How do we study presolar grains?
Presolar grains are very small (smaller than a few millionths of a meter), so sophisticated instrumentation is required to study them. Some of the laboratory instruments and techniques that have been used to analyze presolar grains are described here.
Secondary Ion Mass Spectrometry
Secondary Ion Mass Spectrometry (SIMS) has been used for the majority of the isotopic measurements performed on presolar grains. SIMS works by "sand-blasting" a sample with a beam of ionized cesium or oxygen atoms (primary ions). Atoms from the sample surface are knocked off and some of these get ionized in the process (secondary ions). The secondary ions are passed through a mass spectrometer, which uses electric and magnetic fields to determine their mass. (See http://www.cea.com/tutorial.htm and http://eps.unm.edu/simslab/basics.html for more basic info on SIMS) Shown here is a schematic of Cameca SIMS instruments ("ion probes") used for most presolar grain measurements:
The SIMS technique has the advantage that it is very sensitive. Multiple elements, including those present at minor or trace levels, can be analyzed in single micron-sized dust grains. The elements which have been measured in single presolar grains by SIMS include H, Li, C, N, O, K, Mg, Si, Ca, and Ti.
Some ion probes, like the one shown schematically above, also have the capacity to function as ion microscopes. That is, they can produce a magnified image of the sample surface in a given isotope. This capability has been exploited for presolar grain studies by using a digital camera to digitize the ion images produced on the image detector. By acquiring images in different isotopes, one can then use computer software to rapidly determine isotopic ratios in large numbers of particles simultaneously. For example, shown here are two ion images acquired on oxide grains separated from the Tieschitz ordinary chondrite:
On the left is the distribution of 16O atoms across the sample surface, on the right are 18O atoms. The scale of the image is about 100 microns across. Each blob is a different grain and the color scales are set such that a grain with a terrestrial 16O/18O ratio will look the same in each image. Most of the grains appear the same in the two images, but one (marked by an arrow) is clearly deficient in 18O. This is a presolar aluminum oxide grain. Ion images like these have been used to find most of the known presolar oxide grains in meteorites. This technique has also been used to find very rare sub-types of presolar silicon carbide grains, including X-grains, believed to have originated in supernova explosions.
Laser Noble-gas Mass Spectrometry
Scanning Electron Microscopy
Scanning electron microscopy (SEM) works by focusing a small beam of electrons on a sample surface. These electrons can either cause secondary electrons to be ejected from the atoms in the sample or can be reflected back. In either case, the intensity of the electrons coming off the surface varies according to the chemical composition and surface topography. By moving the beam across the sample in a regular pattern (scanning), very high magnification images can be formed which give a great deal of information about the sample. Moreover, the bombardment of electrons can cause X-rays to be ejected by the sample as well. The energy of the X-rays depends on the type of atoms from which they came and thus one can determine the elements present by looking at the X-ray energy spectrum.
SEMs are used in presolar grain research to characterize the morphology of grains and determine their mineralogy based on their major element abundances. (For examples of SEM images of presolar grains, see Types of Presolar Grains). SEM-X-ray spectroscopy has also been used to automatically identify presolar grains in situ in meteorite slices and on sample mounts prior to SIMS analysis.
Transmission Electron Microscopy
In transmission electron microscopy, a beam of high-energy electrons is passed through a very thin sample. The pattern of electrons that makes it through depends on the crsytal structure of the sample. Thus one can use TEMs to determine the atomic-scale arrangement of a sample. Moreover, one can determine the chemical composition of samples (both elemental abundances and in some cases, what types of molecular bonding is present) by examining in detail the spectrum of energies of X-rays emitted by the sample and the energy lost by the electrons as they pass through the sample.
TEM techniques have been used for a number of different presolar studies. For example, the element carbon can form very different crystal structures, including diamond and graphite. Using the TEM, scientists have determined that some presolar carbon grains are very small diamonds and larger carbon grains are graphitic in structure (see Types of Presolar Grains for an example TEM image of presolar nanodiamonds). In addition, Professor Tom Bernatowiczof Washington University has used a very sophisticated diamond knife to cut presolar grains of graphite and silicon carbide into very thin slices and examined them in the TEM. Through these studies he has discovered that many presolar grains contain sub-grains of different minerals (for example TiC). These results give information about the physical conditions of the stars where the grains formed (pressures, temperatures, etc.).