TNO2024

Fascinating features that change in appearance between different NASA New Horizons images of Pluto were identified in a search for temporal changes. Contrast reversal features (CRs) in Pluto’s Sputnik Planitia appear dark relative to their extremely-high-albedo surroundings in multiple low solar phase angle ($\alpha$) images. These features are barely/not apparent in multiple images at intermediate acquired hours later and are brighter than their surroundings in two high images acquired approximately another hour later. They were stationary over the $\approx 4$-hour period when images that could spatially resolve them were acquired. Thus, although they are barely-to-not observed in some images and also reverse in contrast relative to their surroundings between different images, their changes of appearance are more likely due to changes of imaging geometry than temporal change. Contrast emergent features (CEs) were also identified on Pluto, they are brighter than their surrounding terrain in a high image, like CRs, but not apparent in both intermediate and low images, unlike CRs. Hypotheses for Pluto's CRs and CEs include plumes, clouds, wind streaks, snow, frost, precipitated haze, particle size effects, pits, glints, and geometric effects. The CRs and CEs could be on and/or above and/or within Pluto's surface. These hypotheses were investigated by mapping Pluto's CRs and CEs, determining their color, inspecting their morphology in the highest spatial resolution New Horizons images of Pluto, fitting photometric functions for scattering and reflections from surfaces and atmospheres to CR and CE measurements, and comparison with features on Triton and elsewhere. Acknowledgments: Financial support from the NASA New Frontiers Data Analysis Program (Grant #80NSSC22K1118) and New Horizons project is gratefully acknowledged.