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

TL;DR

This study analyzes Herschel observations of the Pluto-Charon system, revealing a decrease in brightness temperature with wavelength due to low surface emissivity, which impacts thermal modeling of icy bodies.

Contribution

It provides the first detailed analysis of the long-wavelength thermal emission of Pluto-Charon, demonstrating the importance of emissivity effects in interpreting thermal data.

Findings

Brightness temperature decreases from 53 K at 20 μm to 35 K at 500 μm.

Surface emissivity declines from 1 at 20-25 μm to ~0.7 at 500 μm.

Implications for interpreting thermal measurements at centimeter wavelengths.

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