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Abstract Charon displays extensive plains that cover the equatorial area and south to the terminator on the sub-Pluto hemisphere observed by New Horizons. We hypothesize that these plains are a result of Charon's global extension and early subsurface ocean yielding a large cryoflow that completely resurfaced this area leaving the plains and other features that we observe today. The cryoflow consisted of ammonia-rich material, and could have resurfaced this area either by cryovolcanic effusion similar to lunar maria emplacement or a mechanism similar to magmatic stoping where lithospheric blocks foundered. Geological observations, modeling of possible flow rheology, and an analysis of rille orientations support these hypotheses. Highlights • The extensive plains on Charon are a result of global expansion and an early subsurface ocean that flowed as a cryoflow and resurfaced a large area. • The cryoflow most likely was composed of ammonia-rich water ice. • The cryoflow was similar to Lunar maria emplacement or via a mechanism where lithospheric blocks were severely disrupted. [ABSTRACT FROM AUTHOR]

The 2015 New Horizons flyby has produced the first high-resolution maps of morphology and topography of Pluto and Charon, the most distant objects so mapped. Global integrated mosaics of Pluto were produced using both LORRI framing camera and MVIC line scan camera data, showing the best resolution data obtained for all areas of the illuminated surface, ∼78% of the body. A unique feature of the Pluto imaging data set is the observation of terrains illuminated only by light scattered from atmospheric haze, allowing us to map terrains in the southern hemisphere that would otherwise have been in darkness. MVIC 4-color data were combined with the panchromatic map to produce full color global maps. Digital elevation models (DEMs) over ∼42% of Pluto were produced using combinations of MVIC hemispheric scans and LORRI mosaics, from which slopes at scales of ∼1 km can be determined. Pluto can be divided into regions each with distinct topographic signatures, corresponding with major physiographic terrain types. Large areas of Pluto are comprised of low-relief moderately cratered plains units. Deeply pitted and glaciated plains east of Sputnik Planitia are elevated ∼0.7 km. The most dominant topographic feature on Pluto is the 1200-by-2000-km wide depression enclosing the bright Sputnik Planitia ice sheet, the surface of which is 2.5-to-3.5 km deep (relative to the rim) and ∼2 km deep relative to the mean radius. The partial ring of steep-sided massifs, several of which are more than 5 km high, along the western margins of Sputnik Planitia produce some of the locally highest and steepest relief on Pluto, with slopes of 40–50°. The second major topographic feature is a complex, eroded, ridge-trough system ∼300–400 km wide and at least 3200 km long extending north-to-south along the 155° meridian. This enormous structure has several kilometers of relief. It may predate the large impact event forming the basin, though some post-Sputnik Planitia deformation is evident. The large depressed, partially walled plain, Hyecho Palus, lies due southwest of Sputnik Planitia. Near the center of Hyecho Palus lie the circular constructional edifices Wright and Piccard Montes. Wright Mons rises 4.5 km above these plains, with a central depression ∼4.5 km deep, whereas Piccard Mons, best observed in haze-light, rises ∼5.5 km above the plains but has a bowl-shaped central depression ∼5.5 km below the plains for a total relief of up to 11 km, the greatest observed on Pluto. Both of these features are interpreted as constructional (volcanic?) in nature. Additional prominent topographic features include a 2–3 km high and ∼600 km wide dome centered on the illuminated IAU pole and the amoeboidal plateaus of “bladed” terrains in the equatorial region, which rise 2–5 km above local terrains and are the highest standing geologic units on the encounter hemisphere. The mean elevations in the integrated DEM for the two radio occultation areas are consistent with the 5–6 km difference in elevation as determined independently by the radio experiment, and a limb profile near the egress point confirms the presence of elevated bladed terrains in that area. Local relief of 3–5 km at massifs, troughs and pits supports conclusions that the icy shell of Pluto is relatively rigid. Numerous examples of topographic control of ice or frost deposition occur across Pluto, including the distinct coloration of the polar dome, the elevated terrains of eastern Tombaugh Regio, and along the ridge-trough system, where ridge tops and fossae rims are covered in different ices than at lower elevations. The topographic hypsogram of Pluto's encounter hemisphere is strongly bimodal due to the large Sputnik Planitia depression. Otherwise the topographic signature of Pluto is controlled by deviations from the otherwise dominant low plains, including elevated bladed terrain plateaus and the depressed volcanic province including Wright and Piccard Montes. [ABSTRACT FROM AUTHOR]

New Horizons images of Pluto’s companion Charon show a variety of terrains that display extensional tectonic features, with relief surprising for this relatively small world. These features suggest a global extensional areal strain of order 1% early in Charon’s history. Such extension is consistent with the presence of an ancient global ocean, now frozen. [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]

We investigated five large landslides identified in the Serenity Chasma region of Charon. The identification of these landslides involved a search for these features in images taken by cameras onboard the New Horizons spacecraft. Various landslide properties were analyzed based on their morphologies using a digital terrain model of the region. We found that landslides are confined to the walls of the large normal fault scarps that make up Serenity Chasma. Based on extensive landslide runout lengths (L) relative to their drop heights (H), we classified these features as long-runout landslides. By analyzing their geometries, we estimated the friction coefficients of the landslide material (H / L) to be between 0.15 to 0.31 and the runout efficiencies (L / H) to be between 3.2 and 6.8. We also estimated that the specific energy released during landslide motion ranged from 0.8 to 1.3 kJ kg−1. These amounts of energy were too low to have generated significant melt around landslide particles. • We investigated landslides identified in the Serenity Chasma region of Charon. • Friction coefficients of the landslide material are between 0.15 and 0.31. • Landslide runout efficiencies are between 3.2 and 6.8. • The specific energy released during landslide motion ranged from 0.8 to 1.3 kJ kg−1. • Significant melt was not likely generated around landslide particles during motion. [ABSTRACT FROM AUTHOR]

Bladed Terrain on Pluto consists of deposits of massive CH 4 , which are observed to occur within latitudes 30° of the equator and are found almost exclusively at the highest elevations (> 2 km above the mean radius). Our analysis indicates that these deposits of CH 4 preferentially precipitate at low latitudes where net annual solar energy input is lowest. CH 4 and N 2 will both precipitate at low elevations. However, since there is much more N 2 in the atmosphere than CH 4 , the N 2 ice will dominate at these low elevations. At high elevations the atmosphere is too warm for N 2 to precipitate so only CH 4 can do so. We conclude that following the time of massive CH 4 emplacement; there have been sufficient excursions in Pluto's climate to partially erode these deposits via sublimation into the blades we see today. Blades composed of massive CH 4 ice implies that the mechanical behavior of CH 4 can support at least several hundred meters of relief at Pluto surface conditions. Bladed Terrain deposits may be widespread in the low latitudes of the poorly seen sub-Charon hemisphere, based on spectral observations. If these locations are indeed Bladed Terrain deposits, they may mark heretofore unrecognized regions of high elevation. [ABSTRACT FROM AUTHOR]

NASA's New Horizons flyby mission of the Pluto-Charon binary system and its four moons provided humanity with its first spacecraft-based look at a large Kuiper Belt Object beyond Triton. Excluding this system, multiple Kuiper Belt Objects (KBOs) have been observed for only 20 years from Earth, and the KBO size distribution is unconstrained except among the largest objects. Because small KBOs will remain beyond the capabilities of ground-based observatories for the foreseeable future, one of the best ways to constrain the small KBO population is to examine the craters they have made on the Pluto-Charon system. The first step to understanding the crater population is to map it. In this work, we describe the steps undertaken to produce a robust crater database of impact features on Pluto, Charon, and their two largest moons, Nix and Hydra. These include an examination of different types of images and image processing, and we present an analysis of variability among the crater mapping team, where crater diameters were found to average ± 10% uncertainty across all sizes measured (∼0.5–300 km). We also present a few basic analyses of the crater databases, finding that Pluto's craters' differential size-frequency distribution across the encounter hemisphere has a power-law slope of approximately –3.1 ± 0.1 over diameters D ≈ 15–200 km, and Charon's has a slope of –3.0 ± 0.2 over diameters D ≈ 10–120 km; it is significantly shallower on both bodies at smaller diameters. We also better quantify evidence of resurfacing evidenced by Pluto's craters in contrast with Charon's. With this work, we are also releasing our database of potential and probable impact craters: 5287 on Pluto, 2287 on Charon, 35 on Nix, and 6 on Hydra. [ABSTRACT FROM AUTHOR]

Fields of pits, both large and small, in Tombaugh Regio (Sputnik Planitia, and the Pitted Uplands to the east), and along the scarp of Piri Rupes, are examples of landscapes on Pluto where we conclude that sublimation drives their formation and evolution. Our heuristic modeling closely mimics the form, spacing, and arrangement of a variety of Tombaugh Regio's pits. Pluto's sublimation modified landforms appear to require a significant role for (diffusive) mass wasting as suggested by our modeling. In our models, the temporal evolution of pitted surfaces is such that initially lots of time passes with little happening, then eventually, very rapid development of relief and rapid sublimation. Small pits on Sputnik Planitia are consistent with their formation in N 2 -dominated materials. As N 2 -ice readily flows, some other ``stiffer'' volatile ice may play a role in supporting the relief of sublimation degraded landforms that exhibit several hundred meters of relief. A strong candidate is CH 4 , which is spectroscopically observed to be associated with these features, but the current state of rheological knowledge for CH 4 ice at Pluto conditions is insufficient for a firm assessment. [ABSTRACT FROM AUTHOR]

The cameras of New Horizons will provide robust data sets that should be imminently amenable to geological analysis of the Pluto system’s landscapes. In this paper, we begin with a brief discussion of the planned observations by the New Horizons cameras that will bear most directly on geological interpretability. Then we broadly review the major geological processes that could potentially operate on the surfaces of Pluto and its major moon Charon. We first survey exogenic processes (i.e. those for which energy for surface modification is supplied externally to the planetary surface): impact cratering, sedimentary processes (including volatile migration), and the work of wind. We conclude with an assessment of the prospects for endogenic activity in the form of tectonics and cryovolcanism. [ABSTRACT FROM AUTHOR]

Impact craters form as point-like explosions on solar system bodies, excavating a cavity within the surface. Most information about the impactor itself is lost, save its energy, such that the target itself controls much of the final crater size and shape given the energy input. For this reason, impact craters are a useful probe of surface differences across a single body and between bodies. Two properties that are commonly used to compare craters from one body to another are the ratio between crater depth and diameter, and the diameter at which craters transition between simple, bowl-shaped morphologies to more complex morphologies that include flat floors, central peaks, wall terraces, and scalloped rims. Both metrics are important as input into theoretical models and for probing the strength of the surface material. In this work, we have measured these properties on Pluto and Charon and report on the results. However, the analysis is not entirely straightforward; a secondary goal of this work is to explore how to measure and quantify the two properties accurately (depth vs diameter and simple-to-complex transitions). We arrive at different results based on the method, and we report on that variation as a way to reasonably estimate the uncertainty in each quantity. For those reasons, it is difficult to place a single, exact value on each, but we do conclude that the morphology-based simple-to-complex transition occurs at approximately 11–12½ km on Pluto, and 13½–16 km on Charon. We also conclude that the existing, public topography data for at least Charon is likely too low in resolution to measure the simple crater depth vs diameter function accurately, such that a morphometry-based simple-to-complex transition diameter is not quantifiable at this time. • We measured the locations, diameters, depths, and classified morphology for several hundred craters on Pluto and Charon. • We use the measurements to estimate the d (D) function for simple craters on Pluto and complex craters on Pluto and Charon. • We use the measurements to estimate the simple-to-complex crater transition based on numerous different crater features. • Pluto and Charon do not have anomalous d (D) nor simple-to-complex transitions relative to other solar system bodies. [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]