Mid-Infrared (8.1-12.5 μm) Imaging of Mercury

The first images of Mercury to be obtained in thermal emission are discussed. Images at 22 wavelengths between 8.1 and 13.25 μm obtained with MIRAC (Mid-Infrared Array Camera) have been reduced and analyzed to obtain new information regarding the thermal and chemical makeup of Mercury's surface. The region observed is centered at ˜210-250° mercurian longitude, encompassing the location of a ˜1000-km-diameter radar-bright region on Mercury's surface that was not imaged by Mariner 10. We found some evidence supporting ground-based spectroscopy of alkali-rich feldspathoids. However, the large pixel size (at the sub-Earth point the area per square pixel is 28,000 km² - 8 μm; 63,000 km² - 12 μm), the diffraction limit of the telescope, and the small angular diameter of Mercury all conspired to prevent any compositional spatial discrimination on the Earth-facing disk. Our four results are: (1) data fits to rough surface thermal models indicate the surface is smoother at this location (Thetabar ˜8°-11°) than at previous locations (10-75° and 100-160° longitude) measured in the mid-infrared (Thetabar ˜20°); (2) with the spatial resolution of these data, color ratios of images do not reveal any locations of enhanced or depressed flux that can be associated with regions of different rock composition across the disk of Mercury; (3) 18-channel spectra generated from Mercury whole-disk averages exhibit emissivity features over part of the spectral range (8.1-10.5 μm) indicative of bronzite, a pyroxene composed of ˜2% CaSiO₃, 86% MgSiO₃, and 12% FeSiO₃. In addition, the overall spectrum was grossly matched by a spectrum of picrite (an ultra-mafic rock) with 25% MgO, 8% CaO, and 11% FeO, and other minor components including plagioclase feldspar (4). TIMS six-channel simulations have a best match to sodalite (Na₈Al₆Si₆O₂₄C₁₂), a mineral associated with nephaline syenite rocks. On Earth, all of these materials are found in association in the Teschenite Sills of Shiant Isles, Scotland.