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ScienceWeek
2004 9 July C2 PLANETARY SCIENCE: METEORITES AND SOLAR SYSTEM FORMATION
The following points are made by Sara Russell (Nature 2004 428:903):
1) Meteorites that date from approximately the time of the formation of the Solar System -- a little over 4.5 billion years ago -- are testament to the events that occurred before and during planet formation. Most of the interstellar dust that went into forming planetary precursors was melted, vaporized, shocked and, once incorporated into asteroids, further heated and damaged. This has caused the chemistry and isotopic composition of minerals from meteorites to become more homogeneous. But a few mineral survivors predate these events. These "presolar grains" originated around stars that were the predecessors of our own, and made up part of the interstellar medium before collapsing into our Solar System. Several carbonaceous and oxide presolar grains have been identified in meteorite samples.
2) Nagashima et al(1) have uncovered presolar specimens of silicates, the most common rock-forming minerals. This discovery is impressive, because presolar silicates are much more difficult to find than presolar carbonaceous and oxide grains. The latter are resistant to acid and can be separated out of a meteorite by dissolving away the major components -- silicates and metal. The solid residue that survives can then be examined for grains of interest. This technique has been compared (by Edward Anders) to burning down a haystack to find the needle, and is more than a little distressing for meteorite curators. Nevertheless, it is a relatively straightforward way for researchers to find presolar gems.
3) The presolar grains identified so far are all chemically resilient enough to have survived this acid processing: silicon carbide, graphite, aluminum oxide and spinel, at levels of up to a few parts per million. Diamonds, which make up to 0.1% of some meteorites, might also be presolar, but their carbon-isotope composition and variable relative abundance in ancient objects has raised some doubt about this(2). Because these gems condensed around ancient stars, they offer unique insight into how stars synthesize isotopes, how easily different parts of the star mix together and how grains condense in the relatively cool circumstellar region. They also provide a snapshot of the interstellar medium several billion years ago, so we can judge how the composition of our Galaxy has evolved since before the Sun came into existence.
4) As well as studying presolar grains in meteorites, we have recently learned a great deal about mineral grains in space by characterizing them remotely. For example, one of the most surprising findings of the Infrared Space Observatory mission was the great variety of types of star around which fine-grained silicates crystallize. It thus seemed certain that the young Solar System would also have contained interstellar silicate dust, as well as the other known grains, and there should be silicates from a variety of stellar environments in our meteorite collections.(3-5)
References:
1. Nagashima, K., Krot, A. N. & Yurimoto, H. Nature 428, 921-924 (2004)
2. Dai, Z. R. et al. Nature 418, 157-159 (2002)
3. Yurimoto, H., Nagashima, K. & Kunihiro, T. Appl. Surf. Sci. 203-204, 793-797 (2003)
4. Nguyen, A. N. & Zinner, E. Science 303, 1496-1499 (2004)
5. Messenger, S., Keller, L. P., Stadermann, F. J., Walker, R. M. & Zinner, E. Science 300, 105-108 (2003)
Nature http://www.nature.com/nature
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Related Material:
SPACE SCIENCE: ANCIENT SILICATE STARDUST IN A METEORITE
The following points are made by Ann N. Nguyen and Ernst Zinner (Science 2004 303:1496):
1) Presolar grains were isolated in primitive meteorites only 15 years ago. These grains of stardust formed in the atmospheres of evolved stars and in nova and supernova ejecta. They survived processing in the interstellar medium and in the solar nebula, where most material was heated and homogenized to an average composition. Having undergone minimal alteration, they preserve the original isotopic composition of their parent stars and thus provide important information about stellar evolution and nucleosynthesis.
2) The types of presolar grains identified to date include nanodiamonds; silicon carbide; graphite; silicon nitride; and the oxide grains corundum, spinel, and hibonite (1). However, spectroscopic data of young main sequence stars (2,3) and oxygen-rich asymptotic giant branch stars (AGB) (4,5) indicate an abundance of submicrometer amorphous silicate grains and crystalline silicate grains of forsterite, enstatite, and diopside. Surprisingly, circumstellar silicate grains were absent from the collection of identified presolar grains despite attempts to isolate them from meteorites. The question was whether or not these particles were destroyed by processing in the interstellar medium, solar nebula, or the parent body.
3) A major difficulty in identifying anomalous silicate grains is that the Solar System formed under oxidizing conditions, and thus solar system minerals are dominated by oxidized phases such as oxides and silicates. Consequentially, the identification of a few isotopically anomalous O-rich grains in this great sandbox of oxidized phases of Solar System composition requires the analysis of a large number of grains. This difficulty is heightened by the limiting spatial resolution (greater than 1 micron) of the instruments used in previous searches and the expected submicrometer sizes of presolar silicates.
4) In summary: The authors report the discovery of nine presolar silicate grains from the carbonaceous chondrite Acfer 094. The anomalous oxygen isotopic compositions of the grains indicate formation in the atmospheres of evolved stars. Two grains are identified as pyroxene, two as olivine, one as a glass with embedded metal and sulfides (GEMS), and one as an Al-rich silicate. One grain is enriched in Mg-26, which is attributed to the radioactive decay of Al-26 and provides information about mixing processes in the parent star. The authors suggest this discovery opens new means for studying stellar processes and conditions in various Solar System environments.
References (abridged):
1. E. Zinner, in Meteorites, Comets, and Planets, A. M. Davis, Ed. (Elsevier, Oxford, UK, 2004), vol. 1, pp. 17-39
2. C. Waelkens et al., Astron. Astrophys. 315, L245 (1996)
3. K. Malfait et al., Astron. Astrophys. 332, L25 (1998)
4. L. B. F. M. Waters et al., Astron. Astrophys. 315, L361 (1996)
5. K. Demyk, E. Dartois, H. Wiesemeyer, A. P. Jones, L. d'Hendecourt, Astron. Astrophys. 364, 170 (2000)
Science http://www.sciencemag.org
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Related Material:
ON NUCLEAR FOSSILS IN STARDUST
The following points are made by Larry R. Nittler (Science 2004 303:636):
1) In 1952, P.W. Merrill reported that the radioactive element technetium had been observed in a special class of giant stars (1). Because this element has no stable isotopes, Merrill proposed that these stars "somehow produce technetium as they go along". This was direct evidence that elements are produced by nuclear reactions within stars, and it was only a few years later that a comprehensive theory of nucleosynthesis was laid out in largely modern form (2).
2) For decades, two lines of evidence have been used to test nucleosynthesis theories: chemical abundances measured spectroscopically in stars, and the bulk isotopic and elemental composition of the Solar System. Savina et al (3) have reported evidence for now-extinct technetium in microscopic grains of silicon carbide (SiC) extracted from a meteorite. These grains of circumstellar dust predate the solar system and provide a new and powerful way to investigate stellar evolution and nucleosynthesis with a level of detail and precision almost unheard of in nuclear astrophysics.
3) Since they were discovered in the late 1980s, stardust grains in meteorites have provided astrophysical information complementary to that obtained by astronomical observations (4,5). These rare and tiny (less than a few micrometers) grains of minerals such as SiC, graphite, and Al2O3 have isotopic compositions that establish their formation in stellar outflows and ejecta. They survived conditions in the interstellar medium and early solar system and became trapped in asteroids, pieces of which now fall to Earth as meteorites. Each individual grain is essentially a condensed piece of a single star, and each contains a record of a wide array of astrophysical processes.
4) The best studied type of presolar stardust in meteorites is SiC. More than 90% of presolar SiC grains (known as the "mainstream") are believed to have originated in asymptotic giant branch (AGB) stars, one of the last evolutionary stages of stars with mass up to a few times that of the Sun. AGB stars, including the S-type stars studied by Merrill, have a very interesting structure: An inert core is surrounded by thin helium- and hydrogen-burning shells, which are in turn surrounded by a large hydrogen-rich convective envelope. As they evolve, the helium and hydrogen shells alternately undergo nuclear burning. A key result is the release of neutrons in the region between the shells. These neutrons can be slowly captured by nuclei to produce heavier elements, which are then mixed into the stellar envelope during convective episodes. This so-called s-process (for "slow" neutron capture) is the source of many elements heavier than iron in the Universe. Freshly synthesized carbon is also convectively brought into the envelope, eventually resulting in a surface carbon/oxygen ratio greater than 1. At this point, SiC dust is observed to condense in strong stellar winds.
References (abridged):
1. P. W. Merrill, Science 115, 484 (1952)
2. E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle, Rev. Mod. Phys. 29, 547 (1957)
3. M. R. Savina et al., Science 303, 649 (2004)
4. E. Anders, E. Zinner, Meteoritics 28, 490 (1993)
5. L. R. Nittler, Earth Planet. Sci. Lett. 209, 259 (2003)
Science http://www.sciencemag.org
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