Identification of a 10-μm silicate absorption feature in the Acidalia region of Mars

Using the SpectroCam-10 instrument on the Hale 200" telescope at Palomar Observatory, we obtained an extensive set of images and spectra of Mars in the thermal infrared during the 1993 and 1995 oppositions. Images were taken using a set of seven wide-band filters centered at 7.9, 8.8, 9.8, 10.3, 11.7, 12.5, and 17.9 μm. Spatially resolved slit spectra with a spectral resolving power of λ/Δλ = 100 between 7.5 and 13.5 μm were also obtained. Initial analysis of data from the 1993 opposition led to the identification of an absorption feature at 9.2 μm in the Acidalia region. At the time the feature was first identified, it was impossible to determine whether it was caused by a local enhancement of atmospheric dust over Acidalia, or by nonunit emissivity of surface materials in that region. Subsequent observations taken during the 1995 opposition showed that the feature in Acidalia persisted, indicating that it was of surficial origin. Because of its spectral position and shape, we interpret the feature to be the result of the emissivity spectrum of coarse (sand-sized or greater) silicate materials on the surface in Acidalia. While the spectral range available in our data is probably insufficient to make a specific mineralogical interpretation, we find it to be consistent with the emissivity spectrum of a terrestrial basalt sample measured in the laboratory.