Reflectance spectra of olivine-orthopyroxene-bearing assemblages at decreased temperatures: Implications for remote sensing of asteroids

Spectral reflectance measurements of powdered olivine-orthopyroxene mixture s and two ordinary chondrites have been performed in the temperature range between 293 and 80 K. The decrease in temperature produced a number of sign ificant spectral effects. The composite absorption feature due to olivine a nd pyroxene in the 1-mu m region (band I) resolves into distinct bands, pro vided the opx/(opx + ol) ratio is less than 0.5, Since such a splitting mak es 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 interpretati on. The pyroxene absorption band near 2 mu m (band II) moves to shorter wav elength, as expected, and splits into two distinct bands even if the calcic pyroxene abundance is very low. Therefore, a possible detection of a clino pyroxene band beyond 2 mu m in asteroid spectra does not indicate the prese nce of an abundant calcic pyroxene component. The increase in reflectivity of the interband peak near 1.5 mu m with decreasing temperature suggests th at the puzzling turnover of the infrared continuum slope of S-type asteroid s might be caused, at least in part, by their fewer surface temperatures. T he position of a composite 1-mu m 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 increase s moderately. For relatively olivine-rich assemblages these effects offset the data points on the plot of band I center wavelength position versus ban d area ratio. The upward displacement of S(II) and possibly S(III) asteroid subgroups with respect to the olivine-orthopyroxene mixing line on the plo t does not necessarily indicate high contents of calcic clinopyroxene on th eir surfaces. (C) 2000 Academic Press.