Atr-ir Spectroscopy for in Situ Mineral Analysis on Planetary Surfaces: Steps toward a Forward Model

Introduction: Attenuated total reflectance (ATR) is an infrared spectroscopic method that has potential as a tool for in situ quantitative mineralog-ical analysis on planetary lander missions. The sample of interest is placed in close physical contact with a crystal of high refractive index, such as Ge or diamond. Modulated infrared light from an FTIR interferometer enters the crystal at such an angle that when the light hits the interface with the sample, total internal reflection occurs. Light penetrates the sample via an evanescent wave that rapidly decays away from the interface. The beam is attenuated at frequencies corresponding to the fundamental vibra-tional modes and overtones of the sample crystal structure. The resultant ATR spectrum has similar peak positions as an equivalent transmission IR spectrum. ATR-IR potentially presents advantages over currently employed in situ planetary mineralog-ical methods, as it requires minimal sample preparation (grinding) and is effective with fine-grained materials. ATR-IR has been used extensively in the study of polymers and organics ([1,2]) and in a few geological applications ([3,4]), typically for qualitative phase identification. ATR can be used to identify and distinguish between a variety of clays, sulfates and oxides of martian significance [5]. Few studies have attempted quantitative interpretation of ATR spectra of single-phase powders or mixtures [6]. We recently modeled ATR spectra of mineral mixtures using linear deconvolution [7]. Although the decon-volution algorithm was generally successful in identifying the component minerals, modeled proportions were often inaccurate; the spectra were dominated by non-linear grain size and interaction effects. Here we present new spectra of silicates and ongoing efforts to develop a forward model from first principles of ATR for geological samples. Methods and Spectra: We analyzed powders of two minerals: San Carlos olivine (Fo 90) and quartz (var. rose, Riverside, CA). Minerals were ground and sieved to a series of grain sizes (<10 µm, 10-20 µm, 20-45 µm, 45-90 µm). Spectra were collected at Stony Brook University on a Nicolet 6700 FTIR spectrometer equipped with a SmartOrbit ATR accessory , a thermo-electrically cooled DLaTGS de