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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]

During years in which Mars does not experience a Global Dust Storm, the Thermal Emission Spectrometer (TES) instrument on Mars Global Surveyor (MGS) and the Mars Climate Sounder (MCS) instrument on Mars Reconnaissance Orbiter (MRO) have observed three annually-recurring regional-scale dust storms that occur in the southern hemisphere during southern spring and summer. These storms produce dust extinctions in excess of 10−3 km−1 and increase temperatures to over 200 K in the middle atmosphere (50 Pa, or ∼ 25 km). Whereas the first and last storms in the occurrence are located in the middle latitudes, the second storm, known as the "B" storm, is confined to the south pole over the receding CO 2 ice cap. In this work, we reproduce the "B" storm in the new NASA Ames Mars Global Climate Model (MGCM), and we use our simulation to investigate the mechanisms lofting dust into the middle atmosphere during the storm. We find that a series of semi-regular dust pluming events that occur poleward of 70° S loft dust to and above 50 Pa during the "B" storm. These plumes share some characteristics with rocket dust storms and often produce detached dust layers whose subsequent evolution resembles the solar escalator effect. • The "B" regional dust storm is reproducible in the NASA Ames Mars GCM. • Local-scale dust plumes loft dust above 50 Pa during the simulated "B" storm. • Dust radiative-dynamic feedbacks drive the pluming mechanism. • Elevated dust layers present in orbital observations may be consistent with plumes. [ABSTRACT FROM AUTHOR]

The geology and stratigraphy of the feature on Pluto informally named Sputnik Planitia is documented through geologic mapping at 1:2,000,000 scale. All units that have been mapped are presently being affected to some degree by the action of flowing N 2 ice. The N 2 ice plains of Sputnik Planitia display no impact craters, and are undergoing constant resurfacing via convection, glacial flow and sublimation. Condensation of atmospheric N 2 onto the surface to form a bright mantle has occurred across broad swathes of Sputnik Planitia, and appears to be partly controlled by Pluto's obliquity cycles. The action of N 2 ice has been instrumental in affecting uplands terrain surrounding Sputnik Planitia, and has played a key role in the disruption of Sputnik Planitia's western margin to form chains of blocky mountain ranges, as well in the extensive erosion by glacial flow of the uplands to the east of Sputnik Planitia. [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]

Pluto's atmospheric profiles (temperature and pressure) have been studied for decades from stellar occultation lightcurves. In this paper, we look at recent Pluto Global Climate Model (GCM) results (3D temperature, pressure, and density fields) from Bertrand et al. (2020) and use the results to generate model observer's plane intensity fields (OPIF) and lightcurves by using a Fourier optics scheme to model light passing through Pluto's atmosphere (Young, 2012). This approach can accommodate arbitrary atmospheric structures and 3D distributions of haze. We compared the GCM model lightcurves with the lightcurves observed during the 15-AUG-2018 Pluto stellar occultation. We find that the climate scenario which best reproduces the observed data includes a N 2 ice mid latitude band in the southern hemisphere. We have also studied different haze and P / T ratio profiles: the haze effectively reduces the central flash strength, and a lower P / T ratio both reduces the central flash strength and incurs anomalies in the shoulders of the central flash. • We generated model lightcurves based on the state of Pluto's atmosphere simulated with a Global Climate Model (GCM). • We used the Fourier optics method of Young (2012) to generate occultation lightcurves. • Model lightcurves are compared with ground-based observations of the Pluto stellar occultation event on 15 August 2018. • The observed central flash is consistent with GCM simulations of Pluto in which a southern hemisphere N 2 ice band is present. [ABSTRACT FROM AUTHOR]

Chaotic terrains are characterized by disruption of preexisting surfaces into irregularly arranged mountain blocks with a "chaotic" appearance. Several models for chaos formation have been proposed, but the formation and evolution of this enigmatic terrain type has not yet been fully constrained. We provide extensive mapping of the individual blocks that make up different chaos landscapes, and present a morphological comparison of chaotic terrains found on Pluto, Jupiter's Moon Europa, and Mars, using measurements of diameter, height, and axial ratio of chaotic mountain blocks. Additionally, we compare mountain blocks in chaotic terrain and fretted terrain on Mars. We find a positive linear relationship between the size and height of chaos blocks on Pluto and Mars, whereas blocks on Europa exhibit a flat trend as block height does not generally increase with increasing block size. Block heights on Pluto are used to estimate the block root depths if they were floating icebergs. Block heights on Europa are used to infer the total thickness of the icy layer from which the blocks formed. Finally, block heights on Mars are compared to potential layer thicknesses of near-surface material. We propose that the heights of chaotic mountain blocks on Pluto, Europa, and Mars can be used to infer information about crustal lithology and surface layer thickness. • We compared chaotic mountain blocks on Pluto, Europa, and Mars. • We measured the block diameters, heights, and axial ratios. • Mountain block heights on Pluto and Mars increase with increasing block size. • In contrast europan block heights are generally similar, regardless of size for a given chaos region. • Block heights can be used to infer crustal lithology and surface layer thickness. [ABSTRACT FROM AUTHOR]

Observations during the New Horizons (NH) spacecraft flyby of Pluto in July 2015 revealed that Pluto's atmosphere supports an extensive circumplanetary haze with embedded layers, suggesting several possible microphysical and/or dynamical excitation processes. The purpose of this paper is to build upon existing observations and analyses of Pluto's atmosphere—specifically of the complex haze layer structures—to identify wave structure in Pluto's atmosphere. Here three NH/Long Range Reconnaissance Imager (LORRI) image sequences from the flyby at high phase angles (148°–169°) and three different resolutions (0.093 km/pix, 0.96 km/pix, and 3.86 km/pix) are analyzed. Several haze layer characteristics were extracted, namely—slope, amplitude, waveform, and the associated power spectral densities (PSDs); and their variations with local geography. These are then explored in the context of possible wave types in Pluto's atmosphere, such as tidal and orographically driven inertia-gravity (buoyancy) waves. PSD peaks at 8–10 km and 18–22 km vertical wavelength are found in NH images, which is consistent with the perturbations seen in Earth-based stellar occultations of Pluto's atmosphere. The 8–10 km signals are localized to low-latitudes and equatorial regions and the 18–22 km signals are more globally distributed. Haze layer background relative amplitudes were found to be around 0.01–0.04. Slopes of layers were found to be correlated with the emergence and disappearance of a 25 km layer around 30°N. An amplitude increase of oscillations below 30 km altitude exists in the high-resolution image sequence. These findings indicate the possibility of waves in Pluto's atmosphere and motivate further studies of wave dynamics combining NH data with state-of-the-art models of Pluto's atmosphere. These results are important because they can provide strong constraints to models and to the type of waves that can be present in Pluto's atmosphere. [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]