Pluto's non-isothermal surface

We report on repeated far-infrared photometric observations of the Pluto-Ch aron system conducted in 1997 with the Infrared Space Observatory (ISO). Th ese observations have led to the first detection of the system at 150 and 2 00 mu m and to the first clear detection of its thermal lightcurve at 60 mu m (and more marginally at 100 mu m). They definitely prove that Pluto's su rface is not isothermal. The thermal lightcurve is, as expected, roughly an ticorrelated with the visible lightcurve, but not exactly. The data are fit by physical models including Charon and three separate units on Pluto, res pectively dominated by (1) N-2-ice (2) CH4-ice, and (3) tholins. These mode ls are constructed in accordance with information from visible imaging and lightcurves, visible spectroscopy and infrared spectroscopy, and considerat ions on the thermal balance of N-2 and CH4, and they include a thermophysic al description of subsurface conduction and infrared beaming. Charon's cont ribution, which cannot be separated from Pluto's in the observations, is as sumed to be independent of longitude and equivalent to that of a similar to 52 K body. The main implications are that Pluto's surface in units 2 and 3 has a thermal inertia Gamma = (1.5-10) x 10(4) erg cm(-2) s(-1/2) K-1, com parable to that of other icy satellites, and relatively high bolometric emi ssivities (not lower than 0.5 and most likely 0.8-1). Diurnal temperature v ariations must be significant, with maximum dayside temperatures in the ran ge 54-63 K. The value of thermal inertia may be indicative of porosity in t he top centimeters of Pluto's surface. The observations further confirm tha t the far-IR brightness temperatures, though somewhat smaller than indicate d by IRAS, are higher than in the millimeter/submillimeter range. Extending the models to longer wavelengths suggests that a low radio emissivity, as opposed to a mixing of temperatures or a subsurface sounding effect, is the correct explanation. Finally, in spite of large error bars, the 150-mu m f luxes indicated by ISO seem unexpectedly high given the spectral properties of ices in the far-IR. These, and the expected lightcurves of the Pluto-Ch aron system at lambda = 15-60 mu m should be priority measurements for SIRT E. (C) 2000 Academic Press.