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The New Horizons spacecraft, which flew by Pluto on July 14, 2015, revealed the presence of haze in Pluto’s atmosphere that were formed by CH 4 /N 2 photochemistry at high altitudes in Pluto’s atmosphere, as on Titan and Triton. In order to help the analysis of the observations and further investigate the formation of organic haze and its evolution at global scales, we have implemented a simple parameterization of the formation of organic haze in our Pluto General Circulation Model. The production of haze in our model is based on the different steps of aerosol formation as understood on Titan and Triton: photolysis of CH 4 in the upper atmosphere by Lyman- α UV radiation, production of various gaseous species, and conversion into solid particles through accumulation and aggregation processes. The simulations use properties of aerosols similar to those observed in the detached haze layer on Titan. We compared two reference simulations ran with a particle radius of 50 nm: with, and without South Pole N 2 condensation. We discuss the impact of the particle radius and the lifetime of the precursors on the haze distribution. We simulate CH 4 photolysis and the haze formation up to 600 km above the surface. Results show that CH 4 photolysis in Pluto’s atmosphere in 2015 occurred mostly in the sunlit summer hemisphere with a peak at an altitude of 250 km, though the interplanetary source of Lyman- α flux can induce some photolysis even in the Winter hemisphere. We obtained an extensive haze up to altitudes comparable with the observations, and with non-negligible densities up to 500 km altitude. In both reference simulations, the haze density is not strongly impacted by the meridional circulation. With No South Pole N 2 condensation, the maximum nadir opacity and haze extent is obtained at the North Pole. With South Pole N 2 condensation, the descending parcel of air above the South Pole leads to a latitudinally more homogeneous haze density with a slight density peak at the South Pole. The visible opacities obtained from the computed mass of haze, which is about 2–4 × 10 − 7 g cm − 2 in the summer hemisphere, are similar for most of the simulation cases and in the range of 0.001-0.01, which is consistent with recent observations of Pluto and their interpretation. [ABSTRACT FROM AUTHOR]

The topography of Neptune's large icy moon Triton could reveal important clues to its internal evolution, but has been difficult to determine. New global digital color maps for Triton have been produced as well as topographic data for <40% of the surface using stereogrammetry and photoclinometry. Triton is most likely a captured Kuiper Belt dwarf planet, similar though slightly larger in size and density to Pluto, and a likely ocean moon that exhibited plume activity during Voyager 2′s visit in 1989. No surface features or regional deviations of greater than ±1 km amplitude are found. Volatile ices in the southern terrains may take the form of extended lobate deposits 300–500 km across as well as dispersed bright materials that appear to embay local topography. Limb hazes may correlate with these deposits, indicating possible surface–atmosphere exchange. Triton's topography contrasts with high relief up to 6 km observed by New Horizons on Pluto. Low relief of (cryo)volcanic features on Triton contrasts with high-standing massifs on Pluto, implying different viscosity materials. Solid-state convection occurs on both and at similar horizontal scales but in very different materials. Triton's low relief is consistent with evolution of an ice shell subjected to high heat flow levels and may strengthen the case of an internal ocean on this active body. [ABSTRACT FROM AUTHOR]

The surface of Pluto is more geologically diverse and dynamic than had been expected, but the role of its tenuous atmosphere in shaping the landscape remains unclear. We describe observations from the New Horizons spacecraft of regularly spaced, linear ridges whose morphology, distribution, and orientation are consistent with being transverse dunes. These are located close to mountainous regions and are orthogonal to nearby wind streaks. We demonstrate that the wavelength of the dunes (~0.4 to 1 kilometer) is best explained by the deposition of sand-sized (~200 to ~300 micrometer) particles of methane ice in moderate winds (<10 meters per second). The undisturbed morphology of the dunes, and relationships with the underlying convective glacial ice, imply that the dunes have formed in the very recent geological past. [ABSTRACT FROM AUTHOR]

Photochemical aerosols were detected as high as 350 km of altitude in Pluto's atmosphere during the New Horizons fly-by. These aerosols are thought to affect Pluto's climate, by acting as cooling agents, and the colours of Pluto's surface, in particular in the dark regions named Cthulhu and Krun and at the North Pole. Pluto atmospheric and surface models have so far used the optical constants of Titan aerosol analogues (tholins), whereas their chemical composition is known to differ from that of Pluto aerosol analogues. In order to provide a new set of optical constants for Pluto tholins, we synthesized analogues of Pluto's aerosols and determined with spectroscopic ellipsometry their optical constants from 270 to 2100 nm. Three types of samples were produced from N 2 :CH 4 :CO gas mixtures differing in their CH 4 :N 2 mixing ratio, representative of different altitudes in Pluto's current atmosphere or different seasons or epochs of Pluto. Our analysis shows a strong absorption by Pluto tholins in the UV and visible spectral ranges, with k index of a few 10−1 at 270 nm, in agreement with N- and O-bearing organic molecules. Pluto tholins are less absorbent in the near-IR than in the UV–Vis wavelength range, with k of a few 10−3 between 600 and 2100 nm. Our comparative study highlights the dependency of n and k indices to the CH 4 :N 2 mixing ratio. Aerosols formed at different altitudes in Pluto's atmosphere or during different seasons or epochs of Pluto will therefore affect the budget of Pluto radiative transfer differently. The optical constants presented in this study were tested with a Pluto surface model and with a model of light scattering. The surface modelling results highlight the suitability of these optical constants to reproduce Pluto compositional observations in the visible spectral range by MVIC and LEISA. The atmospheric modelling results conclude that Pluto tholins absorb 5 to 10 times less than Titan tholins at 500 nm, and this lower absorption is consistent with Alice observations of Pluto's haze. • Optical constants [ n , k ] of Pluto tholins determined by spectroscopic ellipsometry from UV to near-IR • Strong absorption of UV and visible radiations by Pluto tholins, in agreement with N- and O-bearing organic molecules • Dependency of n and k indices to the altitude or epoch of aerosol formation • Suitability of Pluto tholins optical constants to reproduce Pluto compositional observations by MVIC and LEISA • Absorption in the Vis by Pluto tholins weaker than that by Titan tholins, consistent with Alice observations of Pluto's haze [ABSTRACT FROM AUTHOR]

A prominent fossa trough (Uncama Fossa) and adjacent 28-km diameter impact crater (Hardie) in Pluto's Viking Terra, as seen in the high-resolution images from the New Horizons spacecraft, show morphological evidence of in-filling with a material of uniform texture and red-brown color. A linear fissure parallel to the trough may be the source of a fountaining event yielding a cryoclastic deposit having the same composition and color properties as is found in the trough and crater. Spectral maps of this region with the New Horizons LEISA instrument reveal the spectral signature of H 2 O ice in these structures and in distributed patches in the adjacent terrain in Viking Terra. A detailed statistical analysis of the spectral maps shows that the colored H 2 O ice filling material also carries the 2.2-μm signature of an ammoniated component that may be an ammonia hydrate (NH 3 ·nH 2 O) or an ammoniated salt. This paper advances the view that the crater and fossa trough have been flooded by a cryolava debouched from Pluto's interior along fault lines in the trough and in the floor of the impact crater. The now frozen cryolava consisted of liquid H 2 O infused with the red-brown pigment presumed to be a tholin, and one or more ammoniated compounds. Although the abundances of the pigment and ammoniated compounds entrained in, or possibly covering, the H 2 O ice are unknown, the strong spectral bands of the H 2 O ice are clearly visible. In consideration of the factors in Pluto's space environment that are known to destroy ammonia and ammonia-water mixtures, the age of the exposure is of order ≤109 years. Ammoniated salts may be more robust, and laboratory investigations of these compounds are needed. • A flooded crater and trough are identified in Pluto's Viking Terra. • Water ice and an ammoniated material are found in the filling material. • A red-colored pigment, also found elsewhere, characterizes the filling material. • The morphology and composition in this region are indicative of cryovolcanism. [ABSTRACT FROM AUTHOR]