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Triton and Pluto are believed to share a common origin, both forming initially in the Kuiper Belt but Triton being later captured by Neptune. Both objects display similar sizes, densities, and atmospheric and surface ice composition, with the presence of volatile ices N2, CH4, and CO. Yet their appearance, including their surface albedo and ice distribution strongly differ. What can explain these different appearances? A first disparity is that Triton is experiencing significant tidal heating due to its orbit around Neptune, with subsequent resurfacing and a relatively flat surface, while Pluto is not tidally activated and displays a pronounced topography. Here we present long-term volatile transport simulations of Pluto and Triton, using the same initial conditions and volatile inventory, but with the known orbit and rotation of each object. The model reproduces, to first order, the observed volatile ice surface distribution on Pluto and Triton. Our results unambiguously demonstrate that obliquity is the main driver of the differences in surface appearance and in climate properties on Pluto and Triton, and give further support to the hypothesis that both objects had a common origin followed by a different dynamical history. [ABSTRACT FROM AUTHOR]

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A review of studies on the atmospheres of planets in the solar system performed by Russian scientists in 2019–2022 is presented. This review was prepared in the Commission on Planetary Atmospheres of the National Geophysical Committee for the National Report on Meteorology and Atmospheric Science at the 28th General Assembly of the International Union of Geodesy and Geophysics (IUGG 2023) in Berlin. [ABSTRACT FROM AUTHOR]

Pluto's surface is geologically complex because of volatile ices that are mobile on seasonal and longer time scales. Here we analyzed New Horizons LEISA spectral data to globally map the nitrogen ice, including nitrogen with methane diluted in it. Our goal was to learn about the seasonal processes influencing ice redistribution, to calculate the globally averaged energy balance, and to place a lower limit on Pluto's N2 inventory. We present the average latitudinal distribution of nitrogen and investigate the relationship between its distribution and topography on Pluto by using maps that include the shifted bands of methane in solid solution with nitrogen (which are much stronger than the 2.15-μm nitrogen band) to more completely map the distribution of the nitrogen ice. We find that the global average bolometric albedo is 0.83±0.11, similar to that inferred for Triton, and that a significant fraction of Pluto's N2 is stored in Sputnik Planitia. We also used the encounter-hemisphere distribution of nitrogen ice to infer the latitudinal distribution of nitrogen over the rest of Pluto, allowing us to calculate the global energy balance. Under the assumption that Pluto's nitrogen-dominated 11.5μbar atmosphere is in vapor pressure equilibrium with the nitrogen ice, the ice temperature is 36.93±0.10 K, as measured by New Horizons’ REX instrument. Combined with our global energy balance calculation, this implies that the average bolometric emissivity of Pluto's nitrogen ice is probably in the range 0.47–0.72. This is consistent with the low emissivities estimated for Triton based on Voyager results, and may have implications for Pluto's atmospheric seasonal variations, as discussed below. The global pattern of volatile transport at the time of the encounter was from north to south, and the transition between condensation and sublimation within Sputnik Planitia is correlated with changes in the grain size and CH4 concentration derived from the spectral maps. The low emissivity of Pluto's N2 ice suggests that Pluto's atmosphere may undergo an extended period of constant pressure even as Pluto recedes from the Sun in its orbit.

In the distant outer solar system, carbon monoxide (CO) and nitrogen (N _2 ) ices tend to be colocated in the same deposits due to their similar molecular weights and sublimation properties. For instance, these volatiles are abundant on the surfaces of Pluto and Triton, so knowledge of their phase behavior is necessary for understanding surface evolution and geology. However, it is presently unclear how mixing between CO and N _2 molecules affects the physical properties of such mixtures. Here, we measured the liquidus, solidus, and alpha–beta phase transitions for the N _2 and CO binary system. We observed the liquidus by using visual inspection. The solidus and alpha–beta transitions were measured by using Raman spectroscopy. The laboratory results were later compared to a thermodynamic model, CRYOCHEM 2.0. The liquidus and solidus were consistent with CRYOCHEM 2.0. However, the alpha–beta coexistence region is shown to be narrower in the laboratory results than in the thermodynamic model. Finally, we present a method for deriving the compositions of a sample using Raman spectroscopy (Appendices A.1 and A.2).

The long-term evolution of the outer solar system is subject to the influence of the giant planets, however, perturbations from other massive bodies located in the region imprint secular signatures, which are discernible in long-term simulations. In this work, we performed an in-depth analysis of the evolution of massive objects Eris, 2015 KH _162 , Pluto, and 2010 EK _139 (aka, Dziewanna), subject to perturbations from the giant planets and the 34 largest trans-Neptunian objects. We do this by analyzing 200, 1 Gyr long simulations with identical initial conditions, but requiring the numerical integrator to take different time steps for each realization. Despite the integrator’s robustness, each run’s results are surprisingly different, showing the limitations of individual realizations when studying the trans-Neptunian region due to its intrinsic chaotic nature. For each object, we find orbital variables with well-defined oscillations and limits, and others with surprisingly large variances and seemingly erratic behaviors. We found that 2015 KH _162 is a nonresonant and very stable object that experiences only limited orbital excursions. Pluto is even more stable and we found a new underlying constraining mechanism for its orbit; 2010 EK _139 is not well trapped in the 7:2 mean motion resonance in the long term and cannot be trapped simultaneously in von Zeipel–Lidov–Kozai resonance; and finally, we found that at present Eris’s longitude of perihelion is stationary, tightly librating around 190°, but unexpectedly loses its confinement, drifting away after 150 Myr, suggesting a missing element in our model.

Kuiper Belt objects (KBOs) show an unexpected trend, whereby large bodies have increasingly higher densities, up to five times greater than their smaller counterparts. Current explanations for this trend assume formation at constant composition, with the increasing density resulting from gravitational compaction. However, this scenario poses a timing problem to avoid early melting by decay of ^26 Al. We aim to explain the density trend in the context of streaming instability and pebble accretion. Small pebbles experience lofting into the atmosphere of the disk, being exposed to UV and partially losing their ice via desorption. Conversely, larger pebbles are shielded and remain icier. We use a shearing box model including gas and solids, the latter split into ices and silicate pebbles. Self-gravity is included, allowing dense clumps to collapse into planetesimals. We find that the streaming instability leads to the formation of mostly icy planetesimals, albeit with an unexpected trend that the lighter ones are more silicate-rich than the heavier ones. We feed the resulting planetesimals into a pebble accretion integrator with a continuous size distribution, finding that they undergo drastic changes in composition as they preferentially accrete silicate pebbles. The density and masses of large KBOs are best reproduced if they form between 15 and 22 au. Our solution avoids the timing problem because the first planetesimals are primarily icy and ^26 Al is mostly incorporated in the slow phase of silicate pebble accretion. Our results lend further credibility to the streaming instability and pebble accretion as formation and growth mechanisms.

We estimate the loss of nitrogen from Pluto over its lifetime, including the giant planet instability period, which we term the “Wild Years.” We analyze the orbital migration of 53 simulated Plutinos, which are Kuiper Belt Objects (KBOs) captured into 3:2 mean-motion resonance with Neptune during the instability. This orbital migration brought the Plutinos from 20 to 30 au to their present-day orbits near 40 au along a nonlinear path that includes orbits with semimajor axes from 10 to 100 au. We model the thermal history that results from this migration and estimate the volatile loss rates due to the ever-changing thermal environment. Due to the early Sun’s enhanced ultraviolet radiation, the photochemical destruction rate during the Wild Years was a factor of 100 higher than the present-day rate, but this only results in a loss of ∼10 m global equivalent layer (GEL). The enhanced Jeans escape rate varies wildly with time, and a net loss of ∼100 cm GEL is predicted. Additionally, we model the impact history during the migration and find that impacts are a net source, not loss, of N _2 , contributing ∼100 cm GEL. The 100 cm GEL is 0.1% of the amount of N _2 in Sputnik Planitia. We therefore conclude that Pluto did not lose an excessive amount of volatiles during the Wild Years, and its primordial volatile inventory can be approximated as its present-day inventory. However, significant fractions of this small total loss of N _2 occurred during the Wild Years, so estimates made using present-day rates will be underestimates.

Copyright of Larq Journal for Philosophy, Linguistics & Social Sciences is the property of Republic of Iraq Ministry of Higher Education & Scientific Research (MOHESR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Sputnik Planitia basin, the dominant surface feature of the dwarf planet Pluto, is located very close to the far point of Pluto‐Charon tidal axis. This position is currently believed to be a result of whole body reorientation driven by the combination of (a) the uplift of a subsurface ocean in response to a basin‐forming impact and (b) the nitrogen layer accumulated inside the basin. Since an ice shell made of pure water ice cannot maintain the uplift on timescales of billions of years, the presence of an insulating and highly viscous layer of methane clathrates at the base of the shell has recently been proposed. In this study, we solve the thermo‐mechanical evolution of the ice shell in a 2D spherical axisymmetric geometry and evaluate the gravity anomaly associated with the evolving ice shell shape. Taking into account the effect of impact heating and stress‐dependent rheology of both ice and clathrates, we show that a thick shell (≥200 km) loses the impact heat slowly which leads to fast uplift relaxation of the order of hundreds of million years. On the contrary, a thin shell (∼100 km) cools down quickly (∼10 Myr), becoming rigid and more likely to preserve the ocean/shell interface uplift till the present. These results suggest that a thick ocean may be present beneath Pluto's ice shell. Plain Language Summary: Several icy bodies within our Solar System are believed to have harbored a subsurface ocean below an outer ice shell early in their history. During a few billions of years (Solar System age), such oceans are likely to freeze, unless supplied by additional energy sources or chemical compounds that slow down the crystallization processes. In 2015, the New Horizons mission revealed signs that the dwarf planet Pluto may have also harbored a subsurface ocean. Moreover, the unlikely position of Pluto's dominant surface feature, Sputnik Planitia basin, assumed to be formed by a giant impact, suggests that Pluto's deep ocean may still be present. One of the hypotheses explaining the basin's position assumes that the ocean/shell boundary below the basin is deformed and that its shape is stable on long timescales due to presence of a layer of highly rigid and thermally insulating clathrates. In this paper, we study the effects of more realistic rheology than assumed previously as well as those of heating due to the impact that formed the basin. We show that a rather thin ice shell (∼100 km) is more plausible than thicker shells thus implying that Pluto's ocean may be surprisingly thick (∼200 km). Key Points: We study viscous relaxation of Pluto's impact‐deformed ice shell and evolution of gravity anomaly associated with Sputnik Planitia basinStress‐dependent rheology can substantially speed up relaxation of the ice shell, while impact heating has only a minor effectThe positive gravity anomaly responsible for the basin's position can be preserved only if the ice shell is thin (∼100 km) or the ocean salty [ABSTRACT FROM AUTHOR]

During the 40-year photographic period of astronomical observations, the Baldone Astrophysical Observatory has accumulated more than 22,000 direct and 2,500 spectral images. In 2018, the digitization of direct astrophoto images with a resolution of 1200 dpi was completed. A processing methodology for obtaining the equatorial coordinates and photometric characteristics of the objects recorded in digitized astronegatives using the LINUX/MIDAS/ROMAFOT environment has been developed at the Main Astronomical Observatory of the National Academy of Sciences of Ukraine. Program package with success already works in seven observatories. A description of the operation of this program complex in stages and analysis of the obtained results is given. The steps describe the astrometric and photometric reduction process of digital records, as well as the reduction of the obtained instrumental magnitudes to the Johnson UBVR photometric system. The methodology of characteristic curve construction in the case of one exposure is described in detail. From all digitized astrophotoplates to date Approximately 2200 V film negatives, 300 U plates and several R, B plates were processed. As a result, catalogs of the positions and magnitudes of Pluto, 1848 asteroids and comets were obtained. 31 new positions were recorded in the VizieR Pluto catalog VI/155. It was found that the root-mean-square errors of the reduction of the measured coordinates to the equatorial coordinate system of the Tycho-2 catalog have values σRA,DEC= 0.1-0.2”, and the root-mean-square errors of the reduction of instrumental photometric quantities m to the Johnson system of stellar UBVR-values are also in within σUBVR = 0.1 − 0.2m.

The habitability of oceans within icy worlds depends on material and heat transport through their outer ice shells. Previous work shows a methane clathrate layer at the upper surface of the ice shell of Titan thickens the convecting region, while on Pluto a clathrate layer at the base of the ice shell hinders convection. In this way, the dynamics of clathrate‐ice shells may be essential to the thermal evolution and habitability of ocean worlds. However, studies to date have not addressed the dynamics that determine the location of clathrates within the ice shell. Here, we show that, in contrast to previous studies, clathrates accumulating at the base of the ice shell are entrained throughout the shell. Clathrates are stiffer than ice. As a result, entrainment slows convection and thickens the conductive lid across a range of ocean worlds, potentially preserving sub‐ice oceans but limiting avenues for material transport into them. Plain Language Summary: Icy ocean worlds are promising targets for the discovery of extraterrestrial life in our Solar System. Icy ocean worlds have an outer shell, predominantly composed of ice, which insulates internal oceans where life may exist. Preserving internal oceans depends on the rate of heat loss through the ice shell. Furthermore, the habitability of internal oceans likely depends on the transport of chemical materials from the surface of the ice shell to the ocean below. The outer ice shells of ocean worlds are not exclusively composed of ice. Methane gas released from the core can form methane clathrate hydrates, methane trapped within ice crystals that buoyantly rise to the base of the ice shell. We find, in contrast to previous studies, that clathrates become entrained into the ice shell by convective overturning. Methane clathrates have different physical properties than ice. Entrained clathrates thicken the static region near the surface of the ice shell, limiting avenues for material transport through the ice shell. However, their effect depends on the poorly constrained properties of ice‐clathrate mixtures. Finally, we find that clathrate entrainment may altogether halt convective motion in ice shells across a range of ocean worlds, potentially preserving sub‐ice oceans. Key Points: Methane clathrates formed within ocean worlds become entrained throughout their ice shellsEntrained clathrates slow convection and thicken the conductive lid of outer ice shells on Titan and PlutoClathrate entrainment may stop convective motion in many ocean worlds [ABSTRACT FROM AUTHOR]

Many of the unusual properties of Pluto's orbit are widely accepted as evidence for the orbital migration of the giant planets in early solar system history. However, some properties remain an enigma. Pluto's long-term orbital stability is supported by two special properties of its orbit that limit the location of its perihelion in azimuth and in latitude. We revisit Pluto's orbital dynamics with a view to elucidating the individual and collective gravitational effects of the giant planets on constraining its perihelion location. While the resonant perturbations from Neptune account for the azimuthal constraint on Pluto's perihelion location, we demonstrate that the long-term and steady persistence of the latitudinal constraint is possible only in a narrow range of additional secular forcing which arises fortuitously from the particular orbital architecture of the other giant planets. Our investigations also find that Jupiter has a largely stabilizing influence whereas Uranus has a largely destabilizing influence on Pluto's orbit. Overall, Pluto's orbit is rather surprisingly close to a zone of strong chaos. [ABSTRACT FROM AUTHOR]

Four small moons (Styx, Nix, Kerberos and Hydra) are at present known to orbit around the barycenter of Pluto and Charon, which are themselves considered a binary dwarf planet due to their relatively high mass ratio. The central, non-axisymmetric potential induces moon orbits inconvenient to be described by Keplerian osculating elements. Here, we report that observed orbital variations may not be the result of orbital eccentricities or observational uncertainties, but may be due to forced oscillations caused by the central binary. We show, using numerical integration and analytical considerations, that the differences reported on their orbital elements may well arise from this intrinsic behavior rather than limitations on our instruments. [ABSTRACT FROM AUTHOR]

In the light of NASA's New Horizons mission, the solid-phase behaviour of methane and nitrogen has been re-examined and the thermal expansion coefficients of both materials have been determined over their whole solid temperature range for the first time. Neutron diffraction results indicate that the symmetric Pa3 space group is the best description for the α-nitrogen structure, rather than the long-accepted P213. Furthermore, it is also observed that β-nitrogen and methane phase I show changes in texture on warming, indicating grain growth.

Sputnik Planitia on Pluto is a vast plain consisting of a nitrogen ice deposit filling a broad topographic depression, likely an impact basin. The basin displays a broad, raised rim and is surrounded by numerous extensional fault systems, each with characteristic orientations with respect to the basin center. The nitrogen ice exerts a large mechanical load on the water ice outer shell crust (here also containing the lithosphere). We calculate models of stress and deformation related to this load, varying dimensional, mechanical, and boundary condition properties of the load and Pluto's lithosphere, in order to constrain the conditions that led to the formation of the observed tectonic and topographic signals. We demonstrate that the tectonic configuration is diagnostic of a particular set of conditions that hold for the Sputnik basin and Pluto, including moderate elastic lithosphere thickness (40–75 km, with higher values favored if initial basin topography is compensated) and a basin that was pan‐shaped and shallow (∼3 km) at the time of nitrogen deposition initiation. These tectonic systems show the contributions of both flexural (bending) and membrane (stretching) responses of the lithosphere, with the latter dominating in proportion to the importance of spherical geometry effects. Rim topography may also show an influence of primordial annular trans‐basin ice shell thickening from the impact process. Analysis of stress‐driven cryomagma transport shows that loading stresses can facilitate ascent of cryomagmas in annular zones around the basin, the locations of which overlap the observed distances from Sputnik of several candidate cryovolcanic sites. Plain Language Summary: The bright Sputnik Planitia region on Pluto is a vast plain consisting of a deposit of frozen nitrogen that fills a broad depressed area. The depression is probably the result of a large object colliding with Pluto; such a depression is called an impact basin. The basin displays a broad, raised rim and is surrounded by numerous cracks that reach outward like spokes on a bicycle wheel. The frozen nitrogen pushes down the outer shell of Pluto, which consists of frozen water (ice). The pushing creates stress in the shell that can fracture it. We use computer models to test for the conditions that would create the configuration of cracks seen around Sputnik Planitia. We show that the models strongly favor a particular range for the shell thickness, 40–75 km. Also, for the most successful models the starting shape of the depression resembled that of a frying pan, and was around 3 km in depth. The stresses created in the ice shell actually assist the rising of liquid water through it, an unusual form of volcanism that may be occurring at several sites in the region surrounding Sputnik Planitia. Key Points: The Sputnik Planitia impact basin on Pluto is filled with nitrogen ice, producing stresses that create outward‐radiating fault systemsFor likely load distributions, the pattern of faulting is strongly diagnostic of an elastic lithosphere (ice shell) thickness around 50 kmThe initial basin depth must not have exceeded several km in order to be consistent with the observed topographic level of nitrogen ice [ABSTRACT FROM AUTHOR]

The rotational lightcurves of the Pluto-Charon system were previously believed to be solely attributed to their surfaces. However, a proposed scenario of haze cooling suggests that the atmospheric haze of Pluto could significantly contribute to mid-infrared emission, which calls for a revisit of previous analyses. In this study, we employ a Bayesian retrieval approach to constrain the haze emission from the rotational lightcurves of the Pluto-Charon system. The lightcurves were observed by the Spitzer and Herschel telescopes at 24 and 70 μ m, and were combined with the latest surface albedo maps of Pluto and Charon from the New Horizons spacecraft. Our results show that including the haze emission is consistent with all current observations, with the best-fit haze flux around 1.63 mJy. This is in agreement with the composition of Titan-like tholins. However, the “surface only” scenario, which excludes the haze contribution, can still explain the observations. We conclude that the current data at 24 μ m cannot constrain Pluto’s haze emission due to the degeneracy with Charon’s surface emission. Regardless, some surface properties of Pluto are well constrained by the shape of the lightcurves, with a thermal inertia of approximately 8–10 MKS and a relatively low CH _4 emissivity of 0.3–0.5. We suggest that observations by the JWST telescope at 18 μ m, which can resolve Pluto from Charon, could directly probe the haze emission of Pluto due to the low surface emission at that wavelength.

Ocean worlds, or icy bodies in the outer solar system that have or once had subsurface liquid water oceans, are among the most compelling topics of astrobiology. Typically, confirming the existence of a subsurface ocean requires close spacecraft observations. However, combining our understanding of the chemistry that takes place in a subsurface ocean with our knowledge of the building blocks that formed potential ocean worlds provides an opportunity to identify tracers of endogenic activity in the surface volatiles of Pluto and Triton. We show here that the current composition of the volatiles on the surfaces and in the atmospheres of Pluto and Triton are deficient in carbon, which can only be explained by the loss of CH _4 through a combination of aqueous chemistry and atmospheric processes. Furthermore, we find that the relative nitrogen and water abundances are within the range observed in building block analogs, comets, and chondrites. A lower limit for N/Ar in Pluto’s atmosphere also suggests source building blocks that have a cometary or chondritic composition, all pointing to an origin for their nitrogen as NH _3 or organics. Triton’s lower abundance of CH _4 compared to Pluto, and the detection of CO _2 at Triton but not at Pluto points to aqueous chemistry in a subsurface ocean that was more efficient at Triton than Pluto. These results have applications to other large Kuiper Belt objects as well as the assessment of formation locations and times for the four giant planets given future probe measurements of noble gas abundances and isotope ratios.

Pluto and Charon are the farthest bodies from the Sun that have been visited by spacecraft where more than several dozen impact craters can be accurately identified and measured. These craters form a critical record of outer solar system impactors in the trans-Neptunian environment, so an accurate census of them is important for numerous dynamical applications seeking to understand the Kuiper Belt and its evolution. In 2017, a crater database was published based on several different researchers mapping on various image products from the New Horizons mission. To date, that remains the only work that has tried to provide a complete census of the craters on Pluto and Charon. Meanwhile, Singer et al. mapped on different, more final image products, while Robbins & Singer worked to reconcile their crater populations, demonstrate general agreement, and lay out issues with the previous work from 2017. However, neither attempted to make complete databases of Pluto's or Charon's craters. To come full circle, we present a short report on a revised crater database for both Pluto and Charon that incorporates revised interpretations of geologic features on both bodies; this work supersedes our other Pluto and Charon crater database work.

Clathrate hydrates may represent a sizable fraction of material within the icy shells of Kuiper Belt objects and icy moons. They influence the chemical and thermal evolution of subsurface oceans by locking volatiles into the ice shell and by providing more thermal insulation than pure water ice. We model the formation of these crystalline compounds in conditions relevant to outer solar system objects, using Pluto as an example. Although Pluto may have hosted a thick ocean in its early history, Pluto’s overall heat budget is probably insufficient to preserve liquid today if its outer shell is pure water ice. One previously proposed reconciliation is that Pluto’s ocean has a winter jacket: an insulating layer of methane clathrate hydrates. Unfortunately, assessments of the timing, quantity, and type of clathrate hydrates forming within planetary bodies are lacking. Our work quantifies the abundance of clathrate-forming gases present in Pluto’s ocean from accreted ices and volatiles released during thermal metamorphism throughout Pluto’s history. We find that if Pluto formed with the same relative abundances of ices found in comets, then a buoyant layer of mixed methane and carbon dioxide clathrate hydrates may form above Pluto’s ocean, though we find it insufficient to preserve a thick ocean today. In general, our study provides methodology for predicting clathrate formation in ocean worlds, which is necessary to predict the evolution of the ocean’s composition and whether a liquid layer remains at present.

We map the surface of Pluto using an unsupervised machine-learning technique using the near-infrared observations of the LEISA/Ralph instrument on board NASA’s New Horizons spacecraft. The principal-component-reduced Gaussian mixture model was implemented to investigate the geographic distribution of the surface units across the dwarf planet. We also present the likelihood of each surface unit at the image pixel level. Average I/F spectra of each unit were analyzed—in terms of the position and strengths of absorption bands of abundant volatiles such as N _2 , CH _4 , and CO and nonvolatile H _2 O—to connect the unit to surface composition, geology, and geographic location. The distribution of surface units shows a latitudinal pattern with distinct surface compositions of volatiles—consistent with the existing literature. However, previous mapping efforts were based primarily on compositional analysis using spectral indices (indicators) or implementation of complex radiative transfer models, which need (prior) expert knowledge, label data, or optical constants of representative end-members. We prove that an application of unsupervised learning in this instance renders a satisfactory result in mapping the spatial distribution of ice compositions without any prior information or label data. Thus, such an application is specifically advantageous for a planetary surface mapping when label data are poorly constrained or completely unknown, because an understanding of surface material distribution is vital for volatile transport modeling at the planetary scale. We emphasize that the unsupervised learning used in this study has wide applicability and can be expanded to other planetary bodies of the solar system for mapping surface material distribution.

The flyby of the New Horizons spacecraft in 2015 July revealed an unexpected cold atmosphere of Pluto and confirmed the existence of its atmospheric haze. The observed and simulated vertical profiles of chemical species and microphysical processes suggest that the haze particles in Pluto’s middle and lower atmosphere may contain organic ice condensation. Such organic ice components can potentially affect Pluto’s haze chemistry and optical properties, as well as its energy budget. This study investigates the influence of the ice components on the scattering properties of Pluto’s haze by comparing New Horizons observations and simulated particle scattering properties. Comprehensive tests are performed for various haze particle parameters, including their size, chemical component, ice content, and morphology. Scattering properties of these ice-bearing haze particles are calculated by a discrete dipole approximation method and compared to multispectral observations obtained by four New Horizons instruments in spectral regions ranging from the ultraviolet to the near-infrared. The results indicate that the inclusion of the organic ice component leads to higher ratios of backscattering in the visible to extinction in the ultraviolet and provides better agreement with observations compared to monodispersed homogeneous aggregates. But it alone is not sufficient to explain the observed forward scattering values in the visible and near-infrared. Therefore, other scattering sources and/or mechanisms are still required to explain the full set of scattering observations. Further observations, as well as laboratory measurements and numerical tests, are anticipated to improve our understanding of the morphology and ice content of Pluto’s haze.

We present a new orbit and mass solution for the four small satellites of Pluto: Styx, Nix, Kerberos, and Hydra. We have reanalyzed all available observations of the Pluto system obtained by the Hubble Space Telescope from 2005 to 2019 with the ACS, WFPC2, and WFC3 instruments, as well as the New Horizons LORRI images taken on approach to Pluto in 2015. We have used this high-precision astrometry to produce updated orbits and mass estimates with uncertainties for all four of the small satellites. We find that the masses of Nix and Hydra are smaller than previously published estimates, with a dynamical mass of (1.8 ± 0.4) × 10 ^−3 km ^3 s ^−2 ((2.7 ± 0.6) × 10 ^16 kg) for Nix and (2.0 ± 0.2) × 10 ^−3 km ^3 s ^−2 ((3.0 ± 0.3) × 10 ^16 kg) for Hydra. These masses are 60% and 63% of the mean estimates by Brozovic et al., respectively, although still consistent with their 1 σ uncertainties, and correspond to densities of 1.0 ± 0.2 g cm ^−3 for Nix and 1.2 ± 0.2 g cm ^−3 for Hydra given the moon volume estimates from Porter et al. Although these densities are consistent with a range of ice−rock compositions, depending on the unknown bulk porosity in the moon interiors, the moons’ high albedos and predominantly icy surfaces are most easily explained if their interiors are ice-rich. The tiny masses of Kerberos and Sytx remain very poorly constrained; we find 1 σ upper limits for dynamical mass of 3 × 10 ^−5 km ^3 s ^−2 (5 × 10 ^14 kg) for Styx and 5 ×10 ^−5 km ^3 s ^−2 (8 × 10 ^14 kg) for Kerberos, consistent with densities of

New Horizons showed that Pluto exhibits a wide range of geologic groups, with much of the surface modified by volatile ice processes. Impact craters are a valuable tool to investigate how these regions have evolved, as they record the effects of various modification mechanisms and retain information on the properties of the bedrock ice(s). In this work, we use Pluto’s crater population to quantify the extent of surface modification and identify variations in surface properties on Pluto. In this study, we have measured the relative depth of Pluto’s craters compared to minimally modified water-ice craters to interpret how the craters may have evolved and/or what the information tells us about the surface properties of the bedrock. We found a trend of increasing crater relative depth from southeast to northwest that may reflect the conditions of an ancient surface when a thicker layer of volatile ice may have existed, possibly changed by polar wander after the Sputnik impact. We have identified anomalously deep craters across Pluto’s surface, with a concentration in Cthulhu Macula, suggesting different bedrock-ice properties in this region. Other deep craters may be influenced by extraneous factors, such as impactor speed. Overall, this study expands on our understanding of the evolution and composition of Pluto’s surface and sets a road map for further investigations into Pluto’s surface evolution.

The New Horizons encounter with the Pluto system revealed Pluto to have an extremely spatially variable surface with expansive dark, bright, and intermediate terrains, refractory and volatile ices, and ongoing/recent endogenous and exogenous processes. Albedo is useful for understanding volatile transport because it quantifies absorbed solar energy; albedo may also provide insights into surface processes. Four filters of the New Horizons LORRI and MVIC imagers are used to approximate the bolometric (flux-weighted, wavelength-integrated) albedo. The bolometric hemispherical albedo (local energy balance albedo) as a function of the incidence angle of the solar illumination is measured for both Cthulhu and Sputnik Planitia, which are extensive, extreme dark and extreme bright terrains on Pluto. For both terrains, the bolometric hemispherical albedo increases by >30% from 0° to 90° incidence. The incidence-angle-average bolometric hemispherical albedo of Cthulhu is 0.12 ± 0.01, and that of Sputnik Planitia is 0.80 ± 0.06, where uncertainties are estimates based on scatter from different photometric functional approximations. The bolometric Bond albedo (global energy balance albedo) of Cthulhu is 0.12 ± 0.01, and that of Sputnik Planitia is 0.80 ± 0.07. A map of Pluto’s incidence-angle-average bolometric hemispherical albedo is produced. The incidence-angle-average bolometric hemispherical albedo, spatially averaged over areas north of ≈30° S, is ≈0.54. Pluto has three general albedo categories: (1) very low albedo southern equatorial terrains, including Cthulhu; (2) high-albedo terrains, which constitute most of Pluto’s surface; and (3) very high albedo terrains, including Sputnik Planitia. Pluto’s extraordinary albedo variability with location is also spatially sharp at some places.

We investigate the origins of Triton’s deformed and young surface. Assuming Triton was captured early in solar system history, the bulk of the energy released during capture will have been lost, and cannot be responsible for its present-day activity. Radiogenic heating is sufficient to maintain a long-lived ocean beneath a conductive ice shell, but insufficient to cause convective deformation and yielding at the surface. However, Triton’s high inclination likely causes a significant ( 0.7) obliquity, resulting in large heat fluxes due to tidal dissipation in any subsurface ocean. For a 300 km thick ice shell, the estimated ocean heat production rate (0.3 TW) is capable of producing surface yielding and mobile-lid convection. Requiring convection places an upper bound on the ice shell viscosity, while the requirement for yielding imposes a lower bound. Both bounds can be satisfied with an ocean temperature 240 K for our nominal temperature-viscosity relationship, suggesting the presence of an antifreeze such as NH3. In our view, Triton’s geological activity is driven by obliquity tides, which arise because of its inclination. In contrast, Pluto is unlikely to be experiencing significant tidal heating. While Pluto may have experienced ancient tectonic deformation, we do not anticipate seeing the kind of young, deformed surfaces seen at Triton.

Much of the modern Kuiper belt is thought to be the result of Neptune's migration through a primordial belt of planetesimals, possibly interrupted by an instability amongst the giant planets. While most prior work has employed massless test particles to study this Kuiper belt formation process, we perform simulations here that include the gravitational effects of the primordial planetesimal belt consisting of ∼ 1 0 5 massive bodies. In our simulations, Neptune unlocks from resonance with the other giant planets and begins to migrate outward due to interactions with planetesimals before a planetary orbital instability is triggered, and afterward, residual Neptunian migration completes the formation of the modern Kuiper belt. Compared to past simulations using massless test particles, our present work exhibits a number of notable differences. First, Neptune's planetary resonance unlocking requires the Neptunian 3:2 mean motion resonance to sweep much of the primordial disk interior to 30 au prior to the giant planet instability. This generates a pre-instability population of planetesimals that is significantly lower in semimajor axis, eccentricity, and inclination than predicted by many prior models, and this effect persists after the instability. Second, direct scattering between Pluto-mass bodies and other small bodies removes material from Neptunian resonances significantly more efficiently than the resonant dropout resulting from small changes in Neptune's semimajor axis during scattering between Pluto-mass bodies and Neptune. Consequently, the primordial population of Pluto-mass bodies may be as few as ∼ 200 objects. Finally, our simulation end states display a wide variety of orbital distributions, and straightforward relationships between final bulk Kuiper belt properties and Neptune's migration or initial planetesimal properties largely elude us. In particular, we find that the rapid, stochastic planetary orbital evolution occurring during the giant planet instability can significantly alter final Kuiper belt properties such as its inclination dispersion and the prominence of resonant populations. This complicates efforts to use modern Kuiper belt properties to confidently constrain early solar system events and conditions, including planetary orbital migration and the primordial Kuiper belt's characteristics. • We run Kuiper belt formation simulations with Kuiper belt objects of realistic mass. • The giant planet instability adds a stochastic element to Kuiper belt formation. • It is thus more difficult to infer solar system pre-instability conditions. • Encounters between Kuiper belt objects limit long-term resonant capture with Neptune. • 200 to 1000 Pluto-mass bodies likely existed in the primordial outer solar system. [ABSTRACT FROM AUTHOR]

After Plutus's recovery of his eyesight and the subsequent redistribution of wealth, an old woman, probably a hetaira, is suddenly abandoned by her young lover, who is now financially solvent and in no more need of her (Aristophanes' Plutus, ll. 959-1096). The expression of her complaints --in front of Chremylus, the comic hero, and also of the young man, who approaches the hero's house in order to express his gratitude to the god-- results in an episodic scene devoted to the exposition of material precariousness of her decayed body. Behind this comic denigration, usual against old and ugly persons, there are also exposed various forms of body apprehension very subtly, on which this paper intends to explore. We refer to the affective and sensitive construction of corporality, to the dynamics of body language, to the physiological imprint of immateriality (such as pleasure or pain), among several aspects that prove the author's special interest in the body. [ABSTRACT FROM AUTHOR]

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]

In 2006, the International Astronomical Union (IAU) voted to demote Pluto. Through an analysis of astronomical research, science journalism, and popular science publication, this essay details the roles that narrative has in communicating science both to expert and non-expert audiences. Pluto's academic and public controversy has resulted in scientists' prolific use of narrative to communicate research findings and engage in dialogue with peers and members of the public. The case of Pluto's demotion exemplifies the contribution of narrative science communication to the shaping of scientific practice and cultural conceptualizations of science. [ABSTRACT FROM AUTHOR]

Pluto and Charon are strikingly diverse in their range of geologies, surface compositions, and crater retention ages. This is despite the two having similar densities and presumed bulk compositions. Much of Pluto's surface reflects surface-atmosphere interactions and the mobilization of volatile ices by insolation. Abundant evidence, including past and present N2 ice glacial activity, implies that Pluto has undergone substantial climate evolution. An ancient impact basin contains a massive, convectively overturning N2 ice reservoir, whose position and surrounding tectonics suggest a subsurface ocean. Aligned blades of methane ice hundreds of meters tall, found only at high altitude, likely cover much of Pluto's low latitudes and may be a consequence of obliquity variation–driven volatile migration. Multikilometer-high possible cryovolcanic constructs and apparent fissure eruptions indicate relatively late endogenic activity on Pluto. Pluto's range of surface ages is extreme, whereas Charon's surface, while old, displays a large resurfaced plain and a globally engirdling extensional tectonic network attesting to earlier endogenic vigor. The vast N2 ice sheet Sputnik Planitia controls Pluto's atmosphere and climate, comparable in importance with the role of Greenland and Antarctica on the climate of Earth. Spectacular evidence for erosion such as now-unoccupied glacial valley networks implies a vigorous early climate, and more widespread N2 ice glaciation, on Pluto. Geological activity on both bodies requires or required sustained internal heat release and suggests a past (Charon) or present (Pluto) ammoniated, subsurface ocean. The varieties of geologic experience witnessed on Pluto and Charon should play out among the many and varied dwarf planets of the Kuiper belt. [ABSTRACT FROM AUTHOR]

The origin of the interstellar object 1I/'Oumuamua has defied explanation. We perform calculations of the non‐gravitational acceleration that would be experienced by bodies composed of a range of different ices and demonstrate that a body composed of N2 ice would satisfy the available constraints on the non‐gravitational acceleration, size, and albedo, and lack of detectable emission of CO or CO2 or dust. We find that 'Oumuamua was small, with dimensions 45 m × 44 m × 7.5 m at the time of observation at 1.42 au from the Sun, with a high albedo of 0.64. This albedo is consistent with the N2 surfaces of bodies like Pluto and Triton. We estimate 'Oumuamua was ejected about 0.4–0.5 Gyr ago from a young stellar system, possibly in the Perseus arm. Objects like 'Oumuamua may directly probe the surface compositions of a hitherto‐unobserved type of exoplanet: "exo‐plutos". In a companion paper (Desch & Jackson, 2021) we demonstrate that dynamical instabilities like the one experienced by the Kuiper belt, in other stellar systems, plausibly could generate and eject large numbers of N2 ice fragments. 'Oumuamua may be the first sample of an exoplanet brought to us. Plain Language Summary: 1I/'Oumuamua is very strange and it is hard to explain where it came from. We looked at several different ices and the push they would give 'Oumuamua as they evaporated. We found that the best ice is nitrogen (N2), which would explain many of the things we know about it. 'Oumuamua was small, about half as long as a city block and only as thick as a three story building, but it was very shiny. The shininess is about the same as the surfaces of Pluto and Triton, which are also covered in nitrogen ice. We suggest 'Oumuamua was probably thrown out of a young star system about half a billion years ago. Bodies like 'Oumuamua may allow us to see what the surfaces of a so far unknown type of exoplanet, "exo‐Plutos," are made of. In a following paper (Desch & Jackson, 2021) we show that orbital instabilities in which giant planets move around, as happened in our own outer solar system 4 billion years ago, could make and throw out large numbers of small pieces of nitrogen ice like 'Oumuamua. 'Oumuamua may be the first piece of an exoplanet brought to us. Key Points: A fragment of N2 ice would satisfy all known constraints on 'Oumuamua's size, composition, and accelerationThe presence of N2 ice in the solar system on Pluto makes it likely 'Oumuamua is such a fragmentThe existence of "exo‐Plutos" in other stellar systems must be common [ABSTRACT FROM AUTHOR]

The origin of the interstellar object 1I/'Oumuamua has defied explanation. In a companion paper (Jackson & Desch, 2021), we show that a body of N2 ice with axes 45 m × 44 m × 7.5 m at the time of observation would be consistent with its albedo, nongravitational acceleration, and lack of observed CO or CO2 or dust. Here we demonstrate that impacts on the surfaces of Pluto‐like Kuiper belt objects (KBOs) would have generated and ejected ∼1014 collisional fragments—roughly half of them H2O ice fragments and half of them N2 ice fragments—due to the dynamical instability that depleted the primordial Kuiper belt. We show consistency between these numbers and the frequency with which we would observe interstellar objects like 1I/'Oumuamua, and more comet‐like objects like 2I/Borisov, if other stellar systems eject such objects with efficiency like that of the Sun; we infer that differentiated KBOs and dynamical instabilities that eject impact‐generated fragments may be near‐universal among extrasolar systems. Galactic cosmic rays would erode such fragments over 4.5 Gyr, so that fragments are a small fraction (∼0.1%) of long‐period Oort comets, but C/2016 R2 may be an example. We estimate 'Oumuamua was ejected about 0.4–0.5 Gyr ago, from a young (∼108 yr) stellar system, which we speculate was in the Perseus arm. Objects like 'Oumuamua may directly probe the surface compositions of a hitherto‐unobserved type of exoplanet: "exo‐Plutos." 'Oumuamua may be the first sample of an exoplanet brought to us. Plain Language Summary: Our Kuiper belt originally had much more mass than today, but an instability caused by Neptune's migration disrupted their orbits, ejecting most of this material from the Solar System, and simultaneously causing numerous collisions among these bodies. There were thousands of bodies like Pluto, with N2 ice (like the gas in Earth's atmosphere, but frozen) on their surfaces, and this instability would have generated trillions of N2 ice fragments. A similar fragment, generated in another solar system, after traveling for about a half billion years through interstellar space, would match the size, shape, brightness, and dynamics of the interstellar object 1I/'Oumuamua. The odds of detecting such an object, as well as more comet‐like objects like the interstellar object 2I/Borisov, are consistent with the numbers of such objects we expect in interstellar space if most stellar systems ejected comets and N2 ice fragments with the same efficiency our solar system did. This implies other stellar systems also had Kuiper belts and similar instabilities. There are hints that some N2 ice fragments may have survived in the Oort cloud of comets in our Solar System. 'Oumuamua may be the first sample of an exoplanet born around another star, brought to Earth. Key Points: A dynamical instability in the early Kuiper belt likely ejected ∼1015 N2 ice fragments from thousands of PlutosA fragment of N2 ice from an "exo‐Pluto" would match all known attributes of the interstellar object 1I/'OumuamuaThe dynamical instability in the Kuiper belt experienced by our Solar System may be common among stellar systems [ABSTRACT FROM AUTHOR]

We derive a topography data set from images of Pluto and Charon that contain the body edge (i.e., limb profiles) which will help in understanding the comparative history of the binary system. We use the profiles to derive topographic variance spectra and find that while the variance spectrum of Pluto fits a single power law, Charon's spectrum displays a clear breakpoint at ∼150 km wavelength. Assuming the breakpoint is a result of topographic flexure, we find that Charon's elastic thickness must have been 20 ± 10 km during topography formation. A lack of a breakpoint for Pluto sets a minimum elastic thickness for Pluto of 60 km. We use these elastic thickness estimates to calculate a maximum heat flux of ∼13 m Wm−2 on Pluto during and after topography formation. On Charon, however, we find that the heat flux during topography formation was 35−15+44 m Wm−2. This range of values far exceeds the likely radiogenic heat production and is consistent with either heat released following the Charon‐forming impact event or (more likely) tidal heating during Charon's early history. Plain Language Summary: Studying the topography of planetary bodies provides key insights into the geologic processes of their surfaces and interiors. In this work we develop a topography data set for Pluto and Charon by mapping variations in the height along the worlds' edges in images from New Horizons. We analyze the data to determine roughness using the mean amplitude of mountains and valleys for a range of widths. Pluto shows the expected result of a single slope decreasing in roughness at shorter widths, but Charon has a change in the slope at ∼150 km. Mountains and valleys on Charon wider than this are respectively shorter and shallower than expected. This gives insight into how the landforms on Charon formed as well as the ability of Charon's crust to support variations in elevation. Charon's landforms must have formed at the observed size or decreased over time to have modern amplitudes. Either case implies that Charon had a thinner ice shell, and was relatively hotter, than Pluto in the ancient past. This extra heat is consistent with a Charon‐forming impact or (more likely) tidal heating during the Charon's initial history. Key Points: We derived new limb profile topography datasets of Pluto and Charon based on the body edge in imagesFor Charon, the topographic variance spectrum displays a distinct change in slope at ∼150 km wavelengthCharon's topography records high heat fluxes from either tidal heating or a giant impact [ABSTRACT FROM AUTHOR]

Recent analysis of New Horizons data revealed an ∼1,000‐km‐diameter region of large‐scale lineations antipodal to Sputnik Planitia, the 1,200 × 2,000 km elliptical impact basin on Pluto's anti‐Charon hemisphere. At the available resolution, these lineations are similar to antipodal terrains associated with large impact basins elsewhere in the Solar System. Here, we simulate the Sputnik Planitia‐forming impact and track stress waves through a Pluto‐like target body to the antipode. We find that the Sputnik Planitia‐forming impact is capable of producing significant antipodal terrains and the observed lineations may be extensional graben. The extent and mode of deformation at the antipode, however, is sensitive to ocean thickness and core composition. Simulations that best reproduce the observed terrains imply that Pluto may have hosted a >150‐km‐thick ocean and a hydrated core at the time of impact. Plain Language Summary: Pluto's interior structure remains largely a mystery, including the composition of the core and the existence and extent of a subsurface ocean. Though seismological study is not yet possible on Pluto, the reaction of a planet's interior structure to stress waves produced by large impacts can be used to indirectly probe the planet's interior—seismology by impact. We use impact modeling to simulate the formation of Sputnik Planitia, Pluto's massive impact basin, to determine how the dwarf planet's interior controls transmission of stress waves to the opposite hemisphere. Our results suggest that a large subsurface ocean beneath the ice shell, as well as a core that has experienced substantial alteration from interactions with an ocean above, are critical to match the size of the lineated terrain observed antipodal to Sputnik Planitia. Key Points: We model the formation of Pluto's Sputnik Planitia basin from an impact origin to assess the dwarf planet's interior structureOur simulations illustrate the Sputnik Planitia‐forming impact is capable of producing significant antipodal terrains on PlutoThe extent and mode of deformation at the antipode is most consistent with a thick (>150‐km) ocean and a hydrated core [ABSTRACT FROM AUTHOR]

Pluto's atmosphere is mainly nitrogen and is in solid‐gas equilibrium with the surface nitrogen ice. As a result, the global nitrogen ice distribution and the induced nitrogen condensation‐sublimation flows strongly control the atmospheric circulation. It is therefore essential for Global Climate Models to accurately account for the global nitrogen ice distribution in order to realistically simulate Pluto's atmosphere. Here we present a set of new numerical simulations of Pluto's atmosphere in 2015 performed with a Global Climate Model using a 50‐km horizontal resolution (3.75° × 2.5°) and taking into account the latest topography and ice distribution data, as observed by the New Horizons spacecraft. In order to analyze the seasonal evolution of Pluto's atmosphere dynamics, we also performed simulations at coarser resolution (11.25° × 7.5°) but covering three Pluto years. The model predicts a near‐surface western boundary current inside the Sputnik Planitia basin in 2015, which is consistent with the dark wind streaks observed in this region. We find that this atmospheric current could explain the differences in ice composition and color observed in the northwestern regions of Sputnik Planitia, by significantly impacting the nitrogen ice sublimation rate in these regions through processes possibly involving conductive heat flux from the atmosphere, transport of dark materials by the winds, and surface albedo positive feedbacks. In addition, we find that this current controls Pluto's general atmospheric circulation, which is dominated by a retrorotation, independently of the nitrogen ice distribution outside Sputnik Planitia. This exotic circulation regime could explain many of the geological features and longitudinal asymmetries in ice distribution observed all over Pluto's surface. Key Points: High‐resolution simulations of Pluto's climate show that the circulation is dominated by ∼10‐m/s retrograde winds during most of the yearNitrogen condensation‐sublimation flows in Sputnik Planitia are creating an intense western boundary currentAtmospheric heat flux, transport of tholins, and albedo feedbacks could explain the albedo contrasts observed in Sputnik Planitia [ABSTRACT FROM AUTHOR]

The existence of a subsurface water ocean on Pluto has been inferred on numerous grounds. The characteristics of such an ocean are vital inputs for various types of models that are carried out to illuminate the interior structure and evolution of Pluto, especially in light of the complementary relationship between the dimensions of the ocean and an outer water ice shell. In particular, models of loading of the Sputnik Planitia (SP) impact basin by nitrogen ice infill require knowledge of ocean properties such as density, and previous studies did not consider the potential densification of Pluto's ocean from salinity. Here we carry out analytic and Finite Element Method (FEM) modeling of loading deformations and stresses in an ice shell lithosphere, to determine the conditions, including ocean density, that best account for the observed distributions of tectonic features around the SP basin. Our results, while generally consistent with those of previous studies, provide more complete constraints on the initial basin depth (no deeper than 6 km, with most likely values in the 3–4 km range). Further, we find that increasing density (salinity) generally increases disagreement between model tectonic predictions and observations, and therefore we infer modest salinity-based densification of Pluto's ocean (ρ ocean < 1100 kg/m3) relative to values seen or inferred on other worlds. • The density of Pluto's interior ocean affects the stress state in the icy shell above. • The Sputnik Planitia impact basin is filled with nitrogen ice, stressing the shell. • Ocean density (salinity) affects the response of the shell to loads. • Models with high density predict stress states inconsistent with observed faulting. • The density of Pluto's ocean is likely <1100 kg/m3. [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