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Researchers reach to the skies to reveal the secrets of the stars

Argonne scientists study microscopic interstellar dust grains that meteorites have transported to Earth



University of Chicago researcher Roy Lewis (right), pictured with Andrew Davis, pioneered the meteorite grain-separating technique.

To reach for the stars is no mere figure of speech for scientists at Argonne National Laboratory and the University of Chicago. For them, it is the literal truth.

Using an instrument available only at Argonne, they study microscopic interstellar dust grains that meteorites have transported to Earth. Exploding stars launched these dust grains into space billions of years ago. The grains then got mixed into clouds of interstellar dust that collapsed to form the sun, the planets and meteorites. These grains now reveal details about how stars evolved over billions of years to produce the elements needed to form the Earth and all other objects in the solar system.

"These grains are pieces of stuff that came from individual stars," said Argonne chemist Michael Pellin of Argonne's Materials Science Division. "The neat thing in studying these materials is that each one is its own record, its own story of one star."

The stories all tell the tale of nucleosynthesis, the process by which the universe creates the elements, including iron, gold and silver. But some of the grains tell a story that no one has heard before.

"We've found a new kind of heavy-element nucleosynthesis, which we think will tell us quite a bit about what's going on in supernovae as they explode," said Andrew Davis, a senior scientist in the Department of Geophysical Sciences and the Enrico Fermi Institute at the university.

After the Big Bang After the Big Bang occurred about 14 billion years ago, the universe contained only two elements: hydrogen and helium. Atoms of hydrogen, helium and other light elements fused in the first stars to form elements such as carbon, oxygen and nitrogen, which are essential to life. Exploding stars and dying stars called red giants make elements heavier than iron by adding subatomic particles called neutrons to lighter elements.

"It isn't a terribly efficient process, which is why most of those elements are rare in nature," Davis said.

"Why is gold precious?" Pellin asked. "Well, it's hard to make in stars."

In a collaboration that spans more than a decade, the Argonne-Chicago team has focused its attention on measuring the isotopes of heavy elements in interstellar grains. Isotopes are varieties of an element that differ only in the number of neutrons at their core. They are like fingerprints left behind by certain kinds of stars, Davis said. And Argonne is the only laboratory in the world that can identify these fingerprints, using a technique called resonance ionization mass spectrometry.

"The resonant ionization technique is in fact the only one that allows enough sensitivity to be able to measure the elements to a tenth of a part per billion," said Roy Lewis, a senior scientist at the university's Fermi Institute.

Determining the fingerprints of stars The instrument is housed in an Argonne laboratory thickly forested with electronic hardware, wires and cables. Green and blue laser beams glowed brightly in the laboratory earlier this year during an experiment involving silicon and barium. Lewis begins the sample preparation by attaching many interstellar grains to a soft piece of gold. Then he maps the tiny grains with a scanning electron microscope so that his colleagues can find them again later.

In Pellin's laboratory, another microscope focuses a laser on the sample, vaporizing a portion of it into a cloud of carbon, silicon, molybdenum and many other atoms. Pellin can tune a second laser to a wavelength that will electrically charge one specific type of atom in an experimental sample containing a trillion atoms. After Pellin fires the lasers, an electric field shoots those atoms through a mass spectrometer, which measures their mass and abundance.

"You can't just go to a manufacturer and say, 'I'd like you to build me one of these," Davis said. "There are no commercially available instruments of this sort. But at Argonne, they're really good at building instruments."

In the past several years, this instrument has fueled a series of pathbreaking papers. These papers announced that the team had, for the first time, precisely measured the isotopic compositions of heavy elements such as strontium, zirconium and barium found in interstellar dust grains.

The only such grains available for study come from meteorites. Most interstellar dust grains melted and became mixed up with other material during the solar system's formation. The grains the Argonne-Chicago team studies are among the select few that survived. "These grains are the last pieces of our solar system that didn't get mixed up," Pellin said.

Lewis pioneered the technique of separating the grains from meteorites more than a decade ago with Edward Anders, the university's Horace Horton Professor Emeritus in Chemistry. The process involves dissolving the meteorites in acid, leaving a residue of grains behind.

They know these grains come from interstellar space because of their strange isotopic composition. "It doesn't look anything like the solar system, therefore it has to have come from some other place, some other star," Lewis said.

The grains the team studies typically measure two or three microns in diameter. This is small enough for 15 or 20 of them to fit comfortably across the width of a human hair.

Since 1987, Chicago researchers have identified three types of interstellar grains. The most interesting ones are made of silicon carbide and graphite, Davis said, though the most plentiful ones are diamond. All three types are preserved in the Murchison meteorite, which landed in Australia in 1969.

Most interstellar grains come from red giants. Stars once like the sun, red giants grow huge and shed much of their mass when they reach the end of their lifetimes. Scientists also have traced the grains to supernovae - exploding stars.

Slow neutron-capture called s-process nucleosynthesis takes place deep inside red giants in episodes lasting 10,000 years or more. The more violent rapid neutron-capture r-process nucleosynthesis occurs in supernovae, possibly within a matter of seconds. This process accounts for approximately half of the 117 elements.

But the Argonne-Chicago team has found a pattern that matches neither the s- nor the r-process in their isotopic measurements of molybdenum, a silver-grey metal that helps make stronger steel.

"It may be that there's some kind of neutron burst of some sort, intermediate between r and s process, that might explain the pattern we're seeing," Davis said.

"What we saw was puzzling," Pellin noted. "There's no question it's different and surprising."

The Genesis connection No one yet knows what different and surprising results will flow from NASA's Genesis mission. Launched in August 2001, the spacecraft will collect samples of the solar wind - single atoms and electrically charged particles from the sun - and return them to Earth in 2003. If successful, Genesis will become the first mission to return a sample of extraterrestrial material from beyond the moon. These samples will allow a precise measure of the elemental and isotopic composition of our most important star - the sun.

Pellin and his research group at Argonne will be among the scientists to analyze the samples in an effort to better understand how the planets formed and how the sun works. No element heavier than calcium, which is far lighter than iron, has ever been measured in the sun. Scientists believe that the heavier elements are there, but in the tiniest of amounts.

"At some level, half the periodic table is betting on our instrument," Pellin said.

But until the Genesis samples arrive at Argonne, the researchers will continue to coax more secrets of the universe from their meteorites, one grain at a time.

"We already have a sample-return mission to the stars represented by these grains from meteorites," Davis said. "Nature's doing it on the cheap for us. It can't give us everything we want. We don't know where the grains came from, but we've sampled a huge number of different stars."

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For more information, please contact Steve Koppes (773/702-8366 or s-koppes@uchicago.edu) at the University of Chicago.

 

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