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The Search for Trace Elements in Presolar Dust Grains: A Glance at a Time prior to the Solar System

The solar system as we know it evolved from a cloud of gas and dust known as the protosolar nebula. This matter reservoir, from which our sun, the eight planets, and all other smaller bodies like comets and asteroids formed about 4.6 billion years ago, consisted mainly of the light gases of hydrogen and helium produced during the Big Bang. Only a minor fraction of heavy elements present delivered the raw material that condensed into rock and other solids, the constituents of bodies like the Earth. These heavier elements were produced inside stars during hydrostatic nuclear burning phases and under explosive conditions during supernovae events. The products were released into the interstellar medium as supernova ejecta and also during quiet “burning phases” as stellar winds after complex mixing processes had brought them to the surface of the star. As a whole, the solar system represents a mixture of materials formed with a number of stellar sources contributing.
Material originating from specific nucleosynthetic processes in stars exhibits isotopic fingerprints with parameters that are characteristics of the physical conditions during its formation. Thus, unaltered stellar material allows for checking theoretical models for stellar evolution, the evolution of the galaxy as a whole, grain growth in stellar atmospheres, and convection and mixing processes between different layers in stars. However, the protosolar nebula was exposed to distinct mixing processes which led to isotopic homogenization. Therefore, samples taken from different regions of the solar system reveal essentially the same isotopic ratios, except for changes due to later processes, e.g. radioactive decay. There is, however, a source of material that survived the formation of the solar system without being seriously affected by isotope homogenization. Microscopic dust grains of ”presolar“ origin have been found in so called primitive meteorites since 1987.
Among these are nanodiamonds, silicon carbide, graphite, corundum, spinel, silicon nitride, hibonite, and silicates. Except for the latter, these are rare acid-resistant particles that are routinely isolated from the matrix of a meteorite by aggressive chemical treatment. Our research concentrates on presolar silicon carbide grains with concentrations of a few ppm (parts per million) only relative to the bulk meteorites. A fragment of the Murchison meteorite and a typical isolated SiC sample are shown in Fig. 1. Most of the silicon carbide grains originate from carbon stars (red giant stars that become carbon-rich at the surface in a late stage of their evolution).

Figure 1






Figure 1: Two fragments of the Murchison meteorite (upper part) dropped down onto Australia in 1969. It shows the typical carbonaceous matrix interspersed with ligh,t socalled Calcium Aluminium rich Inclusions (CAIs). These inclusions are believed to be the oldest solid material that formed in the solar system. The lower part shows acid-resistant dust grains extracted from the Murchison meteorite as seen in the scanning electron microscope (SEM). The symmetrically shaped grain in the center is a spinel crystal (probably not presolar), the smaller surrounding grains consist of presolar silicon carbide.





A characteristic is that they contain heavy trace elements that were synthesized by the s-process (slow neutron capture process; one of the two neutron capture processes that are predominantly responsible for synthesis of the nuclei heavier than iron).
To investigate the isotopic ratios of “diagnostic“ trace elements in single grains, methods of choice are SIMS (secondary ion mass spectrometry) and RIMS (resonance ionization mass spectrometry). However these methods have a major disadvantage: They are destructive. Hence, since they are also no multi-element techniques. Considering the small amount of material available, analysis of a small number of isotopes only is possible. Application of these techniques alone may result in destroying the sample grain by doing some type of analysis, without realizing the possible presence of more interesting trace elements, the isotopic analysis of which might have been far more interesting.
A novel approach to solving this problem was developed recently. It is based on a non-destructive chemical screening of ensembles of grains prior to isotopic analysis. In our approach, a large number of grains is investigated in parallel by means of imaging NEXAFS (Near Edge X-ray Absorption Fine Structure) measurements. The experiments are carried out at the WERA beamline of the synchrotron light source ANKA. A sample containing numerous particles is irradiated with monochromatic radiation produced by the synchrotron and the beamline optics. By scanning this radiation over a certain energy interval and mapping the released photoelectrons by means of a Photo Emission Electron Miscroscope (PEEM), the chemical composition of the sample can be determined with a lateral resolution down to ~100 nanometers. The X-ray energies at which photoelectrons are released are characteristic of the chemical elements present in the sample. Different chemical elements and also different compounds of a specific element are identified by the spectra.
Utilizing these as input, the mapped sample area can be ”unmixed“ into separate images that contain information on the lateral distribution of the various elements. In the example, it can be distinguished amorphous carbon (substrate), a mixture of silicon and silicon dioxide (in silicon carbide grains), and chromium (in spinel crystals). Hence, the mineral phases present in the sample can be identified in a non-destructive way. Further work will focus on the extension of this method to higher sensitivities for detecting the presence of trace amounts of elements typical of the s-process. Further experiments will be carry out, these investigations are expected to contribute to a variety of scientific fields of modern astrophysics.

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