Mid-infrared (8.1-12.5 mu m) imaging of Mercury

The first images of Mercury to be obtained in thermal emission are discusse d. Images at 22 wavelengths between 8.1 and 13.25 mum obtained with MIRAC ( Mid-Infrared Array Camera) have been reduced and analyzed to obtain new inf ormation regarding the thermal and chemical makeup of Mercury's surface. Th e region observed is centered at similar to 210-250 degrees mercurian longi tude, encompassing the location of a similar to 1000-km-diameter radar-brig ht region on Mercury's surface that was not imaged by Mariner 10. We found some evidence supporting ground-based spectroscopy of alkali-rich feldspath oids. However, the large pixel size (at the sub-Earth point the area per sq uare pixel is 28,000 km(2)-8 mum; 63,000 km(2)-12 mum), the diffraction lim it of the telescope, and the small angular diameter of Mercury all conspire d to prevent any compositional spatial discrimination on the Earth-facing d isk. Our four results are: (1) data fits to rough surface thermal models in dicate the surface is smoother at this location (Thetabar similar to8 degre es -11 degrees) than at previous locations (10-75 degrees and 100-160 degre es longitude) measured in the midinfrared (Thetabar similar to 20 degrees); (2) with the spatial resolution of these data, color ratios of images do n ot reveal any locations of enhanced or depressed flux that can be associate d with regions of different rock composition across the disk of Mercury; (3 ) 18-channel spectra generated from Mercury whole-disk averages exhibit emi ssivity features over part of the spectral range (8.1-10.5 mum) indicative of bronzite, a pyroxene composed of similar to2% CaSiO3, 86% MgSiO3, and 12 % FeSiO3. In addition, the overall spectrum was grossly matched by a spectr um 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 ass ociated with nephaline syenite rocks. On Earth, all of these materials are found in association in the Teschenite Sills of Shiant Isles, Scotland. (C) 2000 Academic Press.