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In this work, we study the trajectories of the dust particles ejected in the geysers of the moon Enceladus, inside its Hill sphere, where its gravitational influence dominates. Our goal is to determine the properties of the particles that escape from the moon or of those that eventually impact on its surface. The analysis considers the gravity of Enceladus, the Lorentz force due to Saturn's magnetic field and the gas drag from the geysers. The local magnetic field around Enceladus is derived from an interpolation of Cassini data from six flybys. In general, we obtain that grain sizes and launching speeds define whether grains escape, though the launching angle may also play an important role in some cases. Plain Language Summary: Dust grains expelled from the geysers of the small Saturnian moon Enceladus follow particular trajectories determined by the different forces like the gravity of the moon, the gas drag of the geysers and the magnetic field of Saturn. In this work we study the dynamics of these grains inside a region around the moon, called the Hill sphere, where its gravitational influence dominates, in order to determine the effects of the different forces on the grains, as well as the properties of grains, like their size, mass and electric charge. Key Points: We study the trajectories of dust grains from the geysers of Enceladus inside the Hill sphere of the moonWe implement an interpolated magnetic field background to study the trajectories of dust grainsWe study the dynamic properties of dust grains that are able to escape from Enceladus [ABSTRACT FROM AUTHOR]

• -The geology of a newly discovered putative cryovolcanic field is introduced. • -Regional cryotectonic stress fields are reconstructed and compared to global models. • -The potential triggers of cryovolcanic activity in the region is summarized. More and more attention is devoted to the icy moons of the Solar System, including Europa, the second Galilean satellite of Jupiter, since the discovery of potential liquid water and the possibility of extra-terrestrial life harbored in its subsurface ocean below its icy crust. Along with the renaissance of the study of icy satellites, the ongoing missions, such as Europa Clipper and JUICE (JUpiter ICy moons Explorer), are also part of such rejuvenation of icy satellite research. One of the leading research topics connected to Europa is understanding its surface renewal, including the interaction between the subsurface ocean and the icy crust. One of the longest-lasting and still unsettled debates related to Europa is about the nature of the potentially active cryovolcanism, which may play an essential role in the interaction between the surface of the Jovian moon and the underlying subsurface ocean. Our study focuses on the geological-geomorphological characterization of a newly identified putative cryovolcanic field found on the surface of Europa. Various volcanic structures, possibly in multiple stages of maturity, were identified. The executed geological analysis in the surroundings of the volcanoes suggests strong local influence during the formation of cryovolcanic cones working together with the overall global-scale stress fields appearing in the ice crust of Europa. [ABSTRACT FROM AUTHOR]

Cryovolcanism has been invoked to explain numerous features observed on icy bodies. Many of these features show similar morphologies to volcanic features observed on Earth suggesting similar physics involved in their formation. Cryovolcanism lies at the intersection of volcanology and hydrology but as such, no one model from either discipline satisfactorily represents cryolava flow emplacement. We produced a new model for cryolava flow evolution that draws from both disciplines to track the physical, chemical, and thermal states of a hypothetical H2O‐NaCl flow on a Europa‐like body as it evolves away from the vent. This model is currently restricted to compositions on the water‐rich side of this chemical system and only predicts emplacement up to the turbulent to laminar transition. Modeling the laminar regime and a broader compositional space will be dealt with separately. Concentrations between 5 and 23 wt% (H2O‐NaCl eutectic) and initial flow thicknesses of 0.1, 1, 10, and 100 m were set as initial conditions. Model results suggest that flow may reach 40–60 vol% solids before transitioning to laminar flow. The thermal budget for these flows is dominated by the heat loss from vaporization in the low‐pressure environment. This model produces length to thickness aspect ratios, for the given compositions, that are broadly consistent with candidate cryovolcanic features on Ceres and Titan. These first‐order comparisons are not ideal and suggest the need for future modeling of cryovolcanic features in at least two dimensions. Plain Language Summary: Cryovolcanism is the icy volcanism that may occur on the surface icy bodies in the outer solar system. Erupted material will be predominantly aqueous fluid indicating that cryovolcanism lies at the intersection of volcanology and hydrology. However, neither discipline has models that adequately account for all of the parameters necessary for a cryovolcanic context. We produced a model for cryolava flow evolution that draws from both disciplines to track the physical, chemical, and thermal state of a hypothetical H2O‐NaCl flow. Results suggest that these flows may evolve more like a slush rather than a river with an expanding ice cap. These flows may also begin crystallizing while still in the turbulent regime. The transition to laminar flow may therefore be important to constrain, as it could be a point where a morphology or surface expression changes due to changing in behavior in the flow. Extending this model to different compositions and allowing more complicated mixtures may help to draw comparisons to observations of surface flows on icy satellites. Key Points: Reynolds and Stokes number suggest turbulence and particle entrainment may persist to crystal contents >60%Heat loss due to vaporization is the largest heat flux in this context, accounting for >95% of the total thermal budgetThe "life" of a cryolava flow (expressed as distance traveled) may be predominantly in the turbulent regime, whereas a silicate lava flow would be predominantly laminar [ABSTRACT FROM AUTHOR]

Effusive cryovolcanism may have occurred now or in the past on several icy bodies of the outer solar system. Our understanding of the physical properties of potential cryolavas is hindered by the limited range over which experimental data exist at appropriate conditions. The viscosity of the purely liquid phase is poorly constrained, yet, is a required component for understanding subsurface oceans, cryomagma ascent/transport, and subsequent evolution during emplacement. However, limited data exist for the viscosity of brines extending down temperature (<0°C). We have thus measured the viscosities of a broad range of binary aqueous systems at a range of temperatures and concentrations. These include the water-rich side of the eutectic for chloride (Na, K, NH 4), sulfate (Mg, K, NH 4), ammonia, and methanol systems between −35 and 30 °C. For geological applications, the effect of temperature on viscosity is often a larger control than the effect of concentration. We therefore constructed a model of viscosity based on the Vogel-Fulcher-Tammann equation, commonly used for silicate melt viscosities. This semi-empirical, temperature dependent model was parameterized for a given composition to account for the concentration dependence. Our model reproduced the experimental dataset to within 10 % relative uncertainty (typically within 5 %) and reproduces exactly the curve defined for pure water. The model is capable of extrapolating down to sub-ambient temperatures relevant to cryovolcanism without any singularities. Preliminary investigation of ternary systems (H 2 O-NaCl-MgSO 4 & H 2 O-NH 3 -CH 3 OH) suggests this model can be successfully extended to more complicated multi-component systems. This relatively straightforward scalability and the coupled temperature and concentration dependence are key improvements over previous, more complicated models. The viscosity data produced in this study covers a poorly investigated temperature range and can be applied to cryovolcanic processes on icy bodies. [ABSTRACT FROM AUTHOR]

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]

This paper considers the occurrence, diversity, and special features of volcanic and degassing processes on Titan, as well as analyzing the maximum gas concentrations of volatiles in a vertical profile of its interior and atmosphere. We showed that the specific character of Titan volcanic and degassing processes is due to low temperatures, a peculiar material composition, and the ubiquitous presence of a subsurface liquid horizon. Consequently, the common (on Earth) forms of (cryo)volcanic activity are reduced, while the more common forms include (1) gas-lifting degassing, which produces numerous, small, depressions with a higher level of liquid, and which is a probable cause of "magic islands" in Ligeia Mare, as well as (2) limnological discharges of gaseous methane with subsequent intensive cloud formation, occurrence of methane storms, and large-volume precipitation. In turn, both gas-lifting degassing and limnological discharges of endogenous methane are due to the condensate–gas inversion at depths of 1–2 km in Titan's interior. Below that depth, the endogenous fluids are nearly all gasless condensates, while above, these condensates start intensive boiling. We conclude that degassing is the most important relief-forming factor on Titan. [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]

Despite evidence for plumes on Jupiter's moon Europa, no surface features have been definitively identified as the source of the plumes to date. Furthermore, it remains unknown whether the activity originates from near‐surface water reservoirs within the ice shell or if it is sourced from the underlying global ocean. Here we investigate brine pocket migration, studied previously in the context of sea ice on Earth, as a process for transporting brine along thermal gradients. We show that the fracture system located in the center of Europa's Manannán crater is consistent with the formation of a subsurface brine reservoir. After the initial impact, residual aqueous melt concentrated via brine pocket migration as the target material cooled. Freezing and overpressurization then resulted in a cryovolcanic eruption. The volume of the emptied reservoir and the critical composition at the end of migration provide further constraints on the average salinity of Europa's ice shell. Plain Language Summary: Jupiter's satellite Europa has a subsurface ocean covered by an icy shell. We show how small pockets of brine can migrate within the ice from colder areas to warmer areas. This can happen even at very low temperatures, below the point where pure water would freeze, because the water becomes saltier and saltier as it migrates. By looking at an impact crater on Europa, which was initially warm in the center and cooled inward from its colder surroundings, we can study how the water migrated toward the center and formed a central water reservoir. As the final water pocket at the center of the crater started to freeze, the increasing pressure lead to a cryovolcanic eruption that emplaced brine onto the surface to form a prominent "spider" feature before the ice collapsed into the cavity below. Using a digital terrain model of the crater and collapse feature, we estimate how much water erupted and how salty Europa's ice shell is. Key Points: We introduce brine pocket migration as a mechanism for melt mobilization and a driver for cryovolcanism on icy worldsWe apply brine pocket migration to Europa's Manannán crater and its central "spider" (araneiform) fracture system and collapse structureWe show that understanding the concepts of brine pocket migration can also constrain the salinity of Europa's ice shell and ocean [ABSTRACT FROM AUTHOR]

Cassini Radar observations of Titan have revealed diverse landforms resulting from a variety of geologic processes. Many landforms can be unambiguously interpreted as resulting from atmospheric processes (dunes, rivers, and lakes) or impact cratering. Here we argue from morphological evidence such as nested collapses, elevated ramparts, halos, and islands or floor mountains that some of the abundant small depressions in the north polar region of Titan are volcanic collapse craters. A few similar depressions occur near the south pole; the restriction of this volcanism to polar regions is possibly related to predicted warmer and thinner‐than‐normal ice crust at the low‐elevation poles. The close association of the proposed volcanic craters with polar lakes is consistent with a volcanic origin through explosive eruptions, as either maars or calderas. The apparent freshness of some craters may mean that volcanism has been relatively recently active on Titan or even continues today. Plain Language Summary: The Cassini mission revealed many landforms on Saturn's moon Titan that are like those found on Earth. Sand dunes, river valleys, and lakes are all a result of actions by the atmosphere on the surface, driven by solar heating. We demonstrate there is also evidence for internal heat, manifest at the surface as cryovolcanoes, made from melting the water ice crust into liquid water and erupting it onto the surface. These features are roughly round, with raised rims, and they sometimes overlap each other. They are consistent with the shapes of other volcanic landforms on Earth formed by explosion, excavation, and collapse. That these features are at the polar regions, near the lakes of methane, may indicate methane or some other volatile can power them. The features appear relatively fresh, meaning they could still be forming today. Key Points: Landforms with characteristics consistent with a volcanic origin are found at Titan's north polar regionElevated rims, subround shapes, and large sizes indicate an origin by explosion and collapseThe relative youth of the landforms indicates they may be forming today [ABSTRACT FROM AUTHOR]

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 faculae in Occator Crater on dwarf planet Ceres are an accumulation of salts that have been interpreted as cryovolcanic products. Current age estimates from crater counting suggest a maximum 18‐Ma difference between the crater forming impact and the formation of Cerealia Facula, the central and most recent region in the crater. Here we model the thermal evolution of the potential impact‐induced cryomagma chamber beneath Occator Crater and show that it cools in less than 12 Ma. To reach cooling times of 18 Ma requires initial melt volumes exceeding 11,000 km3. However, simulations suggest that smaller initial cryomagma chambers may lead to partial melting of the lower crust. This may allow recharge of the magma chamber by deep brines located in the porous upper mantle of Ceres and may extend the longevity of cryovolcanic activity. Plain Language Summary: We are testing the hypothesis that the bright spots in the center of Occator crater on Ceres are salts extruded from a large brine reservoir in the crust that melted during the asteroid impact that formed Occator Crater. The age difference between the crater and the salt deposits is approximately 18 Ma and it is not clear if the brine can remain molten for such a long time. Our simulations show that an isolated impact‐induced cryomagma chamber will cool in less than 12 Ma. To reach cooling times of 18 Ma requires initial melt volumes exceeding 11000 km3, unrealtic given the available energy of the impactor. However, our simulations show that the crustal brine reservoir might communicate with a deeper brine reservoir in Ceres' mantle. Such recharge could extend the longevity of the impact‐induced cryomagma chamber beneath Occator Crater. Key Points: Likely cooling times for the impact‐induced cryomagma beneath Occator Crater are less than 12 MaCooling times approaching the 18‐Ma age difference between Occator Crater and Cerealia Facula require melt volumes exceeding 11,000 km3Large impact‐induced cryomagma chambers may partially melt lower crust, leading to recharge from a deep mantle brine reservoir [ABSTRACT FROM AUTHOR]

We examine the relevance of tidal heating for large Trans-Neptunian Objects, with a focus on its potential to melt and maintain layers of subsurface liquid water. Depending on their past orbital evolution, tidal heating may be an important part of the heat budget for a number of discovered and hypothetical TNO systems and may enable formation of, and increased access to, subsurface liquid water. Tidal heating induced by the process of despinning is found to be particularly able to compete with heating due to radionuclide decay in a number of different scenarios. In cases where radiogenic heating alone may establish subsurface conditions for liquid water, we focus on the extent by which tidal activity lifts the depth of such conditions closer to the surface. While it is common for strong tidal heating and long lived tides to be mutually exclusive, we find this is not always the case, and highlight when these two traits occur together. We find cases where TNO systems experience tidal heating that is a significant proportion of, or greater than radiogenic heating for periods ranging from100′s of millions to a billion years. For subsurface oceans that contain a small antifreeze component, tidal heating due to very high initial spin states may enable liquid water to be preserved right up to the present day. Of particular interest is the Eris-Dysnomia system, which in those cases may exhibit extant cryovolcanism. [ABSTRACT FROM AUTHOR]

A combination of geological interpretations and thermal-orbital evolution models imply that Pluto's large moon, Charon, had a subsurface water (and possibly ammonia) ocean that eventually froze. Ocean freezing generates large tensile stresses in the upper part of the ice shell and pressurizes the ocean below, perhaps leading to the formation of Charon's large canyons and putative cryovolcanic flows. Here, we identify the conditions in which a freezing ocean could create fractures that fully penetrate its ice shell, linking Charon's surface with its ocean and facilitating ocean-sourced cryovolcanism. We find that current models of Charon's interior evolution predict ice shells that are far too thick to be fully cracked by the stresses associated with ocean freezing. Either Charon's ice shell was <10 km thick when the flows occurred (as opposed to >100 km) or the surface was not in direct communication with the ocean as part of the eruptive process. If Charon's ice shell had been thin enough to be fully cracked, it would imply substantially more ocean freezing than is indicated by the canyons, Serenity and Mandjet Chasma. Due to the low radiogenic heating within Charon and the loss of tidal heating early in its history, a thin ice shell should have been short-lived, implying that ocean-sourced cryovolcanic flows would have ceased relatively early in Charon's history, consistent with interpretations of its surface geology. An additional (and perhaps implausibly large) heat source would be required to generate the substantially larger ocean implied by through-going fractures. We also find that ocean freezing can easily generate deep fractures that do not fully penetrate to the ocean, which may be the foundation of Charon's canyons. When ocean-bearing moons begin to cool down, their oceans can freeze. As new ice accretes to the bottom of the existing ice shell, the added volume of the ice can stress the shell. Pluto's largest moon, Charon, has canyons and cryovolcanic flows that may have formed in response to a freezing ocean. Here, we model the formation of fractures within Charon's ice shell as the ocean underneath it freezes to explore the evolution of Charon's interior and surface. We find that an ocean source for cryovolcanic flows is unlikely because the ice shell would have had to be much thinner than current thermal evolution models imply. However, freezing the ocean may have produced the stresses that formed canyons later in Charon's history. • Charon's ice shell had to be <10 km thick for cooling cracks to penetrate to the ocean. • Cryovolcanism from ocean freezing would have ceased before canyons formed. • If ocean-sourced cryovolcanism occurred, additional extensional features are predicted. • An ammonia-rich ocean does not change these conclusions. • Ocean-sourced cryovolcanism via cooling cracks is incompatible with current models of Charon's thermal evolution. [ABSTRACT FROM AUTHOR]

Jupiter's moon Europa has a relatively young surface (60–90 Myr on average), which may be due in part to cryovolcanic processes. Current models for both effusive and explosive cryovolcanism on Europa may be expanded and enhanced by linking the potential for cryovolcanism at the surface to subsurface cryomagmatism. The success of cryomagma transport through Europa's crust depends critically on the rate of ascent relative to the rate of solidification. The final transport distance of cryomagma is thus governed by initial melt volume, ascent rate, overall ascent distance, transport mechanism (i.e., diapirism, diking, or ascent in cylindrical conduits), and melt temperature and composition. The last two factors are especially critical in determining the budget of expendable energy before complete solidification. Here we use these factors as constraints to explore conditions under which cryomagma may arrive at Europa's surface to facilitate cryovolcanism. We find that 1–5 km radius warm ice diapirs ascending from the base of a 10 km thick stagnant lid can reach the shallow subsurface in a partially molten state. Cryomagma transport may be further facilitated if diapirs travel along pre-heated ascent paths. Under certain conditions, cryolava transported from 10 km depths in tabular dikes or pipe-like conduits may reach the surface at temperatures exceeding 250 K. Ascent rates for these geometries may be high enough that isothermal transport is approached. Cryomagmas containing significant amounts of low eutectic impurities can also be delivered to Europa's surface by propagating dikes or pipe-like conduits. [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: Surface venting is a common occurrence on several outer solar system satellites. Spacecraft have observed plumes erupting from the geologically young surfaces of Io, Triton and Enceladus. Europa also has a relatively young surface and previous studies have suggested that cryovolcanic eruptions may be responsible for the production of low-albedo deposits surrounding lenticulae and along triple band margins and lineae. Here, we have used the projected thicknesses of these deposits as constraints to determine the lifetimes of detectable cryovolcanic plumes that may have emplaced them. In an effort to explore the feasibility of detection of the particle component of plumes by spacecraft cameras operating at visible wavelengths, we present a conservative model to estimate plume characteristics such as height, eruption velocity, and optical depth under a variety of conditions. We find that cryovolcanic plumes on Europa are likely to be fairly small in stature with heights between 2.5 and 26km, and eruption velocities between 81 and 261m/s, respectively. Under these conditions and assuming that plumes are products of steady eruptions with particle radii of 0.5μm, our model suggests that easily detectable plumes will have optical depths, τ, greater than or equal to 0.04, and that their lifetimes may be no more than 300,000 years. Plume detection may be possible if high phase angle limb observations and/or stereo imaging of the surface are undertaken in areas where eruptive activity is likely to occur. Cameras with imaging resolutions greater than 50m/pixel should be used to make all observations. Future missions could employ the results of our model in searches for plume activity at Europa. [Copyright &y& Elsevier]

Enceladus, one of the mid-sized icy moons of Saturn, has an importance to planetary science far greater than its modest 504-km diameter would suggest. Intensive exploration of Enceladus by the Cassini Saturn orbiter has revealed that it is the only known icy world in the solar system with ongoing deep-seated geological activity. Active tectonic fractures at Enceladus's south pole, dubbed "tiger stripes," warmed by internal tidally generated heat, spew supersonic jets of water vapor, other gases, and ice particles into circum- Saturnian space. A subsurface saltwater sea probably exists under the south pole, between the ice shell and the silicate core. Because of evidence that liquid water is probably present at the jet sources, Enceladus is also of great astrobiological interest as a potential habitat for life. [ABSTRACT FROM AUTHOR]

Abstract: Spectro-imaging and radar measurements by the Cassini–Huygens mission suggest that some of the Saturnian satellites may be geologically active and could support tectonic processes. In particular Titan, Saturn''s largest moon, possesses a complex and dynamic geology as witnessed by its varied surface morphology resulting from aeolian, fluvial, and possibly tectonic and endogenous cryovolcanic processes. The Synthetic Aperture Radar (SAR) instrument on board Cassini spacecraft, indicates the possibility for morphotectonic features on Titan''s surface such as mountains, ridges, faults and canyons. The mechanisms that formed these morphotectonic structures are still unclear since ensuing processes, such as erosion may have modified or partially obscured them. Due to the limitations of Cassini–Huygens in the acquisition of in situ measurements or samples relevant to geotectonics processes and the lack of high spatial resolution imaging, we do not have precise enough data of the morphology and topography of Titan. However we suggest that contractional tectonism followed by atmospheric modifications has resulted in the observed morphotectonic features. To test the possibility of morphotectonics on Titan, we provide in this work a comparative study between Cassini observations of the satellite versus terrestrial tectonic systems and infer suggestions for possible formation mechanisms. [Copyright &y& Elsevier]

Abstract: Cassini radar images of Titan''s surface reveal numerous dark circular features in the equatorial region. These may be related to methane sublimation and have been interpreted as pits. In the T8 equatorial swath, we identify 199 individual pits and estimate their diameters. We analyze the observed spatial distribution and size to characterize these features and understand their formation. Chi-square analysis confirms a significant deviation from a random distribution and shows clustering over the entire swath. However, analysis of the densest cluster of pits, a group of 50, shows a more random distribution. Fractal analysis and comparison with a same-sized random set find only a hint of linearity. A Poisson distribution fits the observed pit-sizes, although resolution limits size determination for the smallest pits, those less than 1km in diameter. Models for random pit generation and evolution simulate the observed Poisson distribution of pit sizes, with larger pits forming by the coalescing of smaller, overlapping ones. Pits, estimated to cover 0.5% of the equatorial T8 swath, could be an erosion mechanism that significantly contributes to the negative skew that has been observed in Titan''s hypsography. [Copyright &y& Elsevier]

The surfaces of the Solar System's icy satellites show an extraordinary variety of morphological features, which bear witness to exchange processes between the surface and subsurface. In this paper we review the characteristics of surface features on the moons of Jupiter, Saturn, Uranus and Neptune. Using data from spacecraft missions, we discuss the detailed morphology, size, and topography of cryovolcanic, tectonic, aeolian, fluvial, and impact features of both large moons and smaller satellites. [ABSTRACT FROM AUTHOR]

Abstract: Future planetary exploration of Titan will require higher degrees of on-board automation, including autonomous determination of sites where the probability of significant scientific findings is the highest. In this paper, a novel Artificial Intelligence (AI) method for the identification and interpretation of sites that yield the highest potential of cryovolcanic activity is presented. We introduce the theory of fuzzy cognitive maps (FCM) as a tool for the analysis of remotely collected data in planetary exploration. A cognitive model embedded in a fuzzy logic framework is constructed via the synergistic interaction of planetary scientists and AI experts. As an application example, we show how FCM can be employed to solve the challenging problem of recognizing cryovolcanism from Synthetic Aperture Radar (SAR) Cassini data. The fuzzy cognitive map is constructed using what is currently known about cryovolcanism on Titan and relies on geological mapping performed by planetary scientists to interpret different locales as cryovolcanic in nature. The system is not conceived to replace the human scientific interpretation, but to enhance the scientists’ ability to deal with large amounts of data, and it is a first step in designing AI systems that will be able, in the future, to autonomously make decisions in situations where human analysis and interpretation is not readily available or could not be sufficiently timely. The proposed FCM is tested on Cassini radar data to show the effectiveness of the system in reaching conclusions put forward by human experts and published in the literature. Four tests are performed using the Ta SAR image (October 2004 fly-by). Two regions (i.e. Ganesa Macula and the lobate high backscattering region East of Ganesa) are interpreted by the designed FCM as exhibiting cryovolcanism in agreement with the initial interpretation of the regions by . Importantly, the proposed FCM is shown to be flexible and adaptive as new data and knowledge are acquired during the course of exploration. Subsequently, the FCM has been modified to include topographic information derived from SAR stereo data. With this additional information, the map concludes that Ganesa Macula is not a cryovolcanic region. In conclusion, the FCM methodology is shown to be a critical and powerful component of future autonomous robotic spacecraft (e.g., orbiter(s), balloon(s), surface/lake lander(s), rover(s)) that will be deployed for the exploration of Titan. [Copyright &y& Elsevier]

Abstract: A new model is presented on how chemically driven cryovolcanism might contribute to episodic outgassing at the icy moon Enceladus and potentially elsewhere including Europa and Kuiper Belt Objects. Exposed water ices can become oxidized from radiolytic chemical alteration of near-surface water ice by space environment irradiation. In contact with primordially abundant reductants such as NH3, CH4, and other hydrocarbons, the product oxidants can react exothermically to produce volatile gases driving cryovolcanism via gas-piston forces on any subsurface liquid reservoirs. Radiolytic oxidants such as H2O2 and O2 can continuously accumulate deep in icy regoliths and be conveyed by rheological flows to subsurface chemical reaction zones over million-year time scales indicated by cratering ages for active regions of Enceladus and Europa. Surface blanketing with cryovolcanic plume ejecta would further accelerate regolith burial of radiolytic oxidants. Episodic heating from transient gravitational tides, radioisotope decay, impacts, or other geologic events might occasionally accelerate chemical reaction rates and ignite the exothermic release of cumulative radiolytic oxidant energy. The time history for the suggested “Old Faithful” model of radiolytic gas-driven cryovolcanism at Enceladus and elsewhere therefore consists of long periods of chemical energy accumulation punctuated by much briefer episodes of cryovolcanic activity. The most probable sequence for detection of activity in the current epoch is a long evolutionary phase of slow but continuous oxidant accumulation over billions of years followed by continuous but variable high activity over the past 107–108 years. Detectable cryovolcanic activity could then later decline due to near-total oxidation of the rheologically accessible ice crust and depletion the accessible reductant abundances, as may have already occurred for Europa in the more intense radiation environment of Jupiter''s magnetosphere. Astrobiological potential of Enceladus could correspondingly be higher than at Europa due to a less extreme state of oxidation and greater residual abundance of organics. [Copyright &y& Elsevier]

In this work, we made a geomorphological analysis of the surface of Pluto using LORRI data from the New Horizons flyby, to make an observational evaluation of a previously established convective model as the endogenic source of Sputnik Planitia geologic activity. With corrected high-resolution images and the Digital elevation model (DEM), we defined and described six geological terrains. Sputnik Planitia was examined in more detail using the higher quality images, and within it were defined eighteen geomorphological units. The link between the interpretation of the geoforms and the known composition of some of these units, provided key information to understand the geometries and distributions of the ices that make up the plain, including the classical cell patterns and the small H 2 O blocks trapped in them. The analysis of Sputnik Planitia lead to interesting interpretations about the zone: The convective process is the best explanation for its current configuration; the glaciers of Sputnik Planitia have lost altitude over the last geological times, exhuming rocks of the Cthulhu Regio terrain; and it could be possible that the rough-looking Sputnik Highlands terrain is a degraded form of a previous zone similar to Sputnik Planitia. • The morphology of Sputnik Planitia is consistent with a convective model, as it explains features like the size of the pits. • The small water ice blocks trapped in Sputnik Planitia may be detached fragments of the basement made of Cthulhu Regio ices. • Sputnik Highlands share several properties with Sputnik Planitia, so it could represent a degraded stage of Sputnik Planitia. • The level of the glaciers of the plains has decreased over time, this is evidenced by the exhumation of Cthulhu Regio rocks. [ABSTRACT FROM AUTHOR]

At least two active plumes were observed on Neptune's moon Triton during the Voyager 2 flyby in 1989. Models for Triton's plumes have previously been grouped into five hypotheses, two of which are primarily atmospheric phenomena and are generally considered unlikely, and three of which include eruptive processes and are plausible. These hypotheses are compared, including new arguments, such as comparisons based on current understanding of Mars, Enceladus, and Pluto. An eruption model based on a solar-powered, solid-state greenhouse effect was previously considered the leading hypothesis for Triton's plumes, in part due to the proximity of the plumes to the subsolar latitude during the Voyager 2 flyby and the distribution of Triton's fans that are putatively deposits from former plumes. The other two eruption hypotheses are powered by internal heat, not solar insolation. Based on new analyses of the ostensible relation between the latitude of the subsolar point on Triton and the geographic locations of the plumes and fans, we argue that neither the locations of the plumes nor fans are strong evidence in favor of the solar-powered hypothesis. We conclude that all three eruption hypotheses should be considered further. Five tests are presented that could be implemented with remote sensing observations from future spacecraft to confidently distinguish among the eruption hypotheses for Triton's plumes. The five tests are based on the: (1) composition and thickness of Triton's southern hemisphere terrains, (2) composition of fan deposits, (3) distribution of active plumes, (4) distribution of fans, and (5) surface temperature at the locations of plumes and/or fans. The tests are independent, but complementary, and implementable with a single flyby mission such as the Trident mission concept. We note that, in the case of the solar-driven hypothesis, the 2030s and 2040s may be the last chance for approximately a century to observe actively erupting plumes on Triton. • Locations of Triton's plumes/fans don't strongly favor solid-state greenhouse model. • Solid-state greenhouse model for Triton's plumes is no longer leading hypothesis. • 5 remote sensing tests for distinguishing among published models are presented. • Models for plumes can likely be distinguished with another single-flyby mission. • 2030s & 2040s are opportune time: possibly last chance for ≈ century to see plumes. [ABSTRACT FROM AUTHOR]

In the last 25 years, the explorations of the Voyager and Galileo missions have resulted in an entirely new view of the icy worlds orbiting the giant outer planets. These objects show a huge diversity in their characteristics, resulting from their formation histories, internal processes and interactions with their space environments. This paper will review the current state of knowledge about the icy satellites and discuss the exciting prospects for the upcoming Cassini/Huygens mission as it begins a new era of exploration of the Saturn satellite system. [ABSTRACT FROM AUTHOR]

Images of Europa's surface taken by the Galileo Solid State Imager (SSI) show smooth features measuring a few kilometers, potentially resulting from eruptions of low-viscosity material such as liquid cryomagma. We estimated the volume of four of these smooth features by producing Digital Elevation Models (DEMs) of four Galileo/SSI images. We used the shape-from-shading technique with special care to estimate the uncertainties on the produced DEMs and estimated feature volumes to be between (5.7 ± 0.9) × 1 0 7 m 3 and (2.7 ± 0.4) × 1 0 8 m 3. We discussed the implications for putative sub-surface liquid reservoir dimensions in the case of eruptions induced from freezing reservoirs. Our previous cryovolcanic eruption model was improved by considering a cycle of cryomagma freezing and effusion and by estimating the vaporized cryolava fraction once cryolava spreads onto Europa's surface. Our results show that the cryomagma reservoirs would have to be relatively large to generate these smooth features (1 to 100 km 3 if the flow features result from a single eruption, and 0.4 to 60 km 3 for the full lifetime of a reservoir generating cyclic eruptions). The two future missions JUICE (ESA) and Europa Clipper (NASA) should reach Europa during the late 2020s. They shall give more information on those putative cryovolcanic regions which appear as interesting targets that could provide a better understanding of the material exchanges between the surface, sub-surface and ocean of Europa. • 4 DEMs of smooth features are produced using the AMES StereoPipeline. • Measurements of these feature volumes give 5. 7 ± 0. 9 10 7 to 2. 7 ± 0. 4 10 8 m3. • Cryolava eruption coupled with pre-existing terrain subsidence might generate these features. • 0.4 to 100 km3 cryomagma reservoirs are necessary to put in place the smooth features. [ABSTRACT FROM AUTHOR]

NASA's New Horizons flyby of the Pluto-Charon system in 2015 exposed a multitude of geologically active terrains on the surface of Pluto. Possible evidence of cryovolcanism has been reported in association with crustal scale extensional tectonic structures at Virgil Fossae and at two large circular mountains, informally referred to as Wright Mons and Piccard Mons. Our detailed geologic mapping of Wright Mons and the surrounding region reveals additional circular mounds overlying the main edifice and narrow ravines emanating from the summit, possibly produced by liquid flow. We model the overpressure generated during the freezing of a mixed ammonia-water liquid inside an enclosed chamber in Pluto's ice crust and in a global subsurface ocean. We consider scenarios spanning the possible range of Pluto's ammonia abundance and conclude that stresses generated due to freezing-induced pressurization can exceed the tensile strength of ice and create fractures for the liquid to escape. Our results suggest that freezing in a crustal chamber and the subsurface ocean are both viable mechanisms for cryovolcanism on Pluto and may explain the morphological variety of cryovolcanic formations observed by New Horizons. • Geological mapping of Wright Mons reveals evidence for recent cryovolcanism. • Enclosed freezing pressurization is a viable mechanism for cryovolcanism on Pluto. • Ammonia reduces the pressurization of subsurface liquid reservoirs. • Subsurface ocean and crustal chamber freezing explain Virgil Fossae and Wright Mons. [ABSTRACT FROM AUTHOR]