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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]

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]

Abstract: Recent images from the High Resolution Imaging Science Experiment (HiRISE) camera have shown that slope streaks have relief on the order of a meter or less. This study presents observations of transverse bedforms and infill deposits within slope streak beds that were not previously identified or were uncommon from earlier analyses of HiRISE images. Transverse bedforms are linear to slightly arcuate features oriented transverse to the slope streak bed which may be analogous to terrestrial splash or coarse-grained ripples based on their morphology, wavelength, and amplitude. In addition to the bedforms, there is also evidence that slope streak beds gradually shallow over time by infilling of material. The presence of ripples within slope streaks implies that saltation-capable material is available on the surface today and/or was available in the recent past. Although airfall dust is not a capable saltation source material, aggregates of electrostatically-bound dust that are possibly later cemented by salts may serve as a source. From the results of this study, we hypothesize a sequence of events in a slope streak formation and modification cycle where grains saltate to form ripples along the bed of a slope streak, airfall dust mantling causes gradual fading of the streak, and infill material buries the ripples, eventually reaching the pre-avalanche surface that removes all traces of relief. [Copyright &y& Elsevier]

Abstract: Distinct competent layers are observed in the slopes of eastern Coprates Chasma, part of the Valles Marineris system on Mars. Our observations indicate that the stratigraphy of Coprates Chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers. The strong, competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes. The topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of Coprates Chasma less than 100 m below the southern Ophir Planum surface. The volume of the topmost emplaced layer is at least 70 km3 and may be greater than 2100 km3 if the unit underlies most of Ophir Planum. The broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within Coprates Chasma where the layer is also observed. Rim outcrop morphology and elevation differences between Ophir and Aurorae Plana may be indicative of the easternmost extent of the topmost competent layer. These observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the Valles Marineris is carved, and they present a picture of some of the last volcanic activity in this area. Furthermore, the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material. [Copyright &y& Elsevier]

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]

Highlights: [•] We introduce a technique to identify major sand-moving winds from dune morphology. [•] Major transport vectors can be identified solely from analysis of single images. [•] The technique was validated by testing on the Great Sand Dunes of Colorado, USA. [ABSTRACT FROM AUTHOR]

Estimates of discharge for martian outflow channels have spanned orders of magnitude due in part to uncertainties in floodwater height. A methodology of estimating discharge based on bedforms would reduce some of this uncertainty. Such a methodology based on the morphology and granulometry of flood-formed (‘diluvial’) dunes has been developed by Carling (1996b, in: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: The Context of Palaeohydrology. Geological Society Special Publication No. 115, London, UK, 165–179) and applied to Pleistocene flood-formed dunes in Siberia. Transverse periodic dune-like bedforms in Athabasca Valles, Mars, have previously been classified both as flood-formed dunes and as antidunes. Either interpretation is important, as they both imply substantial quantities of water, but each has different hydraulic implications. We undertook photoclinometric measurements of these forms, and compared them with data from flood-formed dunes in Siberia. Our analysis of those data shows their morphology to be more consistent with dunes than antidunes, thus providing the first documentation of flood-formed dunes on Mars. Other reasoning based on context and likely hydraulics also supports the bedforms'' classification as dunes. Evidence does not support the dunes being aeolian, although a conclusive determination cannot be made with present data. Given the preponderance of evidence that the features are flood-formed instead of aeolian, we applied Carling''s (1996b, in: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: The Context of Palaeohydrology. Geological Society Special Publication No. 115, London, UK, 165–179) dune-flow model to derive the peak discharge of the flood flow that formed them. The resultant estimate is approximately 2×106 m3/s, similar to previous estimates. The size of the Athabascan dunes'' in comparison with that of terrestrial dunes suggests that these martian dunes took at least 1–2 days to grow. Their flattened morphology implies that they were formed at high subcritical flow and that the flood flow that formed them receded very quickly. [Copyright &y& Elsevier]

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]

The highlands region north and east of Sputnik Planitia can be subdivided into seven terrain types based on their physiographic expression. The northern rough uplands are characterized by jagged uplands and broad troughs, and it may contain a deeply-eroded ancient mantle. Dissected terrain has been interpreted to have been eroded by paleo-glaciation. The smooth uplands and pits terrain contains broad, rolling uplands surrounding complexes of pits, some of which contain smooth floors. The uplands are mantled by smooth-surfaced deposits possibly derived from adjacent pits through low-power explosive cryovolcanism or through slow vapor condensation. The eroded smooth uplands appear to have originally been smooth uplands and pits terrain modified by small-scale sublimation pitting. The bright pitted uplands features intricate texturing by reticulate ridges that may have originated by sublimation erosion, volatile condensation, or both. The bladed terrain is characterized by parallel ridges oriented north–south and is discussed in a separate paper. The dark uplands are mantled with reddish deposits that may be atmospherically deposited tholins. Their presence has affected long-term landform evolution. Widespread pit complexes occur on most of the terrain units. Most appear to be associated with tectonic lineations. Some pits are floored by broad expanses of ices, whereas most feature deep, conical depressions. A few pit complexes are enclosed by elevated rims of uncertain origin. [ABSTRACT FROM AUTHOR]

Modern N 2 ice glaciers flow from highlands to the east of the 750 × 1400 km 2 lowland of Sputnik Planum [ SP ] and merge with the ices of similar composition on SP. We explore the possibility that glaciation may be fed by N 2 sublimation from SP followed by redeposition on the highlands. The uplands to the northeast, north, and west of SP have been erosionally sculpted into a variety of dissected terrains that feature linear depressions (valleys), locally in dendritic networks. We interpret these dissected terrains to have been carved by N 2 glaciers formerly covering the uplands. Depositional glacial landforms (moraines, eskers, outwash) have not been identified, however. N 2 glaciation would have a different erosional manifestation because the substrate (porous water ice and CH 4 -rich mantles) probably has lower density than N 2 , and also because of the lack of freeze-thaw weathering. If sufficiently thick (1–4 km), N 2 glaciers might have experienced basal melting. Past flow of N 2 glaciers from the highlands into SP may have detached and transported the prominent mountainous water ice mountains along the western border of SP . [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]

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]

Europan domes are positive relief features that are typically circular to elliptical in planform shape, and have characteristic diameters <16 km. Although it cannot be ruled out that many of these domes may have been formed from the intrusion of diapirs into Europa's crust, a subset of domes have relatively smooth surfaces that do not mimic the surrounding terrain. These domes appear to obscure the preexisting terrain and have distinct margins which may be lobate or rounded. If all domes on Europa's surface represented structures where the icy crust had simply been "punched up" by diapiric intrusions, uplifts with these distinct morphologies would not be expected to exist. In this study, we revisit the hypothesis that a subset of europan domes formed in a manner similar to lava domes on Earth and Venus. Previously, we modeled dome formation as a consequence of the extrusion of viscous cryolava. However, that approach only allowed for the investigation of late-stage eruptive processes far from the vent and provided little insight into how cryovolcanic fluids may have arrived at the surface. Consideration of cryolava dome emplacement as fluids erupt onto Europa's surface is therefore pertinent. A volume flux approach, in which dome formation is modeled as fluid extrudes onto the surface at a constant rate, has been successfully applied to the formation of lava domes on Venus. That study showed that neglecting to consider changes in fluid rheology while a constant flux of lava is actively extruded onto the surface may result in overestimates, by several orders of magnitude, of initial lava viscosities at the time of eruption. Obtaining accurate viscosity estimates for Europa's cryovolcanic fluids is a critical step in understanding the properties of near-surface fluids that have participated in subsurface-surface exchange in the geologically recent past. To place improved constraints on the rheology and composition of europan cryolavas, and to better gauge the potential for dome formation on Europa via effusive eruptions, we apply this new volume flux approach to the formation of putative europan cryolava domes. We present a perturbation solution to the generalized form of the Boussinesq equation for fluid flow in a cylindrical geometry and explore dome formation while fluid is continuously extruding onto the surface. We find that at the time of eruption, dome-forming cryolavas may have had viscosities of 101–103 Pa s. These viscosity values suggest that cryolavas may be briny slurries composed of a mixture of water, salts, and ice crystals, rather than pure water (viscosity ~10−3 Pa s) or simple brines (viscosities between 10−3 and 10−1 Pa s). Nevertheless, the derived bulk viscosities indicate that dome-forming cryolavas have a rheology more similar to basalt than typical higher-viscosity andesite to rhyolitic dome-forming lavas on Earth. Several of the domes in our study may be connected to liquid reservoirs in Europa's crust, and subsurface-surface exchange may be ongoing today. As such, these domes represent compelling targets for multispectral imaging, radar sounding, and surface sampling by future missions to Europa. • We model the formation of cryolava domes on Europa while fluid is erupting from the vent at a constant rate. • Cryolava viscosities may be on the order of 10–103 Pa s at the time of eruption. • Dome-forming cryolavas may be salty, crystal-rich slurries with a rheology similar to terrestrial basalt. • Cryolava viscosities may represent differences in the thermocompositional states of various subsurface fluid reservoirs. • Cryolava viscosities may be up to 5 orders of magnitude less than those reported from previous modeling. [ABSTRACT FROM AUTHOR]

Abstract: Fine-resolution (500 m/pixel) Cassini Visual and Infrared Mapping Spectrometer (VIMS) T20 observations of Titan resolve that moon''s sand dunes. The spectral variability in some dune regions shows that there are sand-free interdune areas, wherein VIMS spectra reveal the exposed dune substrate. The interdunes from T20 are, variously, materials that correspond to the equatorial bright, 5-μm-bright, and dark blue spectral units. Our observations show that an enigmatic “dark red” spectral unit seen in T5 in fact represents a macroscopic mixture with 5-μm-bright material and dunes as its spectral endmembers. Looking more broadly, similar mixtures of varying amounts of dune and interdune units of varying composition can explain the spectral and albedo variability within the dark brown dune global spectral unit that is associated with dunes. The presence of interdunes indicates that Titan''s dunefields are both mature and recently active. The spectrum of the dune endmember reveals the sand to be composed of less water ice than the rest of Titan; various organics are consistent with the dunes'' measured reflectivity. We measure a mean dune spacing of 2.1 km, and find that the dunes are oriented on the average in an east–west direction, but angling up to 10° from parallel to the equator in specific cases. Where no interdunes are present, we determine the height of one set of dunes photoclinometrically to be between 30 and 70 m. These results pave the way for future exploration and interpretation of Titan''s sand dunes. [Copyright &y& Elsevier]