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Old dust and a rough youth – how tiny metal crystals tell about the
early Solar System

By Dennis Harries (26.10.2011)

About 4.56 billion years ago, in an arm of the Milky Way galaxy, an extensive cloud of gas and dust began to coalesce and collapse into a hot, disk-shaped protoplanetary nebula that should become the Sun and the solar system several tens of million years later. Today, little evidence of this time remains, since geological processes have strongly and persistently reshaped the Earth and other planetary bodies. Solid evidence of the first million years is contained in meteorites that arrive on Earth from the asteroid belt between Mars and Jupiter. In particular, so called chondrites have become treasure chests of information on the history of the solar system, because many physical and chemical properties indicate them to be almost unchanged since their formation at its birth.

Fig. 1: Artist’s impression of a protoplanetary disk similar to the solar nebula 4.56 billion years ago (Credit: ESO/L. Calçada/M. Kornmesser). On the right: Fragments of a CM2 chondrite that formed in the solar nebula at that time.[Bildunterschrift / Subline]: Fig. 1: Artist’s impression of a protoplanetary disk similar to the solar nebula 4.56 billion years ago (Credit: ESO/L. Calçada/M. Kornmesser). On the right: Fragments of a CM2 chondrite that formed in the solar nebula at that time.

In many cases chondrites have posed more questions than they have actually answered. Chondritic rocks are complex mixtures of components, which formed under highly variable conditions. Some of these components are rich in refractory mineral aggregates that presumably formed at temperatures in excess of 1500 °C, while others contain water-bearing minerals that formed at temperatures between 0 and 200 °C. Yet others, so called CM-type carbonaceous chondrites, provokingly contain both types of components and also abundant organic matter, such as amino acids, some of which do not occur in biological matter on Earth. Many similar observations indicate that the first 10 million years of the solar system were dominated by a rather chaotic assortment of processes reflecting processes from 2000 °C down to 100 °C within a short time span.

As part of my studies under supervision of Falko Langenhorst (now University of Jena) I am investigating metallic minerals and sulfides in CM chondrites by using transmission electron microscopy (TEM) and focused ion beam (FIB) sample preparation, which uses a highly focused Ga+ ion beam inside a scanning electron microscope (SEM) to cut out thin TEM samples with sub-µm precision.

In collaboration with the University of Mainz (T. Berg), the Max Planck Institute of Chemistry (D. Schwander, U. Ott,), and the Senckenberg Research Institute and Natural History Museum (H. Palme), we are involved in a project aiming at the study of refractory metal nuggets (RMNs) found in the CM chondrite Murchison, which fell in Australia in 1969. RMNs are tiny, typically less than 400 nm small grains of refractory metal alloys containing mainly the elements Ru, Os, Ir, Mo, W, Fe, and Ni. When these grains were observed for the first time by Berg et al. [1], there was considerable excitement, because their compositions matched closely the element pattern predicted for the first solids that would condensate from a hot gas in the solar nebula at temperatures between approximately 1150 and 1350 °C at a pressure of 10-4 bar. While similar grains are not uncommon in carbonaceous chondrites, the Murchison RMNs are unique, because they still contain the expected proportions of Mo and W, which are elements easily lost by oxidation. This observation led to the conclusion that these tiny dust grains represent a rare case of material having survived the rough first million years of the solar system unaltered (and equally the following, rather boring 4560 million years).

Fig. 2: (a) Secondary electron SEM image of a single RMN. (b) TEM image of a FIB cross section through a RMN. (c) Convergent beam electron diffraction pattern of a sliced RMN.[Bildunterschrift / Subline]: Fig. 2: (a) Secondary electron SEM image of a single RMN recovered from the Murchison CM2 meteorite. (b) TEM image of a FIB cross section through a RMN, straight crystal faces are clearly visible. (c) Convergent beam electron diffraction pattern of a sliced RMN, showing intensity distributions that are characteristic for a hexagonal close packed crystal structure.

One open question remaining was whether these chemically very variable grains condensed all in the same crystal structure or in different structure types – information highly relevant to the validity and refinement of condensation models. In order to elucidate this question, we used the FIB to cut approximately 80 nm thin slices from 15 RMNs, some of which just measured 230 nm in diameter. Electron diffraction in the TEM then revealed that all of these grains are monophase, single crystals with solely hexagonal close packed (hcp) structures – quite surprising, given that many of them are dominated (up to 67%) by elements, which by themselves do not crystallize in hcp structures, such as Mo, Ir or Fe. Moreover, imaging of the grains’ cross sections showed very well developed crystal shapes in almost all cases, supporting the idea that these crystals slowly grew from a gas phase while freely floating in space. Although it is still unclear how the RMNs could have nucleated in the extremely diluted gas of the solar nebula, our findings validates the condensation origin of these grains and the cooling rates derived from them: Starting at 1350 °C the growth of the RMNs occurred over a time interval of about 400 years while the gas cooled down by about 200 °C[1]. Considering that the formation of the solar system took place on a time scale of millions of years, this process was amazingly rapid – another indication for a fairly rough youth.                             

[1] Berg T., Maul J., Schönhense G., Marosits E., Hoppe P., Ott U., and Palme H. (2009) Direct evidence for condensation in the early solar system and implications for nebular cooling rates. The Astrophysical Journal 702, L172-L176.

Dennis Harries
* 1981, Seesen am Harz

  • seit 2008
  • Doktorand am Bayerischen Geoinstitut, Universität Bayreuth. Forschung über Strukturen und reaktives Verhalten von Monosulfiden, Studien an Sulfiden und Metallen in extraterrestrischen Materialien.
  • 2002 - 2008
  • Studium der Geowissenschaften an der Georg-August-Universität Göttingen. Fachrichtung Geochemie. Entwicklung von Referenzmaterialien für Mikrobereichsanalytik und Thermochronologie.

Preise und Auszeichnungen
  • Gordon A. McKay Award der Meteoritical Society 2011
  • Paul-Ramdohr-Preis der Deutschen Mineralogischen Gesellschaft 2011