Photometric Properties of Nix and Hydra from New Horizons and the Hubble Space Telescope

Abstract:

New Horizons observations provide the first spatially-resolved images of Pluto’s small satellites, enabling the study of how albedo and scattering properties vary across their surface. The successful application of photometric models to characterize the physical properties of airless planetary surfaces requires observations which span the broadest possible range of viewing and illumination geometries, from full disk to thin crescent. While New Horizons observed the Pluto system at phase angles no smaller than 15°, full-disk Hubble Space Telescope images (HST Program 13367, M. Buie, PI) at phase angles between 0.06° and 1.7° constrain the phase function near opposition. Together with New Horizons disk-resolved images, we use the near-opposition phase function to test whether ejecta exchange between Pluto’s satellites (Stern 2009, Icarus 199, 571) affects their photometric properties. Using the size of the small satellites measured by New Horizons, we determine the disk-integrated geometric albedo by extrapolating the HST observations to zero phase. New Horizons observations at larger phase angles reveal surface roughness and directional scattering properties. Finally we place the photometric properties of Pluto’s small satellites in context with those of similar size in other planetary systems.