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The VNIR Spectral Properties of Iron Oxide/Oxyhydroxide Mixtures and Applications to Iron Oxides in the Mawrth Vallis Region of Mars

Introduction: Iron oxides/oxyhydroxides (FeOx) are present nearly everywhere on the surface of Mars [e.g. 1,2]. They are characterized in the visible/near-infrared (VNIR) region by a strong electronic absorption centered near 0.85-0.95 μm and additional absorptions that shape the character of the visible reflectance spectrum and their color [3,4]. The objective of this study is to measure the spectral properties of mixtures of FeOx minerals in order to better constrain the types and abundance of FeOx on Mars. Of particular interest is understanding the spectral character of mixtures containing magnetic minerals, as the Martian surface soil is magnetic [5]. Samples: Mixtures were prepared by weighing aliquots of the components and gently stirring the particles then shaking them in a <125 μm sieve. Table 1 lists the mixture compositions of magnetite (Mt: finely particulate, from Aldrich, sieved <125 μm), maghemite (Mh: synthetic, finely particulate, sieved <125 μm), hematite (Hm: <5 μm particle size, from Aldrich; note that subsequent Mossbauer measurements have determined substantial maghemite in this sample [6]) and ferrihydrite (Fh: finely particulate, from Iceland [7] sieved <45 μm). VNIR spectra were measured using an ASD spectrometer under ambient lab conditions of the particulate samples poured onto a black Teflon dish.