  Thermal observations of the Pluto-Charon system acquired by the Herschel
Space Observatory in February 2012 are presented. They consist of photometric
measurements with the PACS and SPIRE instruments (nine visits to the Pluto
system each), covering six wavelengths from 70 to 500 $\mu$m altogether. The
thermal light curve of Pluto-Charon is observed in all filters, albeit more
marginally at 160 and especially 500 $\mu$m. Putting these data into the
context of older ISO, Spitzer and ground-based observations indicates that the
brightness temperature (T$_B$) of the system (rescaled to a common heliocentric
distance) drastically decreases with increasing wavelength, from $\sim$53 K at
20 $\mu$m to $\sim$35 K at 500 $\mu$m, and perhaps ever less at longer
wavelengths. Considering a variety of diurnal and/or seasonal thermophysical
models, we show that T$_B$ values of 35 K are lower than any expected
temperature for the dayside surface or subsurface of Pluto and Charon, implying
a low surface emissivity. Based on multiterrain modeling, we infer a spectral
emissivity that decreases steadily from 1 at 20-25 $\mu$m to $\sim$0.7 at
500~$\mu$m. This kind of behavior is usually not observed in asteroids (when
proper allowance is made for subsurface sounding), but is found in several icy
surfaces of the solar system. We tentatively identify that a combination of a
strong dielectric constant and a considerable surface material transparency
(typical penetration depth $\sim$1 cm) is responsible for the effect. Our
results have implications for the interpretation of the temperature
measurements by REX/New Horizons at 4.2 cm wavelength.
