University scientists anticipate samples from space missions

Scientists at the University’s Enrico Fermi Institute and at Argonne National Laboratory have been working with meteorites to hone an experimental technique well-suited for analyzing samples of extraterrestrial materials being collected from space.

The National Aeronautics and Space Administration and its partners have four missions either already launched or on the drawing boards with the goal of returning to Earth samples of a comet, dust from outside the solar system, pieces of an asteroid and Mars, and charged particles from the sun.

Not since the last Apollo mission in 1972 have scientists faced the prospect of bringing such a rich haul of material from sample-return missions into their laboratories.

“I’m an optimist. I think there is going to be a steady stream of samples coming back,“ said Andrew Davis, Senior Scientist in the Enrico Fermi Institute and Geophysical Sciences. “It isn’t going to be just these four missions, and then we’ll have nothing for another 35 or 40 years, like after Apollo.“

No one knows yet which research teams will receive the coveted material for analysis, but University researchers, building on three decades of milestone research on meteorites and lunar rocks, expect to make a strong case for their share. Meanwhile, their ongoing research on interstellar dust grains transported to Earth via meteorites is revealing new details about how stars evolved billions of years ago and produced the elements needed to support life on Earth.

“Understanding the origin of this material has, in my view, a deep philosophical interest,“ said Roy Lewis, a Senior Scientist in the Enrico Fermi Institute. “To quote Carl Sagan, ‘you and I are star stuff.’ Our sun is a second- or third-generation star. This is why it has the elements we need for life.“

In the past three years, the research team, which includes Argonne chemist Michael Pellin and Robert Clayton, Enrico Fermi Institute Director and the Enrico Fermi Distinguished Service Professor in Chemistry and Geophysical Sciences, has published a series of path-breaking papers. These papers announced that the team had, for the first time, precisely measured the isotopic compositions of heavy elements found in interstellar dust grains transported to Earth via meteorites.

Isotopes are different varieties of an element, such as silicon, that differ only in the number of neutrons in their nuclei. That is where Argonne’s Pellin comes in with a technique called resonant ionization mass spectrometry.

In Pellin’s instrument, a laser “zaps“ a sample, producing a cloud of atoms from all of the sample’s different elements. Lasers tuned to specific wavelengths are then used to ionize––electrically charge––the atoms of only one element. These ions then are analyzed with a mass spectrometer that measures the element’s isotopic proportions.

“No other laboratory in the world has the ability to measure the isotopic abundances of certain trace elements within tiny samples that Pellin can with his resonant ionization technique,“ Lewis said.

During his 10-year collaboration with Chicago scientists, Pellin has specially adapted the technique for measuring the abundance of elements in interstellar dust grains.

These grains are forged in the seething cauldrons of exploding stars and dying stars called red giants.

Lewis pioneered the technique of separating the grains from meteorites more than a decade ago with Edward Anders, the Horace Horton Professor Emeritus in Chemistry. They knew these grains come from interstellar space because their isotopic composition dramatically differs from anything ever observed within the solar system.

The grains the Chicago-Argonne team studies typically measure two or three microns in diameter––small enough for 15 or 20 of them to fit comfortably across the width of a human hair. If the space missions are successful, the team’s ability to measure such small samples could come in handy.

The Stardust mission, launched by NASA in February 1999, is designed to collect both cometary dust and interstellar dust particles and return them to Earth in 2006. The samples probably will measure anywhere from less than a micron to 100 microns in diameter.

Using nondestructive techniques, scientists first will want to make every observation of the samples they can that will yield valuable chemical and mineralogical data.

“But once you’ve done all that, you want to know the isotopic composition of the grains, and that’s where we think we might be able to come in with the techniques being developed at Argonne,“ Davis said.

NASA’s Genesis mission will capture similarly minute charged particles from the sun and return them to Earth in 2003. Missions to asteroid Nereus, scheduled for a 2002 launch, and Mars, which will launch no sooner than 2005, together may return no more than a pound or two of samples.

“Sample size is important because they’re not going to bring very much back,“ Davis said. Noted Lewis: “The smaller the sample you need to do the measurement, the better.“

But even if material from the sample-return missions never comes their way, 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 is 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.“