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

E. Lellouch, P. Santos-Sanz, S. Fornasier, T. Lim, J. Stansberry, E. Vilenius, C. Kiss, T. Müller, G. Marton, S. Protopapa, P. Panuzzo, R. Moreno

Research output: Contribution to journalArticle

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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 (TB) 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 TB 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.

Original languageEnglish
Article numberA2
JournalAstronomy and Astrophysics
Volume588
DOIs
Publication statusPublished - Apr 1 2016

Fingerprint

Charon
Pluto (planet)
Pluto
thermal emission
emissivity
wavelength
brightness temperature
wavelengths
allowances
sounding
asteroids
transparency
solar system
asteroid
light curve
horizon
temperature measurement
observatories
coverings
penetration

Keywords

  • Kuiper belt objects: individual: Charon
  • Kuiper belt objects: individual: Pluto
  • Methods: observational
  • Planets and satellites: surfaces
  • Techniques: photometric

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

The long-wavelength thermal emission of the Pluto-Charon system from Herschel observations. Evidence for emissivity effects∗. / Lellouch, E.; Santos-Sanz, P.; Fornasier, S.; Lim, T.; Stansberry, J.; Vilenius, E.; Kiss, C.; Müller, T.; Marton, G.; Protopapa, S.; Panuzzo, P.; Moreno, R.

In: Astronomy and Astrophysics, Vol. 588, A2, 01.04.2016.

Research output: Contribution to journalArticle

Lellouch, E, Santos-Sanz, P, Fornasier, S, Lim, T, Stansberry, J, Vilenius, E, Kiss, C, Müller, T, Marton, G, Protopapa, S, Panuzzo, P & Moreno, R 2016, 'The long-wavelength thermal emission of the Pluto-Charon system from Herschel observations. Evidence for emissivity effects∗', Astronomy and Astrophysics, vol. 588, A2. https://doi.org/10.1051/0004-6361/201527675
Lellouch, E. ; Santos-Sanz, P. ; Fornasier, S. ; Lim, T. ; Stansberry, J. ; Vilenius, E. ; Kiss, C. ; Müller, T. ; Marton, G. ; Protopapa, S. ; Panuzzo, P. ; Moreno, R. / The long-wavelength thermal emission of the Pluto-Charon system from Herschel observations. Evidence for emissivity effects∗. In: Astronomy and Astrophysics. 2016 ; Vol. 588.
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AU - Lim, T.

AU - Stansberry, J.

AU - Vilenius, E.

AU - Kiss, C.

AU - Müller, T.

AU - Marton, G.

AU - Protopapa, S.

AU - Panuzzo, P.

AU - Moreno, R.

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N2 - 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 (TB) 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 TB 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.

AB - 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 (TB) 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 TB 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.

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KW - Techniques: photometric

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