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Paper No. 53-5
Presentation Time: 2:55 PM
COMPOSITION AND MORPHOLOGY FROM RADIOMETRY AND EMISSIVITY AT 4.2 CM OBTAINED DURING THE NEW HORIZONS PLUTO ENCOUNTER
The radio brightness temperatures of Pluto and Charon were measured at a wavelength of 4.2 cm, during the New Horizons encounter of the Pluto system on July 14, 2015, (a) on the dayside on approach, (b) near closest approach to Pluto, and (c) for the nightside on departure. These measurementsused the on-board Radio Science Experiment (REX). Pluto and Charon were unresolved for (a), but the measurements near closest approach (b), scanned two chords across the Pluto disk with a resolution of about 12% of Pluto’s diameter. Further, the nightside measurements, during the REX radio occultation, scanned across the Pluto at a resolution close to 50% of Pluto’s diameter. Separate full-disk observations of Pluto and Charon were done on departure. Connecting the brightness temperatures to composition and morphology involves knowledge of the emissivity, not only for the surface materials, but also the composition and structure of the subsurface, considering the low absorptivity of the materials on Pluto and Charon at 4.2 cm wavelength. Independently, the surface material composition on Pluto’s dayside was imaged at high resolution by the New Horizons on-board infrared imaging spectrometers [1,2], but only to a depth of the order of 0.1 cm. Adding to the uncertainties, the emissivity of materials identified by the spectral images, such as tholins and the ices of nitrogen and methane, at temperatures in the range 30-40 K is poorly known, and further confounded by the lack of knowledge of the subsurface. Models of emissivity, derived from sparse laboratory experiments and extended via numerical computation, are introduced for choices of surface and subsurface composition and structure. Sorting out possible physical temperatures for Pluto and Charon is a task that relies on fitting the observations within the constraints of emissivity, surface composition, topography and physical plausibility.
[1] Stern S. A., et al., The Pluto System: Initial Results from its exploration by New Horizons, Science 350, id.aad1815 (2015).
[2] Protopapa S., et al., Pluto's global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data. Icarus 287, 218-228 (2017).