The fourth floor of Compton Hall houses numerous moon rocks. To keep these irreplaceable samples secure, access to the area is restricted. In this secluded environment, Ernst Zinner, Ph.D., research professor of physics and of earth and planetary sciences in Arts and Sciences and a fellow of the McDonnell Center for the Space Sciences, works undisturbed on the subject of dust.
Zinner's dust isn't the household stuff that accumulates in corners. It isn't even from Earth.
"These grains of dust are so anomalous, so different from what you find in the solar system, that we know they were not formed here," Zinner said.
In fact, they were formed billions of years ago in the atmospheres of dying stars. "As a star dies," Zinner explained, "its atmosphere begins to expand and cool. Then ions turn into atoms, atoms form molecules and, eventually, molecules condense into grains."
The dust then is ejected into outer space, where it collects with gas and dust from other stars to form cold, dark clouds.
More than 4.5 billion years ago, one such cloud collapsed to form our solar system, and the dust -- literally pieces of distant and long-dead stars -- was preserved in meteorites. Not quite 10 years ago, "stardust" was recovered from meteorites for the first time, and Zinner has led the way in the analysis of these grains since.
"Before the isolation of stardust from meteorites, all astronomy was done from a distance," he said. "Now that we have stardust here in the laboratory, we are able to perform experiments on material from actual stars and get information that couldn't be obtained any other way. The most interesting information comes from the isotopic composition of the grains."
Isotopes are versions of an element that have different numbers of neutrons and, consequently, different masses. In the same way that a zoologist studies a set of footprints to learn about the animal that made them, Zinner and his colleagues study the isotopes in a grain to learn about the parent star -- its mass, age, composition and other characteristics.
The ion microprobe is a mass spectrometer. Like a prism, which divides light into its different colors, the ion microprobe divides matter into its different isotopes. Zinner said it works in the following way: First, the microprobe bombards the grain with high-speed ions and knocks, or "sputters," secondary ions out of the sample. The secondary ions then are focused into a narrow beam and passed through a magnetic field, which deflects the ions. Because the lightweight ions are deflected more than the heavy ones, the beam fans out into several beams, each of which contains only one type of isotope.
Zinner equipped the ion microprobe with an efficient system for measuring the intensity of the isotope beams and developed many analysis techniques. An important advance for the analysis of grains is a technique called isotope imaging, which was developed by Robert M. Walker, Ph.D., the McDonnell Professor of physics and director of the McDonnell Center; former postdoctoral research associate Peter Hoppe, Ph.D.; and graduate student Larry Nittler. Instead of measuring grains one by one, isotope imaging enables scientists to measure several grains simultaneously.
"Typically, we put a few thousand grains on a piece of gold foil and then defocus the bombarding ion beam so it sputters about 20 of the grains at once," Zinner said. The ions from each grain travel through the microprobe separately and land on the detector in a spot that correlates to the position of the grains on the foil. In Zinner's words: "The detector receives an image of the grains in mass-selected ions."
Automating the process has made it possible to measure as many as several thousand grains per day. Zinner said the single grain measurements have turned out to be much more interesting than the bulk measurements.
"Single grain measurements have revealed tremendous variety among the grains and, of course, among their stellar origins," Zinner said. "The range of isotopic compositions has turned out to be greater than anyone had expected." Stellar sources include red giant stars and supernovae, which are massive stars that end their lives in gigantic explosions.
Zinner, who is described as "tireless" and "determined" by students and colleagues, takes evident pleasure in his work and in communicating its foundations to others. Although his interests clearly lie in the deeper points, he is receptive to even the simplest questions and delves into the material enthusiastically. His students benefit from his clear, down-to-earth presentation of the subject.
Nittler, who has worked with Zinner for the past four years, said: "Ernst explains things well and has a great scientific sense, which he conveys to his students. He has the unique ability to teach how one does good science."
Nittler adeptly has applied the skills he acquired from Zinner to his thesis work (also done with Walker): the study of aluminum oxide, or "corundum," grains. Nittler has reasoned that because of the unusual oxygen isotopic ratios of these grains, they most likely came from red giants -- low-mass stars like the sun that are old enough to have started expanding but still young enough to be rich in oxygen. He also has measured the oxygen and aluminum isotopes in the grains and shown that, according to theoretical models of red giants, some of the grains were formed in stars that lived as long as 6.5 billion years. Because the solar system is more than 4.5 billion years old, those stars must have been born at least 11 billion years ago. This requires the age of the Milky Way and, for that matter, the Universe -- both of which must be older than the stars -- to be at least 11 billion years old.
Nittler, who will defend his doctoral thesis next week, said he is glad to have had the opportunity to do this research and that it would not have been possible without Zinner's measurement techniques. "Ernst is truly one of the fathers of this field," Nittler said. "His developments on the ion microprobe are the reason we are able to do these studies. Twenty years ago, no one would have imagined that we would be able to study these grains on such a small scale."
For many astronomers, however, the grains are remarkable because they pose serious challenges to prevailing notions of the structure and evolution of stars. Donald Clayton, Ph.D., a professor in the Department of Physics and Astronomy at Clemson University, said: "Zinner's data show a lot of surprising and puzzling relative abundances of the isotopes, many of which don't fit standard models of how stars work. The data is right, however, so it's up to the astronomers to figure out how to interpret it and refine the models."
Yet Clayton went on to say that astronomers have been slow to get going. "Large numbers of astronomers are still unaware of this new data, while others don't quite know what to make of it," Clayton said. "The results are exciting, but astronomers have yet to come to grips with what it all means."
Together with Walker and Thomas Bernatowicz, Ph.D., research professor of physics and of earth and planetary sciences, Zinner organized this week's presolar grains conference to address these problems. He hopes the conference will promote familiarity with the grain data and bring about an exchange among scientists from each of the relevant disciplines: the physicists and chemists who analyze the grains, the astrophysicists who develop theoretical models of stars, and the astronomers who observe the grains in stellar atmospheres and cold, dark clouds.
"I came to the States back in 1965 because I decided I needed a change," Zinner said. He had spent the previous 10 years in Vienna and wanted to continue to live in a big city. With Washington University as his choice for graduate school, that big city meant St. Louis. However, Zinner became concerned about St. Louis' reputation when he met a man in Vienna who was from Manhattan. When Zinner told the man he was going to St. Louis, the New Yorker blurted: "Oh, you poor man. Don't you know St. Louis is in the middle of the cultural desert?"
Although St. Louis may not be quite as cosmopolitan as New York or Vienna, Zinner is not dissatisfied. True to his reputation as a hard worker, he said he hardly finds enough time to take advantage of the cultural offerings here. But that is not to say Zinner spends all his time in the laboratory. On the contrary, he plays the harpsichord once a week in an informal baroque ensemble and currently is trying to remain one step ahead of his 8-year-old son, Max Giacobini Zinner (named after the comet Giacobini-Zinner), as they learn the cello together.
Zinner's wife, Brigitte Wopenka, Ph.D., senior research scientist in earth and planetary sciences, shares not only her husband's Austrian roots but also his scientific inclination. She and her colleagues have developed techniques for determining the composition of geological samples using a laser Raman microprobe. In one of her few collaborations with her husband, Wopenka has used this instrument to determine the crystal structure of presolar graphite grains -- which puts limits on the conditions under which the grains were formed.
When asked what he thought of the recent evidence for life on Mars, Zinner said he is not convinced. "I am waiting for more proof," he said.
What is needed is proof as solid as stardust.
-- Debra Daugherty
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