Contrast Reversal and Emergent Features on Pluto

Abstract Features that change in appearance between different NASA New Horizons images of Pluto are analyzed. Contrast reversal features (CRs) appear to transition from darker to brighter than their surroundings, more likely due to changes of imaging geometry, specifically increasing solar phase angle, than temporal change. Contrast emergent features (CEs) are not apparent in low- and intermediate-solar-phase-angle images but brighter than their surroundings in high-solar-phase-angle image(s), also more likely due to the increase of solar phase angle than temporal change. Hypotheses for Pluto’s CRs and CEs include plumes, clouds, wind streaks, snow, frost, precipitated haze, lags, pits, glints, slopes, and particle size, compaction, and geometric effects. The CRs and CEs could be on and/or above Pluto’s surface and/or within its shallow subsurface. These hypotheses are investigated via mapping, color, morphology, photometry, and comparison with Triton’s fans. Pluto’s CRs and CEs are likely different manifestations of the same or similar phenomena. They are likely not diffuse deposits of Pluto’s dark red equatorial material(s) on or above or within volatile-ice-rich regions, sunglints, or geometric effects from spatially resolved slopes. They are consistent with less backward-scattering and more forward-scattering surfaces and shallow subsurfaces than their surroundings, as well as optically thin, preferentially forward scattering between the surface and global haze, including atmospheric (e.g., cloud) and surface-coating (e.g., condensation deposit) features. Surface and/or shallow subsurface features, possibly atmospheric derived, are favored over atmospheric features.