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Near‐infrared observations of the Pluto Charon system were captured with the Palomar High Angular Resolution Observer (PHARO) adaptive optics (AO) system on the 200‐inch Hale telescope during the historically small solar phase angles in 2018–2019. Both objects exhibit large opposition surges of ∼30%–35% in the last half degree of solar phase angle, which is among the largest observed for icy moons and other Kuiper Belt Objects. In addition, Pluto's surge is exceptionally steep. Pluto's unusual phase curve may be due to an unusual surface texture caused by seasonal volatile transport and active geologic processes. These observations enable accurate determination of Pluto's geometric albedo in the JHK filter system, which we find to be 0.86 ± 0.04, 0.59 ± 0.05, and 0.39 ± 0.04 for Pluto, respectively, and 0.68 ± 0.06 for Charon in the J filter. Plain Language Summary: The New Horizons spacecraft revealed Pluto to be a geologically active, complex world. It never observed small solar phase angles, when the disk of Pluto is fully illuminated. In 2018 and 2019, Pluto reached a historically small solar phase angle that will not be repeated for 161 years. Observations at Palomar Observatory show that both hemispheres of Pluto, as well as its main moon Charon, exhibit a surge of over 30% in brightness at opposition, suggesting an unusual surface texture due to activity and volatile transport. Key Points: Pluto reached an historically small solar phase angle that will not be repeated for 161 years and that complements New Horizons observationsObservations with the adaptive optics system at Palomar Observatory during this time successfully captured both hemispheres of PlutoPluto exhibits an unusually large opposition surge that may be due to active geology and an unusual surface texture [ABSTRACT FROM AUTHOR]

Abstract: During the Cassini mission the Saturnian moons Dione and Tethys showed intriguing and multiple clues suggesting residual geologic activity that might be detectable as an atmosphere, plume, or even heat signature. These clues included an atmospheric aura around Dione, injection of particles into Saturn's magnetosphere, mysterious red streaks on Tethys, and possible cryovolcanic features on Dione. A concerted effort that was strengthened in the latter stages of the mission included the acquisition of stellar occulations by the Ultraviolet Imaging Spectrograph (UVIS) of both Dione and Tethys and high solar phase observations of Dione by the Visible Infrared Mapping Spectrometer (VIMS) to detect forward scattering from grains in a plume. Analysis of these observations shows no evidence for even a low level of activity on either moon. In addition, infrared images at 2.65 μm obtained throughout the mission were scrutinized for the reappearance of an atmosphere‐like aura, with negative results. [ABSTRACT FROM AUTHOR]

Abstract: Near‐infrared spectral maps of Saturn's north polar vortex obtained at high spatial resolution provided by Cassini/Visual Infrared Mapping Spectrometer (VIMS) reveal localized ammonia clouds composed of unusually large particles exceeding 13 μm in radius, the largest cloud particles documented during the Cassini mission. Taken under near‐optimum direct polar lighting and viewing conditions within a month of the summer solstice, these small (~200 km in breadth) discrete clouds are located within the eye of the polar vortex, which otherwise is unusually clear of observable aerosols in reflected sunlight, with total 2‐μm opacity <0.04 versus >1.0 elsewhere on Saturn. The dichotomy of large‐particle condensate cloud features—indicative of convective upwelling—within a large (~ 2000‐km diameter) nearly aerosol‐free region of downwelling characteristic of the core of a polar vortex reveals surprising polar dynamics on Saturn. [ABSTRACT FROM AUTHOR]

Abstract: We report about visible and infrared albedo maps and spectral indicators of Dione's surface derived from the complete Visual and Infrared Mapping Spectrometer (VIMS) data set acquired between 2004 and 2017 during the Cassini tour in Saturn's system. Maps are derived by applying a photometric correction necessary to disentangle the intrinsic albedo of the surface from illumination and viewing geometry occurring at the time of the observation. The photometric correction is based on the Shkuratov et al. (2011, https://doi.org/10.1016/j.pss.2011.06.011) method which yields values of the surface equigonal albedo. Dione's surface albedo maps are rendered at five visible (VIS: 0.35, 0.44, 0.55, 0.7, and 0.95 μm) and five infrared (IR: 1.046, 1.540, 1.822, 2.050, and 2.200 μm) wavelengths in cylindrical projection with a 0.5° × 0.5° angular resolution in latitude and longitude, corresponding to a spatial resolution of 4.5 km/bin. Apart from visible and infrared albedo maps, we report about the distribution of the two visible spectral slopes (0.35–0.55 and 0.55–0.95 μm) and water ice 2.050 μm band depth computed after having applied the photometric correction. The derived spectral indicators are employed to trace Dione's composition variability on both global and local scales allowing to study the dichotomy between the bright‐leading and dark‐trailing hemispheres, the distribution of fresh material on the impact craters and surrounding ejecta, and the resurfacing of the bright material within the chasmata caused by tectonism. [ABSTRACT FROM AUTHOR]