Reflectance Spectra of Olivine–Orthopyroxene-Bearing Assemblages at Decreased Temperatures: Implications for Remote Sensing of Asteroids

Abstract

Spectral reflectance measurements of powdered olivine–orthopyroxene mixtures and two ordinary chondrites have been performed in the temperature range between 293 and 80 K. The decrease in temperature produced a number of significant spectral effects. The composite absorption feature due to olivine and pyroxene in the 1-μm region (band I) resolves into distinct bands, provided the opx/(opx+ol) ratio is less than 0.5. Since such a splitting makes it possible to better constrain the mineral abundances, we suggest that spectral data of future space missions acquired at large insolation angles (low surface temperatures) may be useful for the compositional interpretation. The pyroxene absorption band near 2 μm (band II) moves to shorter wavelength, as expected, and splits into two distinct bands even if the calcic pyroxene abundance is very low. Therefore, a possible detection of a clinopyroxene band beyond 2 μm in asteroid spectra does not indicate the presence of an abundant calcic pyroxene component. The increase in reflectivity of the interband peak near 1.5 μm with decreasing temperature suggests that the puzzling turnover of the infrared continuum slope of S-type asteroids might be caused, at least in part, by their lower surface temperatures. The position of a composite 1-μm absorption band (band I) is subject to an offset with decreasing temperature. The direction of the wavelength shift depends on the opx/(opx+ol) ratio. The band II/band I area ratio increases moderately. For relatively olivine-rich assemblages these effects offset the data points on the plot of band I center wavelength position versus band area ratio. The upward displacement of S(II) and possibly S(III) asteroid subgroups with respect to the olivine–orthopyroxene mixing line on the plot does not necessarily indicate high contents of calcic clinopyroxene on their surfaces.