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 μm. 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 μm, 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 θ of 20°. A strong 5-μm emission feature was present during both observing periods. We suggest the 5-μm excess may be a result of near surface thermal gradients in regolith materials with a 30 to 100-μm 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 feldspathoidal surface composition.