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 o
f Mercury between 5-7.5 mu m, a region not accessible from ground-based tel
escopes, and are also the first observations of Mercury to be made while fl
ying on the Kuiper Airborne Observatory (KAO). They were obtained with the
High-efficiency Infrared Faint Object Grating Spectrometer (HIFOGS). Our ro
ugh surface thermal model which best fit the KAO data has a Hapke <(theta)o
ver 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 ne
ar surface thermal gradients in regolith materials with a 30 to 100-mu m gr
ainsize, but cannot entirely rule out an observational artifact resulting f
rom our instrumentation or the telescope on the KAO, Other features in the
spectra are consistent with a feldspathic and feld-spathoidal surface compo
sition. (C) 1998 Academic Press.