The long-wavelength thermal emission of the Pluto-Charon system from Herschel observations. Evidence for emissivity effects^⋆

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: emmanuel.lellouch@obspm.fr
² Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ European Space Astronomy Centre (ESAC), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁶ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁷ Konkoly Observatory of the Hungarian Academy of Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
⁸ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁹ GEPI, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France

Received: 2 November 2015
Accepted: 21 December 2015

Abstract

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 μm altogether. The thermal light curve of Pluto-Charon is observed in all filters, albeit more marginally at 160 and especially 500 μm. 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 ~53 K at 20 μm to ~35 K at 500 μm, 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 μm to ~0.7 at 500 μm. 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 ~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.