Research

1. Presolar Grains in Meteorites and IDPs

Primitive meteorites and interplanetary dust particles (IDPs) contain small quantities (ppb-ppm) of presolar dust grains that formed in the winds of late-type stars or in the ejecta of stellar explosions (Fig. 1) [1,2]. These pristine samples represent stardust which can be analyzed with high precision in the laboratory. Their isotopic compositions and mineralogical properties provide a wealth of information on many astrophysical aspects such as stellar nucleosynthesis and evolution, Galactic chemical evolution, grain growth in stellar environments, interstellar chemistry, and on the inventory of stars that contributed dust to the Solar System. Presolar dust grains identified to date include diamonds, silicon carbide (SiC, Fig. 2), graphite, silicon nitride (Si3N4), corundum  and other forms of Al2O3, spinel (MgAl2O4), hibonite (CaAl12O19), and silicates (GEMS, olivine, pyroxene). The most refractory of these grains can be separated by chemical and physical treatments. This holds, e.g., for SiC, the best studied presolar mineral phase. Silicates, on the other hand, are destroyed by the chemical treatments used to isolate presolar grains from meteorites. Presolar silicates can be found by O-isotopic mapping of thin sections of meteorites and IDPs because they are tagged by large O-isotopic anomalies (Fig. 3). This technique became available only recently with the invention of the NanoSIMS ion probe.

The isotopic compositions of the major and of the most abundant trace elements in submcirometer- and micrometer-sized grains can be measured by secondary ion mass spectrometry (SIMS). Currently, our institute operates two SIMS instruments (ion probes), a modified Cameca IMS3f and a Cameca NanoSIMS 50. The NanoSIMS was delivered in 2001 and use of this new generation ion probe has opened a new window to astronomy. Examples for important breakthroughs are: (i) Discovery of silicate stardust in interplanetary dust particles and primitive meteorites. (ii) Extension of isotopic studies to submicrometer-sized presolar SiC and spinel grains separated from primitive meteorites by chemical and physical treatments. (iii) Identification of isotopic heteorogeneities within micrometer-sized presolar grains. (iv) Coordinated NanoSIMS-TEM studies made it possible to obtain simultaneous information on the mineralogy and isotopic composition of presolar materials.

Comets are believed to represent the most primitive matter in the Solar System. In 1999 NASA has launched the STARDUST satellite, a mission dedicated to cometary sample return. In 2004 STARDUST collected dust from comet WILD-2 which returned to Earth in January 2006. Isotope measurements with the NanoSIMS play a keyrole in the laboratory study of this new type of extraterrestrial matter [3].

Figure 1. The different phases in the life cycle of presolar (stardust) grains: (1) Formation around evolved stars. (2) Passage through the interstellar medium and incorporation into the molecular cloud from which our Solar System formed. (3) Survival inside small planetary bodies (asteroids, comets). (4) Transport to the Earth by meteorites or sample return missions. (5) Isotopic and mineralogical studies in terrestrial laboratories.

Figure 2. SEM photograph of a presolar silicon carbide grain from the Murchison meteorite. This grain is less than 1 micrometer in size and formed more than 4.57 billion years ago most likely in the wind of a red giant star.

Figure 3. 16O, 17O, and 17O/16O ion images of a 9 x 9 µ2-sized area in the matrix of the Acfer 094 meteorite. Two presolar silicate grains, about 300 nm in size, can be recognised by large enrichments in 17O.

2. Aerosol Particles

Sulfur isotope ratios of atmospheric aerosol particles can provide detailed information with regard to the origin and the transport of sulfur in the environment. The new Cameca NanoSIMS 50 ion microprobe technique permits analysis of individual aerosol particles with volumes down to 0.5 µm3 and a precision for δ34S of 3-10 ‰ (2 σ). This technique sets new standards in the analysis of isotope ratios in atmospheric aerosol and allows to directly compare chemical and isotopic compositions of individual aerosol particles, to identify internal and external mixtures and to investigate reactions of anthropogenic gases with natural aerosol such as sea salt and mineral dust [4, 5].

1) Hoppe P. and Zinner E. (2000) Presolar dust grains from meteorites and their stellar sources. J. Geophysical Res.105, 10371-10385.

2) Hoppe P. (2008) Reservoir for Comet Material: Circumstellar Grains. Space Sci. Rev. 138, 43-57.

3) McKeegan K. D. et al. (2006) Isotopic compositions of cometary matter returned by Stardust. Science 314, 1724-1728.

4) Winterholler B., Hoppe P., Andreae M. O., and S. Foley (2006) Measurement of Sulfur Isotope Ratios in Micrometer-Sized Samples by NanoSIMS. Applied Surface Science, 252, 7128-7131.

5) Sinha B. W., Hoppe P., Huth J., Foley S., and Andreae M. O. (2008) Sulfur isotope analyses of individual aerosol particles in the urban aerosol at a central European site (Mainz, Germany). Atmos. Chem. Phys. 8, 7217-7238.