Presolar grains discovery

The discovery happened by accident. Lewis and Anders were frustrated by their failure to find the carrier of anomalous xenon in carbonaceous chondrites. They decided to try an extreme treatment to see if they could dissolve the carrier. They treated a sample of the colloidal fraction of an Allende residue with the harshest chemical oxidant known, hot perchloric acid. The black residue turned white, and to their surprise, when they measured it, the anomalous xenon was still there The residue consisted entirely of carbon, and when they performed electron diffraction measurements on it, they found that it consisted of tiny (nanometer sized) diamonds. After a detailed characterization that included chemical, structural, and isotopic studies, they reported the discovery of presolar diamond in early 1987 (Lewis et al., 1987). The 23-year search for the carrier of CCFXe (Xe-HL) was over, and the study of presolar grains had begun. 

Almost immediately after the discovery of presolar grains, it was clear that they could only be found in the most primitive chondrites, those that had suffered the least amount of thermal metamorphism. Further work showed that the abundances of presolar grains, when normalized to the content of fine-grained matrix where the grains reside, correlated strongly... 

Lewis, R. S., Tang, M., Wacker, J. F., Anders, E. and Steel, E. (1987) Interstellar diamonds in meteorites. Nature, 326, 160-162. The paper describing the discovery of the first presolar grains. 

The cosmic chemical memory interpretation was advanced by the writer as a superior way to think of these isotopic anomalies. This picture argued that the early solar system was not hot enough to vaporize the entirety of most solids but only their volatile parts and portions of their refractory Ca-and-Al-rich minerals. The refractory parts had survived to that time from their earlier condensation as stardust and were fused into the CAI assemblages found today in the meteorites. That fusion occurred while the gas that was vaporized from a dust-rich presolar mixture recondensed as the main minerals of the CAIs. The refractory cores, being stardust that had condensed even earlier within individual stars and supernovae, contain the isotope ratios from those distinct sources. When these cores were fused into the CAIs found today, the chemistry remembered the isotope ratios of the source presolar grains, so thatsolar-system rocks (CAIs) remembered their isotopic parentage. Hence the name cosmic chemical memory. See l60 for a fuller account of the historical role played by the experimental discovery of l60-rich minerals within the CAIs, and of how the memory of l60-richness was saved. 

It is clear that the discovery of presolar grains and their detailed study have opened a new and fruitful field of astrophysical research. 

The discovery of presolar grains in meteorites has, for the first time, enabled the precise chemical and isotopic analysis of interstellar material (e.g., Anders and Zinner, 1993 Chapter 1.02). The huge variations in the isotopic compositions of all the elements analyzed in presolar grains is in stark contrast to the basically uniform isotopic composition of solar system materials (see Figure 1). This uniformity would have required an effective isotopic homogenization of all the material in the solar nebula, i.e., gas and dust, during the early stages of the formation of the solar system. 

The discovery of presolar grains was made possible by the development of chemical procedures in which carbonaceous meteorites were subjected to a stringent acid digestion regime. Carbon compounds such as diamonds, SiC and graphite were isolated in this manner and identified through their distinctive pattern and anomalous noble gas component. These carbonaceous phases are samples of interstellar matter which provide a window into the prehistory of the Solar System. 

The surface morphology of grains has been studied by secondary electron microscopy (SEM) (Hoppe et al., 1995). Such studies have been especially useful for pristine SiC grains that have not been subjected to any chemical treatment (Bernatowicz et al., 2003). Finally, the transmission electron microscope (TEM) played an important role in the discovery of presolar SiC (Bernatowicz et al., 1987) and internal TiC and other subgrains in graphite (Bernatowicz et al., 1991). It has also been successfully applied to the study of diamonds (Daulton et al., 1996) and of polytypes of SiC (Daulton et al., 2002, 2003). 

Continuing instrumental developments allow us to make new and more measurements on the grains and likely lead to new discoveries. For example, the NanoSIMS features high spatial resolution and sensitivity, making isotopic analysis of small grains possible, and this capability has already resulted in the discovery of presolar silicate grains in IDPs (Messenger et al., 2003)... 

Besides neon, xenon has turned out to be the element most diagnostic in its isotopic composition. An important role has been played by Xenon-HL, which was the first of the nucleosynthetic isotope anomalies to be discovered (Reynolds and Turner 1964). The HL component has received its name for the simultaneous overabundance of the heavy xenon isotopes (= Xe-H) and the light xenon isotopes (= Xe-L). Because the H part originally was more reliably determined, Xenon-HL was first believed to be associated with fission, possibly of a superheavy element (e.g., Anders et al. 1975), but in the end the search for its host phase led to the discovery of the existence of grains of presolar origin in primitive meteorites (Lewis et al. 1987). 

It was fortuitous that the resistance to acid digestion of presolar carbonaceous material led to their discovery. In the digestive procedures adopted, silicates and oxides are destroyed. It is hypothesized that most interstellar grains in the solar nebula are oxides, but apart from corundum (AI2O3) grains it has not been possible to isolate interstellar oxides. Furthermore the abundance of corundum grains is very small, although they do show isotopic anomalies in Mg, 1 0 and s-process Ti. 

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