Thermal properties of Pluto's and Charon's surfaces from Spitzer observations

We report on thermal observations of the Pluto-Charon system acquired by the Spitzer observatory in August-September 2004. The observations, which consist of (i) photometric measurements (8 visits) with the Multiband Imaging Photometer (MIPS) at 24, 70 and 160μm and (ii) low-resolution spectra (8 visits) over 20-37μm with the Infrared Spectrometer (IRS), clearly exhibit the thermal lightcurve of Pluto/Charon at a variety of wavelengths. They further indicate a steady decrease of the system brightness temperature with increasing wavelength. Observations are analyzed by means of a thermophysical model, including the effects of thermal conduction and surface roughness, and using a multi-terrain description of Pluto and Charon surfaces in accordance with visible imaging and lightcurves, and visible and near-infrared spectroscopy. Three units are considered for Pluto, respectively covered by N₂ ice, CH₄ ice, and a tholin/H₂O mix. Essential model parameters are the thermal inertia of Pluto and Charon surfaces and the spectral and bolometric emissivity of the various units. A new and improved value of Pluto's surface thermal inertia, referring to the CH₄ and tholin/H₂O areas, is determined to be Γ_Pl=20-30Jm^-2s^-1/2K^-1 (MKS). The high-quality 24-μm lightcurve permits a precise assessment of Charon's thermal emission, indicating a mean surface temperature of 55.4±2.6K. Although Charon is on average warmer than Pluto, it is also not in instantaneous equilibrium with solar radiation. Charon's surface thermal inertia is in the range Γ_Ch=10-150 MKS, though most model solutions point to Γ_Ch=10-20 MKS. Pluto and Charon thermal inertias appear much lower than values expected for compact ices, probably resulting from high surface porosity and poor surface consolidation. Comparison between Charon's thermal inertia and even lower values estimated for two other H₂O-covered Kuiper-Belt objects suggests that a vertical gradient of conductivity exists in the upper surface of these bodies. Finally, the observations indicate that the spectral emissivity of methane ice is close to unity at 24μm and decreases with increasing wavelength to ∼0.6 at 100μm. Future observations of thermal lightcurves over 70-500μm by Herschel should be very valuable to further constrain the emissivity behavior of the Pluto terrains.