Meteorites provide potential for identification
Several types of crystalline dust grains have been extracted from meteorites that were formed in the molecular gas environment, includ- ing silicates, nanodiamonds, silicon carbide, graphite, silicon nitride, corundum, hibonite, and spinel. Laboratory investigation of the me- teorite samples with instruments such as electron microscopes, ion microprobes, atomic force micro- scopes, synchrotron microprobes, and laser probe mass spectrometers will provide an extraordinary op- portunity to examine interstellar grains at the highest possible level of detail. The comparison of these materials with primitive meteorites and collected interplanetary dust samples will provide the basis for examining the pre-solar solids that were involved in solar system for- mation. These data will provide fundamental insight into the mate- rials, processes, and environments that existed during the origin and early evolution of the solar system over a wide range of distance from its center.
Morlok is working to establish a database of Infrared spectra of crystalline silicates found in pri- mitive meteorites. These spectra can be compared to spectra from astronomical observations of dust in molecular clouds, discs around young solar systems, and interstellar wind ejected from supernovas. Morlok is one of the first to go be- yond the use of terrestrial minerals and standards for use in finger- printing remote sensing spectra.
The first challenge that he faces is separating the common and well- known materials that constitute the vast majority of most meteorites from the tiny organic and inorganic grains that might shed light on the formation of the solar system. He uses mechanical methods and an optical microscope to separate out the grains. Grains that are in the milligram size range can be ana- lyzed directly with a conventional Infrared spectrometer. But many grains have dimensions in the tens of microns, about one-fifth to one- tenth the width of a human hair, and their weight is measured in nanograms. Once he has separated
the grain, he then grinds the material and places it in a diamond compres- sion cell that flattens soft samples for measurement by transmission. The sample is compressed to op- timum transmission thickness, which also has the effect of making the sample area larger.
Move to Infrared microscopy
“Analyzing nanogram samples on a conventional FT-IR spectrometer would be very time-consuming and require a great deal of skill, particu- larly to centralize the sample in the disk,” Morlok said. “Several years back, we switched to Infrared micro- scopy, which greatly reduces the amount of sample preparation. Coupling an FT-IR instrument to an IR microscope provides identical information to that of traditional IR spectroscopy but allows the IR light and therefore the measurement area, to be much smaller, enabling the measurement of microscopic sam- ples or very small areas within sam- ples. Fourier transform processing yields clear spectra in very little time.”
Morlok’s supervisor, Professor Monica Grady, recognized the need for an FT-IR microscope. So Morlok
Figure 2. The AutoIMAGE microscopy and Spectrum One systems in the laboratory at the Natural History Museum of London.