Brightness temperature (TB) of the Pluto system. Most thermal observations from ISO, Spitzer, Herschel, and ground-based telescopes are gathered. As data were acquired at different epochs spanning ~25 yr, the TB are rescaled by 1/ to a common epoch (September 2004, rh = 30.847 AU). Solid lines: eight Spitzer/IRS spectra over 21−37 μm, taken in Aug.-Sept. 2004 at east longitudes of 33 (yellow), 78 (gray), 122 (light blue), 168 (dark blue), 213 (red), 257 (green), 302 (pink), and 348 (black). Filled circles: Spitzer/MIPS photometric measurements, taken at similar longitudes (37, 80, 127, 172, 218, 264, 307, and 351, same color codes) in September 2004. The Spitzer data are taken from Lellouch et al. (2011, hereafter Paper I). Open circles: additional Spitzer/MIPS data at 71.42 μm from April 2007 (see Fig. 13 from Paper I). Triangles: Herschel data at 70, 100, 160, 250, 250, and 500 μm from this work. Filled squares: data from ISO at 60 and 100 μm taken in 1997 (Lellouch et al. 2000a). The Herschel, Spitzer 2007 and ISO data are reinterpolated to the eight longitudes observed by Spitzer in 2004. The ISO 100 μm (resp. Spitzer 2007) data are shifted by 3 μm (resp. 2 μm) for easier legibility. The comparison between the open and filled circles at 71 μm illustrates the “Pluto fading” witnessed by Spitzer from 2004 to 2007. Additional data (averaged over longitudes, no error bar) from ISO at 150 and 200 μm, and from unpublished Spitzer 156 μm observations taken in October 2008, are shown as filled squares and open circles respectively, under the labels “Mean ISO” and “Mean MIPS 2008”. For ground-based datasets sampling more than one longitude, only the average TB is plotted. The SMA-measured TB refers to Pluto only. The dotted lines show thermophysical model fits (see text), calculated for the conditions of September 2004. Gray dotted line: parameters are from Case 4 in Table 2. Blue dotted line: same, but with spectral emissivities = 1. This latter case still produces brightness temperatures that decrease with wavelength, a consequence of the spatial mixing of different surface temperatures.
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