Telescopic Raman Measurements of Glasses of Mineral Compositions to a Distance of 10 Meters

Introduction: Reflectivity spectra of Martian bright regions and laboratory experiments on synthetic glasses have indicated the presence of glass on the surface of Mars [1-3]. Glass formation on Mars can be from both volcanic and impact processes. Because of the broad nature of the reflectance bands, the exact composition and amount of glass on the surface of Mars is still a subject of discussion. The Martian meteorites (SNC, Shergottite-Nakhlite-Chassignite) contain diaplectic glass “maskelynite” formed by shock melting of feldspars. For example, in Shergottite the composition of maskelynite is normally in the range anorthite (An) 45 % to An 60 % (An 45 to An60), where An=CaAlSi3O8, with a few sodic outliners. Potassium is found to increase with albite (Ab, NaAlSi3O8) content [4-5]. Raman spectroscopy of isochemical minerals and glasses [e.g., 6-7], and micro-Raman measurements on thin-sections of Martian meteorite, have shown that Raman spectroscopy can easily distinguish between crystalline and glassy phases [8-9]. In this work, an improved version of a previously presented pulsed telescopic Raman system [10] has been used to explore the possibility of detecting and characterizing glasses of mineral compositions at a distance of ~10m. Remote Raman measurements are carried out on synthetic glasses along the join An100 to An31, as well as on a glass sample of fused quartz. Equipment: A pulsed telescopic Raman instrument (Fig. 1) for remote analysis of minerals on planetary surfaces is a modified version of an ealier instrument [10] developed at the University of Hawaii. The system consists of a 127mm telescope (Meade ETX125 Maksutov Cassegrain, 125mm clear aperture, 1900mm focal length), a frequency doubled mini Nd:YAG laser source (Model ULTRA CFR, Big Sky Laser, 532nm, 35mJ, 20Hz), and a HoloSpec f/2.2 spectrometer equipped with a gated intensified detector (PI Model I-MAX-1024-E). The width of the laser pulses was approximately 8ns, and the beam divergence was less than 8mrad. Typical slit widths for Raman spectral measurements were 100 to 200μm. Samples were placed at distances ranging 8 to 10m from the telescope. Samples: The fused quartz sample used for telescopic Raman measurement was a UV quality fused quartz circular (50mm diameter and 3mm thick) optical window. Samples of glasses along the join An-Ab were prepared at the Corning Inc., New York. Five compositions of glasses, An100, An75, An61, An50 and An31, were prepared using high purity oxides and carbonates in appropriate amounts. The mixtures were fused at