A Tale of Two Hemispheres: Scattering Properties of Pluto's Sputnik Planitia and Cthulhu Macula Revealed by Near-Opposition Photometry

A solar system body is at "true" opposition at node crossings when the Earth transits the solar disk as viewed from the object. In this configuration, the object is visible at the smallest solar phase angles and when combined with observations at larger phase angles, the resulting reflectance measurement can be related to the optical, structural, and thermal properties of the regolith and its inferred collisional history. Pluto was at true opposition when it crossed the line of nodes for the first time in 87 years in July 2018 and, owing to the eccentricity of its orbit, won't be at true opposition again for another 161 years. Due to its 6.4-day rotation period, only the Charon-facing hemisphere was visible at opposition in 2018. At the subsequent opposition in 2019, Pluto was still observable at small phase angles and presented the opposite hemisphere to Earth-based viewers. The most prominent features on this anti-Charon hemisphere are Tombaugh Regio and Sputnik Planitia, the vast, nitrogen-filled, high albedo basin observed at high spatial resolution by the New Horizons (NH) spacecraft during its 2015 flyby. Since the sub-Earth latitudes were 55° and 56° N in 2018 and 2019, respectively, observations in both years include much of Lowell Regio, Pluto's bright northern polar cap. Here we present and compare solar phase curves of each of these hemispheres acquired between 2015 and 2019 with HST's Wide Field Planetary Camera 3's UVIS F555W filter (0.53 microns). Since Lowell Regio dominates 60% of the anti-Charon hemisphere and 80% of the Charon-facing hemisphere, the shapes of the two solar phase curves are similar; however, any differences in their shapes reveal differences in the scattering properties and physical surface characteristics of the terrains unique to each. The NH encounter hemisphere is ~0.1 mag brighter than the Charon-facing hemisphere in the F555W filter. No part of Sputnik Planitia is visible in the Charon-facing hemisphere, so any differences between its solar phase curve and that of the anti-Charon hemisphere are mainly due to the scattering properties of Pluto's darker equatorial regions. Preliminary results indicate that the slope of the anti-Charon hemisphere's phase curve between phase angles 0.3° and 1° (0.025 mag/°) is shallower than that of the Charon-facing hemisphere (0.035 mag/°). When combined with NH LORRI images acquired at higher phase angles, the quantitative analyses of the resulting solar phase curves reveal differences in the structure and texture of regolith particles on Tombaugh Regio and Sputnik Planitia and those on Pluto's dark equatorial regions, including Cthulhu Macula.