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8 Presolar grains are so small (typically about 1 µm) and constitute such a minor fraction of meteorites by weigh (typically less than five parts per million) that their presence was overlooked when the first measurements were made on gram-sized samples. Hydrogen isotopic anomalies were measured in meteorites as early as 1954 but it was not until the early 1970s that isotopic evidence of presolar grains was found. As with other important developments in meteoritics (for example, the discovery оf the decay products of extinct radionuclides), isotopic studies of noble gases were the key to understanding.
In a landmark paper David Black identified several isotopically distinct components in meteorite samples heated to progressively higher temperatures. One sample was very enriched in neon-22, a discovery that led Black to suggest that surviving presolar material was its host.
9 Subsequent work using various physical and chemical treatments of primitive meteorites produced samples that were even richer in 22Ne, buttressing the idea that an unknown mineral phase carried essentially pure 22Ne. One possible mechanism for the production of such an isotopically distinct component is the incorporation of radioactive 22Na as a minor constituent of condensing dust grains. This nuclide, which has a halflife of 2.6 years, decays to give pure 22Ne. Stepwise heating experiments showed that there were at least two separate carriers of the 22Ne-rich gases, one of which released Ne at low temperatures and the other did so at high temperatures.
10 Studying xenon and krypton in addition to neon, Edward Anders and Roy Lewis discovered a remarkable fact: Isotopically exotic noble gases were not removed by the acid dissolutions that destroyed most of the meteorite.
Using noble gases as tracers of presolar grains the Chicago group developed. procedures for producing mineral separates rich in various exotic components. Their technique included dissolving most of the meteorite in a variety of solvents—a process that Anders has described as "burning down the haystack to find the needle." One such separate, which was characterized by enrichments in both the heavy and light isotopes of xenon, was found to consist primarily of nano-diamonds with a median grain size of 1-3 nm. This discovery was the first identification of a specific carrier phase for a component of exotic noble gas.
11. At the time of this discovery in 1987, the Chicago scientists began collaborating with our group in St. Louis. We had a variety of instruments specifically tailored for microanalytic studies of extraterrestrial materials and had been actively searching for presolar grains—principally in interplanetary dust particles, but also in acid residues of primitive meteorites. As luck would have it, we concentrated at first on the meteorite Allende, which we now know is essentially devoid of circumstellar grains large enough to measure individually. Although a large SiC grain had been found in one of the residues, it proved to have a normal isotopic composition and was undoubtedly a terrestrial contaminant.
12 Fortunately, the same was decidedly not true of the abundant, micrometer-sized SiC grains found in the Chicago separate of the Murray meteorite, which are rich in 22Ne. Ernst Zinner, who is the director of the Washington University ion probe laboratory, pioneered the development of techniques to make isotopic measurements on these small grains. He quickly found that each SiC grain had silicon and carbon isotopic ratios that differed by astonishingly large amounts from the Solar System average. With this discovery, the laboratory study of individual grains of stardust was born.
13 In 1990, Sachiko Amari and Roy Lewis's physical and chemical processing of the Murchison meteorite at Chicago indicated that the presolar carriers of the 22Ne released at low temperatures were micrometer-sized spherules of graphite (see figure 1). Amazingly, grain-to-grain carbon isotopic ratios of these spherules varied by more than three orders of magnitude! That SiC and graphite were indeed the carriers of the 22Ne-rich components was proven by our colleagues Robert Nichols and Charles Hohenberg, who successfully measured the Ne isotopes in individual grains.
figure 2. isotopic pattern of barium measured by Frank Podosek and coworkers in collections of presolar silicon carbide grains (pink), compared with predictions made by Roberto Gallino and coworkers for the composition of barium created in s-process nucleosynthesis (gray). Gallino found that the Ba composition could be reconciled with s-process theory only by revising the neutron capture cross sections for 135Ba, 136Ba and 137Ba—a proposition that was vindicated in lab experiments. What the figure shows is that when the revised cross sections are used, the neutron exposure—as indicated by the width of the gray bars and the arrows—must have been lower for the presolar grains than for average Solar System material.
