evidence that particles fragment
quite extensively as they project into the aerogel. With the development
of the keystone technique, researchers have been able to further refine
techniques to remove the fragmented particles. These fine-grained particles
must be recovered for comprehensive analysis of cometary material.
are determining which method will best remove micrometer- and submicrometer-size
particle fragments from the
tracks within keystones. Focused-ion-beam microscopy is one promising
method being used
to extract 0.1-micrometer-thick sections of a particle fragment.
The thin sections are then examined using the transmission electron
NanoSIMS ion microprobe, and synchrotron infrared microscopy. Because
the focused-ion-beam method destroys most of the particle fragment,
or two sections can be harvested from each sample. The advantage
of this method is that researchers can extract particles as small
as 100 nanometers
or less from targeted regions and cut them into thin sections.
Livermore scientists take samples from specific isotopically anomalous
hot spots, that is, areas where the highest concentration of a
given isotope is found. Using this approach, researchers can correlate
and mineralogy with nanoscale precision.
(a) Focused-ion-beam microscopy is used
to extract 0.1-micrometer-thick sections of an interplanetary
dust particle from a sample of aerogel (shown in the inset).
(b) The thin sections are then examined using the transmission
Preparing for the Return
will analyze Wild 2’s dust, and many of them
will travel to Livermore to use the Laboratory’s SuperSTEM. Until
then, Livermore will continue to refine extraction and specimen preparation
techniques. Bradley notes, “For the first time in 30 years, we will
be analyzing returned samples, and Livermore will be busy preparing a large
quantity of specimens.” In addition to collecting particles, Stardust
has an optical navigation camera that has captured images of Wild 2’s
nucleus. Mission planners were surprised when the first pictures relayed
back to Earth showed a large, circular nucleus rather than the expected
potato shape seen in comets thus far.
In January 2006,
Stardust is programmed to eject its reentry capsule, which will
parachute to the Utah desert southwest of Salt Lake City. The
much-anticipated return of the capsule will perhaps yield more
surprises. Scientists are excited about what Wild 2’s dust may reveal about the
origins of life on Earth.
Key Words: Comet Wild 2, focused-ion-beam microscopy, interplanetary
dust particles, nano secondary-ion mass spectrometry (nanoSIMS),
Stardust, SuperSTEM (scanning transmission electron microscope).
For further information contact John Bradley
(925) 423-0666 (firstname.lastname@example.org).
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