Mercury:: Thermal modeling and mid-infrared (5-12 μm) observations

We present and use a new rough-surface thermal model to aid in the analysis of new mid-infrared spectral measurements of Mercury from 5-12.5 mu m. The model calculates spatially resolved thermal emission from slowly rotating, airless bodies. The Mercury data contain the first spectral measurements of Mercury between 5-7.5 mu m, a region not accessible from ground-based telescopes, and are also the first observations of Mercury to be made while flying on the Kuiper Airborne Observatory (KAO). They were obtained with the High-efficiency Infrared Faint Object Grating Spectrometer (HIFOGS). Our rough surface thermal model which best fit the KAO data has a Hapke <(theta)over bar> of 20 degrees. A strong 5-mu m emission feature was present during both observing periods. We suggest the 5-mu m excess may be a result of near surface thermal gradients in regolith materials with a 30 to 100-mu m grainsize, but cannot entirely rule out an observational artifact resulting from our instrumentation or the telescope on the KAO, Other features in the spectra are consistent with a feldspathic and feld-spathoidal surface composition. (C) 1998 Academic Press.