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B.N. Khare, D.P. Cruikshank (NASA Ames), E.T. Arakawa, P.S. Tuminello (ORNL), M.E. Milham (Edgewood CBC)
For the development of systems for the detection of possible biological constituents on bodies in our solar system such as on Mars and Europa, the optical properties of water ice, microbes, and a variety of rocks and glasses are required to identify their contribution to a signal received by an optical or infrared detection system. We have been measuring the optical constants of many organic materials, such as tholins, kerogens, Murchison, poly HCN, solid and liquid hydrocarbons (CH4, C2H6 etc.), and inorganic materials such as solid and liquid sulfur, terrestrial rocks and glasses, over several decades previously at Cornell University and at present at NASA Ames in collaboration with the team at ORNL. In view of NASA's current goal to search for and detect life elsewhere, we have concentrated our efforts on measuring the optical constants of bacillus subtilis spores [Tuminello et al., Applied Optics 36, 2818 (1997)], bacillus cereus spores, kaolinite, montmorillonite, illite, a mixture of 1/3 each of kaolinite, montmorillonite, and illite (simulated dust), and egg albumin. Here we present the optical constants of the class of clay minerals called illite from 0.2 to 2.5 \mum from measurements made on optical quality thin films on transparent substrates and on sapphire semicylinders, and on particles suspended in the proper (index-matching) liquid. The group of mica-like clay minerals that constitute illite are typical of marine shales and soils derived from them, and are intermediate in composition and structure between muscovite and montmorillonite. These clays and minerals along with their crystalline form may have played an important catalytic effect in interacting with the organic molecules in the evolution of first life. We compare our result to those of Egan and Hilgeman [In, "Optical Properties of Inhomogeneous Materials; Applications to Geology, Astronomy, and Engineering," New York: Academic Press, 1979], which are too low, as they did not collect all of the scattered light at short wavelengths, where scattering effects are severe.