  The Moon's optical characteristics in visible and long-wavelength infrared
(LWIR) have long been observed with our eyes or with instruments. What the
mid-infrared (MIR) Moon looks like is still a mystery. For the first time we
present detailed appearance of the MIR Moon observed by a high-resolution
geostationary satellite and reveal the essence behind its appearance. The
appearance of the MIR Moon is opposite to its normal visible appearance. In
addition the MIR Moon shows limb darkening. Both the absolute and the relative
brightness distribution of the MIR lunar disk changes with the solar incidence
angle. The signatures of the MIR Moon are controlled by both the reflection and
emission of the lunar surface. We also show first-ever brightness temperature
maps of the lunar disk without needing a mosaic, which better show the
temperature variation across the lunar disk. They reveal that the relationship
between brightness temperature and solar incidence angle i is cos1/bi, and the
power parameter is smaller than the Lambertian temperature model of cos1/4i
observed for lunar orbit-based measurements. The slower decrease of the
brightness temperature when moving away from the sub-solar point than the
Lambertian model is due to topographic effects. The brightness temperature is
dominated by albedo and the solar incidence angle and influenced by the
topography. Our results indicate that the Moon in the MIR exhibits many
interesting phenomena which were previously unknown, and contains abundant
information about lunar reflection and thermal emission for future study.
