Raman imaging reveals details of Mars meteorite composition

At the same time the Mars Pathfinder mission continues to send images and information that its x-ray spectrometer-containing Sojourner probe obtains from the Martian surface (see Laser Focus World, Aug. 1997, p. 20), ongoing studies in earthbound laboratories are analyzing allocations of the meteorite ALH84001. As one of only 12 known Mars meteorites, ALH84001 has been in the "spotlight" since last year, when laser-ablation/mass spectrometry investigations indicated the presence of carbonaceous compounds, which triggered wide speculations that, in the past, life may have been present on Mars. Now, researchers Timothy Prusnick and Patrick Treado at ChemIcon (Pittsburgh, PA), in conjunction with Allan Treiman of the Lunar & Planetary Institute (Houston, TX), are applying ChemIcon's Falcon Raman microscope to further analyze ALH84001.

Raman scattering in the Falcon is stimulated with visible radiation from a Spectra-Physics (Mountain View, CA) Millennia frequency-doubled diode-pumped Nd:YVO₄ laser (see p. 91). Wavelength selection of the scattered light is performed with a liquid-crystal tunable filter (Cambrige Research and Instrumentation, Cambridge, MA). ChemIcon president Patrick Treado says, "This imaging system is the only Raman technology ever developed that combines the spectral performance of a conventional Raman spectrometer and the spatial resolving power of optical microscopy." The Falcon has the ability to analyze samples in situ under ambient conditions, and, as a result, chemical information can be obtained at high spatial resolution (up to 250 nm) without requiring sample preparation or extensive sample handling.

ALH84001 is an igneous orthopyroxenite, and ongoing research is characterizing the spatial distribution and molecular composition of inorganic and carbonaceous materials in its allocations (the sample shown is about 3 mm along the long axis). Initial studies have focused on surveying images and Raman spectra of the predominantly orthopyroxene inorganic host (middle and right, respectively). According to Treado, characterizing the precise distribution of carbonaceous minerals and organic compounds in relation to mineral host structures (that is, cracks, grain boundaries, and surface features) at high spatial resolution could contribute fundamentally to the understanding of biotic or pre-biotic chemistry on Mars.

NASA recently released several more allocations of ALH84001 to ChemIcon for further studies. Treado and his colleagues discussed these ongoing experiments in several presentations last month at the Federation of Analytical Spectroscopy Societies conference (Providence, RI). The in situ capability of the Falcon imaging system is also useful for monitoring samples in extreme conditions such as reactors, steam generators, and radiation-contaminated environments. It is Treado's hope that, "in the not too distant future, the surface of Mars may be accessible to Raman imaging as well."