Integrating Near and Mid-Infrared Microspectroscopy with Experimental Petrology

One of the challenges of interpreting data gathered by remote sensing of planetary surfaces is that many of the samples in our spectral libraries that we use for comparison are necessarily terrestrial. Some contamination is easily recognized, such as alteration to hydrated minerals or contamination by organic material. In some cases the contamination is more subtle, such as when small amounts of ferric iron are present, altering the visible wavelength continuum of nominally ferrous silicates. Measurement of meteorite samples (e.g., 1, 2-5) and lunar samples (e.g. 6, 7) provides critical ground-truth data, but often the amount of samples allocated for study are smaller than can be measured in many labs. Experimentally created samples, as well, often produce only small amounts of powder that are difficult to measure without specially designed spectrometers such as the RELAB. FTIR Microspectroscopy has been investigated as an option for studying very small samples in reflectance, but scattering properties of the sample and the intensity of the illumination can result in extremely low signal spectra (8). Fortunately, a more intense source and modified optics allow significantly more signal in the near-infrared, enabling spectral characterization of diagnostic features even in reflectance. Spot sizes on the order of 50-100um allow individual grains or mounted sample charges to be targeted and measured, as long as the sample is slightly roughened to minimize specular reflectance off of the target. This new capability affords us the opportunity to collect spectra for experimental materials created for understanding igneous and metamorphic processes on other planets. Currently, we are incorporating these techniques with two studies aimed at understanding volcanism and mantle processes on Mercury. In the first study, we are conducting a series of melting experiments to explore the mineral composition of the Northern Volcanic Plains of Mercury. In the second, we are investigating reactions between solid Fe-free silicates and iron sulfide at very low oxygen fugacities. We will present experimental results paired with spectral measurements of the samples for each of the experimental runs and evaluate them in the context of data acquired by the MESSENGER spacecraft's Mercury Atmospheric and Surface Composition Spectrometer Visible and Infrared Spectrograph. References: 1. T. Hiroi et al., EPS 53, 1071 (2001). 2. T. Hiroi et al., Science 293, 2234 (2001). 3. R. G. Mayne et al., MAPS 40, 5174 (2005). 4. T. J. McCoy et al., MAPS 34, 735 (1999). 5. J. M. Sunshine et al., Science 320, 514 (2008). 6. Pieters, et al., Icarus 184, 83 (2006). 7. Isaacson et al., MAPS 46, 228 (2011). 8. Klima and Pieters, JGR 111, 01005 (2006).