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Presolar grains

Presolar grains are tiny crystalline grains which are found within the fine grained matrix of primitive meteorites and are assumed to be older than the solar system. They are probably formed in supernovae explosions or stellar outflows of red giant stars. After formation they were incorporated into the molecular cloud from which the solar nebula was separated to form our solar system. Because presolar grains consists 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:

In the 1960s neon and xenon components with unusual isotopic ratios were discovered in primitive meteorites leading to the conclusion that presolar minerals within these meteorites exist which carry these noble gas components.
In 1987 diamond and silicon carbide grains are found to be carriers of these noble gases [1,2] and other isotopic abnormalities are found within these grains.
Because presolar grains formed not within 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 within 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 which consist of diamonds have only a size of a few nanometers and 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 micrometers. Presolar grains represent material from outside our solar system. Thus the study of presolar grains provides information about the nucleosynthesis and stellar evolution and is, in particular, useful to test models of supernovae explosions.

References [1] Lewis R.S., Tang M., Wacker J.F., Anders E. and Steel E. (1987) Interstellar diamonds in meteorites, Nature 326, 160-162.

[2] 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.

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