Presolar grains
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Presolar grains are tiny crystalline grains found in the fine-grained matrix of primitive meteorites, and are assumed to be older than the solar system. They were probably formed in supernovae or the stellar outflows of red giant stars, and then incorporated into the molecular cloud from which the solar nebula separated to form our solar system. Because presolar grains consist of refractory minerals, they survived the collapse of the solar nebula, and also the subsequent formation of planetesimals.
There are different types of presolar grains. Presolar grains consisting of the following minerals have been found:
- diamond (C)
- graphite (C)
- silicon carbide (SiC)
- titanium carbide (TiC)
- silicon nitride (Si3N4)
- corundum (Al2O3)
- spinel (MgAl2O4)
- hibonite ((Ca,Ce)(Al,Ti,Mg)12O19)
- titanium oxide (TiO2)
In the 1960s, neon and xenon components with unusual isotopic ratios were discovered in primitive meteorites, leading to the conclusion that there are presolar minerals carrying these noble gas components which exist within these meteorites.
In 1987 diamond and silicon carbide grains were found to be carriers of these noble gases [1,2], and other isotopic abnormalities were found within these grains.
Because presolar grains did not form in our solar system, the isotopic composition of their elements is usually different from the isotopic composition of solar-system matter. Thus presolar grains can be identified using the isotopic composition of the elements in the grains.
Presolar grains are investigated using scanning or transmission electron microscopes (SEM/TEM), and mass spectrometric methods (noble gas mass spectrometry, resonance ionization mass spectrometry (RIMS), secondary ion mass spectrometry (SIMS, NanoSIMS)). Presolar grains that consist of diamonds are only a few nanometers in size, and are therefore also called nanodiamonds. Because of their small size, nanodiamonds are hard to investigate and, although they are among the first presolar grains discovered, relatively little is known about them. The typical sizes of other presolar grains are in the range of micrometres. Presolar grains represent material from outside our solar system. Thus the study of presolar grains provides information about nucleosynthesis and stellar evolution, and is particularly useful in testing models of supernovae explosions.
References
- [1] Ahnert-Rohlfs E. (1954) Vorläufige Mitteilung über Versuche zum Nachweis von Meteoritischem Staub, Mitteilung der Sternwarte Sonneberg 45
- [2] Lewis R.S., Tang M., Wacker J.F., Anders E. and Steel E. (1987) Interstellar diamonds in meteorites, Nature 326, 160-162.
- [3] Bernatowicz, T., Fraundorf, G., Ming, T., Anders, E., Wopenka, B., Zinner, E., and Fraundorf, P. (1987) Evidence for interstellar SiC in the Murray carbonaceous meteorite, Nature 330, 728.
External links
- http://presolar.wustl.edu/work/grains.html
- http://www.astro.uiuc.edu/~akspeck/witch-stuff/Research/chapter3/node3.html
- http://www.ciw.edu/lrn/psg_main.html
- http://www.psrd.hawaii.edu/July97/Stardust.html
- http://www.stecf.org/~ralbrech/amico/intabs/ottu.html
- http://www.mpch-mainz.mpg.de/~kosmo/sims/sternenstaub/sternenstaub.htm (images of presolar grains, text in German)