Fits of the Spitzer 2004 (black points at top) and Herschel 2012 (all other points) brightness temperatures of the Pluto-Charon system using a seasonal+diurnal model. The seasonal thermal inertia is 2000 MKS. Solid lines: model assuming that radiation originates at the surface at all Herschel wavelengths (case 4b in Table 2). Dashed lines: same, but using the wavelength-dependent absorption coefficient from Fig. 5 (case 6b).