University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2004 • 107-109

Development of Advanced Raman Spectroscopic Methodology for Analysis of Cometary Materials
Viktor G. Hadjiev
TCSAM

Michael Zolensky
NASA-JSC

Andrew Coleman

Abstract--Researchers are engaged in the development of Raman microprobe methodology for identification and analysis of cometary material to be delivered to Earth by the NASA Comet Stardust Missions in early 2006. UH scientists have demonstrated that mineralogical characteristics of coma grains embedded in aerogel can be revealed by means of Raman microprobe imaging.

Ultimate goals of planetary scientific research are (1) to understand the origin of the solar nebula and the forces that formed Earth and the other planets, (2) to determine how individual planets, satellites, and small bodies have evolved, (3) to determine the processes that led to the diversity of solar system bodies, and (4) to determine the specific environmental conditions that may contribute to the origin of life. The Stardust Discovery Mission is expected to address most of these goals through a return to Earth of coma grains from comet Wild II and interplanetary dust samples. The Stardust mission successfully completed the comet sample collection phase on January 2, 2004. Coma grains from comet Wild II were collected by imbedding them into low-density silica aerogel with the grains plunging into the aerogel at 6.2 km/sec. Experiments and similar collections on the Mir space station show that the impact of 1-to-50 micron- sized grains will make carrot-shaped tracks, measuring up to 1 cm in length.^1,2 The captured grains come to rest at the end of the carrot tracks and are rather easily observed in the aerogel.

Goals of the project
The main goal of the project is to provide experimental methodology for rapid mineralogical characterization^3,4 of comet Wild II coma grains when returned to Earth, while the grains are still embedded in aerogel. Only this capability will permit us to recognize the captured grains which have been least altered by collection into the aerogel (occurring at 6.2 km/sec), permitting the time-consuming, painstaking detailed analyses to be performed on the most critical samples in the most efficient manner. Given the severe time constraints that Stardust analyses will be operating under, the capability to quickly recognize the small subset of the thousands of captured particles appropriate for complete examination is critical to the success of this mission. In addition, there is a need for studying not only the elemental composition of the grains but also the mineralogical composition.

The advantages of the Raman microprobe approach to cometary material are that Raman spectroscopy is (1) rapid to perform, (2) applicable to micron-sized or larger grains of diverse and fine-grained (nanometer-scale) mineralogy, (3) performable in-house, and (4) applicable to samples still encased in aerogel. Development of Raman microprobe methodology is thus central to the preliminary analysis of Stardust coma samples, and it is critical to the success of the Hayabusa asteroid mission projected to return to Earth in 2007. The methodology is also expected to reveal important information about mineralogical and/or other content of the comet coma particles.

Results
In this study we used a JY Horiba LabRam HR multi-channel spectrometer coupled to an Olympus optical microscope. Raman microprobes offer technique that have been used extensively for studying various minerals, in particularly small inclusions therein. The LabRam HR Raman system has advanced chemical mapping capabilities,⁵ that is, one can visualize the distribution of certain chemical structures or minerals on the surface of the grain. For the specific goals of the project, this technique needs to be refined and further developed toward establishing procedures for fast detection of particles in aerogel, building up of a spectral library for identification of minerals and inclusions identification, and Raman imaging of coma grains for visualization of their mineral distribution and contents.

We are organizing a Raman spectra database (GRAMS DB) with an integrated spectra identification engine (Spectral ID, GRAMS). More than 100 spectra of minerals have been measured, loaded in the database, and compared with those in the existing database.^6,7 Table 1 lists some of those minerals being incorporated into the spectra library.

Table 1. Major Minerals Spectra to be Catalogued

One of the important findings achieved in the project is that the confocal Raman microprobe enables a complete mineralogical analysis of the cometary grains while they are embedded in the aerogel, that is, nondestructively and without exposing the grains to ambient air.

Note the demonstration in the figures. Figure 1 shows the optical microscope image of an Olivine grain embedded in aerogel approximately 6 mm below the surface of the aerogel sample surface. Olivine has two sharp Raman peaks at 822 and 855 cm^-1. We performed a Raman mapped imaging of the grain by recording the Raman spectra taken from points on the grain surface at a 2-microns distance from each other. In Fig. 2, we show the Raman intensity distribution of the 855 cm^-1 line. The variation in Raman intensity at different points is attributed to the roughness of the surface; the points are not in the same focal plane. Besides the Raman mapping of mineral content, this result shows that we can also record the surface topologygraphy of a single mineral sample by this technique. In a more elaborated attempt (not shown here), we recorded a series of 2D images of the surface at various focal planes. Currently, we are working on the software needed for rendering 3D topographic images of the grain surface.

We also performed a successful mapping of the mineralogical composition of multi mineral grains.

Figure 1. Optical microscope image of an Olivine grain embedded in aerogel matrix. The grain was located at ~ 6 mm below the aerogel surface, at which the microscope picture was taken. The image reveals an apparent blur due to the diffuse elastic light scattering from the aerogel matrix.	Figure 2. Raman mapped image of the grain pictured in Fig. 1. The image represents the Raman scattering intensity distribution of the 855 cm-1 Olivine line. The effective refractive index of the aerogel is close to one, which makes this material a convenient matrix for the confocal Raman microprobe of particles embedded in it.

References
¹R. A. Barrett, M. E. Zolensky, F. Hörz, D. J. Lindstrom, and E. K. Gibson, "Suitability of Silica Aerogel as a Capture Medium for Interplanetary Dust," Proc., 19th Lunar and Planetary Science Conference. Eds. G. Ryder and V. Sharpton (1992): 203-12.
²F. Hörz, M. Zolensky, R. Bernhard, T. See and J. Warren, "Impact Features and Projectile Residues in Aerogel Exposed on Mir," Icarus 147 (2000): 559-79.
³E. Jessberger, et al."Properties of Interplanetary Dust: Information from Collected Samples," in Interplanetary Dust. Eds. E. Grun, B. Gustafdon, S. Dermott, and H. Fechtig. Springer Press, 2001. 253-94.
⁴A. Rubin, "Mineralogy of Meteorite Groups," Meteoritics and Planetary Science 32 (1997): 231-47.
⁵V. G. Hadjiev, S. Arepalli, P. Nikolaev, S. Jandl, and L. Yowell, "Enhanced Raman Microprobe Imaging of Single Wall Carbon Nanotubes," Nanotechnology 15.5 (2004): 562-67.
⁶G. R. Rossman, Mineral Spectroscopy Server. April 25, 2005. California Institute of Technology. March 11, 2005 <http://minerals.gps.caltech.edu/> (Raman spectra acquired at Caltech and from other sources).
⁷K.-P. Kelber, Raman Spectroscopy (RS): Links for Mineralogists. March 21, 2004. University of Würzburg. March 11, 2005 <http://www.uni-wuerzburg.de/mineralogie/links/tools/raman.html>.

Publications
Hadjiev, V. G., A. P. Coleman, and M. Zolensky. "Raman Imaging of Cometary Grains Embedded in Aerogel," (in preparation, 2005).

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Institute for Space Systems Operations - Y2004 Annual Report
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