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Exploration of the Solar System has revealed that the surfaces of many icy bodies have been resurfaced by cryovolcanism: a process during which liquid and vapour are released from the surface into extremely cold and low pressure conditions. Water is one of the most commonly released liquids, and its stability and behavior under such conditions are thus of special interest. When exposed to low pressure, water boils, but it may also start freezing at the phase boundary due to evaporative cooling, as indicated by previous studies. There is only limited insight into how exactly the multiple phase transitions interact and what parameters control the dynamics of the system. To overcome this knowledge gap, we performed experiments in which we simulated the release of water at low pressure and low temperatures, such as could be encountered at local conditions at the surface of an icy moon.We used the Mars Simulation Chamber at The Open University (UK), in which a 60 x 40 cm container containing 5 and 17 litres of water was exposed to a reduced atmospheric pressure of ~4.5 mbar. Deionised water was mixed with a small amount of NaCl to achieve a salinity of 0.5% and was precooled to ~3.8°C to be close to the freezing point. Experiments were documented by video cameras situated around the container and the temperature inside the chamber and of the water was recorded by thermocouples.At the beginning of each experiment, the atmospheric pressure was gradually reduced from ambient, which triggered boiling within the entire volume of water and evaporative cooling in its uppermost layer. This caused a gradual drop in the water temperature down to the freezing point, forming pieces of floating ice. The area where ice was present slowly grew and within timescales of a few minutes the entire surface of the container was covered with ice. However, the ice layer was broken into blocks with uneven surfaces. This was due to active boiling below the freezing layer of the water, with the intense formation of vapour bubbles which were capable of breaking and/or uplifting the ice. Once the fracture(s) developed, trapped vapour was released and deflation followed. Experimental results show that the process was more intense when larger amounts of water were used within the container, which significantly disrupted the freezing of water in those experiments and affected the final topography of the ice layer.Our experiments show that water phase transition during effusive cryovolcanic eruptions are likely to be a highly complex process due to boiling causing major ice fracturing and the formation of topographical anomalies on the frozen surface.

Volcanism based on melting rocks (silicate volcanism) is long known on Earth and has also been found on Jupiter’s moon Io. Remnants of this type of volcanism have been identified also on other bodies in the solar system. Energy sources powered by accretion and the decay of radioactive isotopes seem to be dominant mainly inside larger bodies, which have enough volume to accumulate and retain this energy in significant amounts. On the other hand, the impact of tidal forces allows even tiny bodies to melt up and pass into the stage of cryovolcanism. The dependence of tidal heating on the size of the object is minor, but the masses of and the distances to accompanying bodies as well as the inner compositions of the heated body are central factors. Even though Io as an example of a body supporting silicate volcanism is striking, the physics of tidal forces might suggest a relatively high probability for cryovolcanism. This chapter aims at considering the parameters known and objects found so far in our solar system to give insights into where in our system and other planetary systems cryovolcanism might be expected.

A link between exposures of water (H${}_{2}$O) ice with traces of an ammoniated compound (e.g., a salt) and the probable effusion of a water-rich cryolava onto the surface of Pluto has been established in previous investigations (Dalle Ore et al. 2019). Here we present the results from the application of a machine learning technique and a radiative transfer model to a water-ice-rich exposure in Kiladze area and surroundings on Pluto. We demonstrate the presence of an ammoniated material suggestive of an undetermined but relatively recent emplacement event. Kiladze lies in a region of Pluto's surface that is structurally distinct from that of the areas where similar evidence points to cryovolcanic activity at some undetermined time in the planet's history. Although the Kiladze depression superficially resembles an impact crater, a close inspection of higher-resolution images indicates that the feature lacks the typical morphology of a crater. Here we suggest that a cryolava water carrying an ammoniated component may have come onto the surface at the Kiladze area via one or more volcanic collapses, as in a resurgent volcanic caldera complex. Large regions east of Kiladze also exhibit the presence of H${}_{2}$O ice and have graben-like structures suggestive of cryovolcanic activity, but with existing data are not amenable to the detailed search that might reveal an ammoniated component., Comment: 28 pages, 11 figures, submitted to Icarus

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.

Geological activity on icy planets and planetoids includes cryovolcanism. Until recently, most research on terrestrial permafrost has been engineering-oriented, and many related phenomena have received too little attention. Although fast processes in the Earth’s cryosphere were known before, they have never been attributed to cryovolcanism. The discovery of a couple of tens of meters wide crater in the Yamal Peninsula aroused numerous hypotheses of its origin, including a meteorite impact or migration of deep gas as a result of global warming. However, the origin of the Yamal crater can be explained in terms of cryospheric processes. Thus, the Yamal crater appears to result from collapse of a large pingo, which formed within a thaw lake when it shoaled and dried out allowing a large talik (that is layer or body of unfrozen ground in a permafrost area) below it to freeze back. The pingo collapsed under cryogenic hydrostatic pressure built up in the closed system of the freezing talik. This happened before the freezing completed, when a core of wet ground remained unfrozen and stored a huge amount of carbon dioxide dissolved in pore water. This eventually reached gas-phase saturation, and the resulting overpressure came to exceed the lithospheric confining stress and the strength of the overlying ice. As the pingo exploded, the demarcation of the crater followed the cylindrical shape of the remnant talik core.

INTRODUCTION Classic volcanism prevalent on terrestrial planets and volatile-poor protoplanets, such as asteroid Vesta, is based on silicate chemistry and is often expressed by volcanic edifices (unless erased by impact bombardment). In ice-rich bodies with sufficiently warm interiors, cryovolcanism involving liquid brines can occur. Smooth plains on some icy satellites of the outer solar system have been suggested as possibly cryovolcanic in origin. However, evidence for cryovolcanic edifices has proven elusive. Ceres is a volatile-rich dwarf planet with an average equatorial surface temperature of ~160 K. Whether this small (~940 km diameter) body without tidal dissipation could sustain cryovolcanism has been an open question because the surface landforms and relation to internal activity were unknown. ### RATIONALE The Framing Camera onboard the Dawn spacecraft has observed >99% of Ceres’ surface at a resolution of 35 m/pixel at visible wavelengths. This wide coverage and resolution were exploited for geologic mapping and age determination. Observations with a resolution of 135 m/pixel were obtained under several different viewing geometries. The stereo-photogrammetric method applied to this data set allowed the calculation of a digital terrain model, from which morphometry was investigated. The observations revealed a 4-km-high topographic relief, named Ahuna Mons, that is consistent with a cryovolcanic dome emplacement. ### RESULTS The ~17-km-wide and 4-km-high Ahuna Mons has a distinct size, shape, and morphology. Its summit topography is concave downward, and its flanks are at the angle of repose. The morphology is characterized by (i) troughs, ridges, and hummocky areas at the summit, indicating multiple phases of activity, such as extensional fracturing, and (ii) downslope lineations on the flanks, indicating rockfalls and accumulation of slope debris. These morphometric and morphologic observations are explained by the formation of a cryovolcanic dome, which is analogous to a high-viscosity silicic dome on terrestrial planets. Models indicate that extrusions of a highly viscous melt-bearing material can lead to the buildup of a brittle carapace at the summit, enclosing a ductile core. Partial fracturing and disintegration of the carapace generates slope debris, and relaxation of the dome’s ductile core due to gravity shapes the topographic profile of the summit. Modeling of this final phase of dome relaxation and reproduction of the topographic profile requires an extruded material of high viscosity, which is consistent with the mountain’s morphology. We constrained the age of the most recent activity on Ahuna Mons to be within the past 210 ± 30 million years. ### CONCLUSION Cryovolcanic activity during the geologically recent past of Ceres constrains its thermal and chemical history. We propose that hydrated salts with low eutectic temperatures and low thermal conductivities enabled the presence of cryomagmatic liquids within Ceres. These salts are the product of global aqueous alteration, a key process for Ceres’ evolution as recorded by the aqueously altered, secondary minerals observed on the surface. ![Figure][1] Perspective view of Ahuna Mons on Ceres from Dawn Framing Camera data (no vertical exaggeration). The mountain is 4 km high and 17 km wide in this south-looking view. Fracturing is observed on the mountain’s top, whereas streaks from rockfalls dominate the flanks. Volcanic edifices are abundant on rocky bodies of the inner solar system. In the cold outer solar system, volcanism can occur on solid bodies with a water-ice shell, but derived cryovolcanic constructs have proved elusive. We report the discovery, using Dawn Framing Camera images, of a landform on dwarf planet Ceres that we argue represents a viscous cryovolcanic dome. Parent material of the cryomagma is a mixture of secondary minerals, including salts and water ice. Absolute model ages from impact craters reveal that extrusion of the dome has occurred recently. Ceres’ evolution must have been able to sustain recent interior activity and associated surface expressions. We propose salts with low eutectic temperatures and thermal conductivities as key drivers for Ceres’ long-term internal evolution. [1]: pending:yes

Der kleine Saturnmond Enceladus ist eines der faszinierendsten und spannendsten Objekte in unserem Sonnensystem. Die NASA Raumsonde Cassini entdeckte beim Vorbeiflug eine anomale Zone erhöhter Temperatur in der Südpolarregion, und geysirartige Fontänen aus Wasserdampf und Wassereisteilchen (engl. "Plume"), die von Bruchlinien in der Südpolarregion, den sogenannten "Tiger-Streifen", ausgehen, und die bis in eine Höhe von 500 km zu beobachten sind. Die Entdeckung dieser kryovulkanischen Geysire macht Enceladus einzigartig: er ist der einzige Eismond unseres Sonnensystems auf dem aktuell eine bekannte permanente geologische Aktivität stattfindet. Die Fontänen aus Wasserdampf und Eispartikeln erreichen Geschwindigkeiten von bis zu 1000 m/s, und übersteigen die Fluchtgeschwindigkeit von Enceladus (v_esc = 239 m/s). Der Nachweis von Natriumsalzen und Siliziumhaltigen Molekülen im Plume legt nahe, dass die Eispartikeln aus einem flüssigen Wasserreservoir stammen, welches in Kontakt mit dem heißen, felsigen Kern von Enceladus steht. Es gibt starke Anzeichen, dass dieses flüssige Reservoir ein unterirdischer Ozean ist, der 8-30 km dick ist und sich in der Südpolarregion unter einem Eismantel von 2-40 km Dicke befindet. Neuere Arbeiten deuten darauf hin, dass es sich dabei nicht nur um einen lokalen polaren See handelt, sondern vielmehr sogar um einen globalen unterirdischen Ozean. Es ist durchaus möglich, dass am Grund dieses unterirdischen Ozeans hydrothermale Quellen existieren, analog den "Schwarzen" bzw. "Weißen Rauchern" am Grund der Tiefsee auf der Erde. Die hydrothermalen Quellen auf der Erde und die in deren Umgebung stattfindenden chemischen Reaktionen, die die Existenz von chemolithotroph aktiver Bakterien und Archaeen ermöglicht, stellen eine potentielle Erklärung für den Ursprung des irdischen Lebens dar. Aus diesem Grund ist der kleine Mond Enceladus für die Astrobiologie von großem Interesse, da er einen äußerst interessanten Kandidaten für mögliches außerirdisches Leben in unserem eigenen Sonnensystem repräsentiert. Der Wasserdampf in den Fontänen entsteht durch Verdampfung in der Tiefe in Kammern an der Oberfläche des unterirdischen Wasserreservoirs, und bewegt sich im Eismantel durch ein System von Brüchen, Rissen, Spalten und Röhren, quasi einem Rohrleitungs-System (engl. "Plumbing System") im Eis, aufwärts. In dieser Arbeit wurde anhand eines Models einer zylindrischen Eisröhre, welche von einem Eismantel umgeben ist, der Wärmeverlust entlang eines Eisspalt-Kanals analysiert. Der Wärmeverlust beim Aufsteigen durch die Eiskanäle ist ein wesentlicher Aspekt des sogenannten "Plumbing-System Problems", denn falls dieser zu groß ist, drohen die Kanäle zuzufrieren. Doch die Risse und Spalten, durch die Wasserdampf und Eispartikel vom unterirdischen Ozean bis zur Oberfläche strömen, müssen lange genug offen bleiben, d.h. sie dürfen nicht zu schnell zufrieren. Die Differentialgleichungen, die den Wärmetransfer bzw. den Wärmeverlust des Wassers beim Transport durch eine Eisröhre beschreiben, wurden in dieser Arbeit aufgestellt, und es wurden Lösungen für diese Differentialgleichungen für verschiedene Umgebungsparameter und Randbedingungen ausgearbeitet. Der Verlauf der Temperatur T_out(L) des Wassers/Wasserdampfes entlang der Eisröhre wurde als Funktion der Kanallänge L für verschiedene Werte der Anfangstemperatur T_in(L) und diverse Werte des Röhrenradius r berechnet, um zu bestimmen, welche Kombinationen von Röhrenlängen L und Röhrendicken w = 2r stabile Parameter darstellen, bei denen der Eiskanal nicht zufriert. Die in dieser Arbeit durchgeführten Berechnungen veranschaulichen, dass bei kleineren Eisröhrenradien r < 1 m die Temperatur steil abfällt, wohingegen bei größeren Radien r > 1 m der Abfall moderater ausgeprägt ist. Weiters zeigen die Modellrechnungen dieser Arbeit, dass bei Röhrenradii r >~ 1m Wasser entlang eines Kanals von mehreren km Länge flüssig bleibt. Diese Resultate sind qualitativ in Übereinstimmung mit der aktuellen Literatur, wo Röhrendurchmesser w = 2r ~ 1-2 m vorgeschlagen werden, um ein Nicht-Zufrieren der Eiskanäle zu garantieren. Der Hauptteil dieser Arbeit ist der Untersuchung des Durchflusses des Wasserdampfes durch das aus Brüchen, Rissen, Spalten und Röhren bestehende "Plumbing-System" gewidmet. Die physikalischen Größen, die diesen Durchfluß beschreiben, nämlich Druck- und Geschwindigkeits-Verteilungen, wurden mittels numerischer Simulationen der Navier-Stokes Gleichungen bestimmt, welche die Dynamik viskoser Flüssigkeiten beschreiben. Die durch Risse und Brüche entstandenen Eiskanäle wurden als 2-dimensionale bzw. 3-dimensionale geometrische Objekte modelliert, die verwendeten Geometrien erstrecken sich von einfachen Formen wie Rechtecken und Zylindern bis zu komplexeren Ausprägungen, welche Verengungen, Einschnürungen, Knicke, Verzweigungen, und 'Zick-Zack' Linien beinhalten, wobei letztere Formen tatsächliche physikalische Strukturen, wie sie möglicherweise auf Enceladus vorkommen, realistischer abbilden. Für die numerischen Simulationen wurde in dieser Arbeit die Open-Source Berechnungs-Plattform FEniCS verwendet, welche eine Vielzahl von Modulen zum Lösen partieller Differentialgleichungen bereitstellt, die auf der Methode der finiten Elemente basieren. Mit Hilfe dieses Werkzeuges wurden die diskretisierten Navier-Stokes Gleichungen für verschiedene 2-dim und 3-dim Eiskanal-Geometrien auf numerischem Wege gelöst. Es wurden die Geschwindigkeits- und Druckverteilungen von Flußröhren bestimmt, deren Längen von L = 20 m (nur zur Kalibrierung), 100 m and 1000 m und deren Durchmesser von w = 0.6 - 6 m variieren. Während einfache Geometrien (Rechtecke, Zylinder) naturgemäß triviale Resultate liefern (die Geschwindigkeit im inneren Teil der Röhre ist konstant und unabhängig von der z-Koordinate), zeigen die Ergebnisse dieser Arbeit für komplexere Geometrien, die Knicke und Einschnürungen beinhalten, sehr klar, dass die Geschwindigkeit u in der Umgebung dieser Knicke und Verengungen deutlich erhöht ist, was eine Folge des Venturi-Effektes ist, der den Geschwindigkeitsanstieg eines durch eine verengte Röhre strömenden Fluids beschreibt. Dieser Effekt ist für eine Form mit einer Einschnürung stärker ausgeprägt als für eine Geometrie mit nur einem einfachen Knick. Die höchsten Geschwindigkeits-Werte sind innerhalb der Einengungen eines Eiskanals lokalisiert, die Maximalgeschwindigkeit v_max (definiert als Maximalwert des Geschwindigkeitsfeldes einer bestimmten einzelnen Computersimulation) liegt in der Größenordnung von mehreren 100 m/s. Die numerisch anspruchsvollste und komplexeste Eiskanal-Struktur, die in dieser Arbeit im 2-dimensionalen Fall untersucht wurde, ist eine Geometrie, welche eine Verzweigung und mehrere Einengungen und Knicke beinhaltet (in der Arbeit informell als 'Zick-Zack' Schlot bezeichnet). Eine solche Struktur enthält Elemente, die in einem echten Eismantel durchaus vorkommen können, und die somit einem physikalischen Eisbruchkanal näherungsweise entsprechen könnte. Die höchsten Geschwindigkeiten in solchen 'Zick-Zack' Schloten treten an den Ausstoß-Punkten auf, wo der Wasserdampf eruptiv an der Oberfläche austritt. Die Simulationen dieser Arbeit liefern an diesen Extremstellen Werte von 100 m/s bis zu Spitzenwerte von mehr als 1000 m/s, welche die Entweichgeschwindigkeit von Enceladus, v_esc = 239 m/s, klar überschreiten. Diese Zahlenwerte für die Maximalgeschwindigkeit der Plume-Jets sind mit Resultaten aus der Literatur konsistent, die mit diversen, (auch im Vergleich zu dieser Arbeit) qualitativ unterschiedlichen Methoden ermittelt wurden. Eine DVD ist der Arbeit beigelegt, die Videos der 3D-Simulationen enthält.

International audience; We report infrared spectrophotometric variability on the surface of Saturn's moon Titan detected in images returned by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini Saturn Orbiter. The changes were observed at 7°S, 138°W and occurred between October 27, 2005 and January 15, 2006. After that date the surface was unchanged until the most recent observation, March 18, 2006. We previously reported spectrophotometric variability at another location (26°S, 78°W). Cassini Synthetic Aperture RADAR (SAR) images find that the surface morphology at both locations is consistent with surface flows possibly resulting from cryovolcanic activity (Wall et al., companion paper, this issue). The VIMS-reported time variability and SAR morphology results suggest that Titan currently exhibits intermittent surface changes consistent with present ongoing surface processes. We suggest that these processes involve material from Titan's interior being extruded or effused and deposited on the surface, as might be expected from cryovolcanism.

During the Dawn mission, observations at Ceres reveal numerous interesting post-impact modifications in and around craters. These modifications contain the deposition of extended plains material with pits, multiple lobate flows, and widely dispersed deposits that form a diffuse veneer on the preexisting surface.

Before the arrival of the New Horizons probe at the Pluto-Charon system, we developed a series of models that predicted that Kuiper Belt Objects, even as small and as cold as Charon, have experienced internal ice-rock differentiation and possibly cryovolcanism. Confronting these predictions is a wide array of spectroscopy, imagery, and other data from New Horizons. In this article we compare the predictions against the new observations, and find that they largely support the expected history of the Pluto system and the evolution of Charon. Following the collision of two partially differentiated impactors with radii ≈1000 km, a disk of material formed around Pluto, from which Charon and Pluto’s other moons formed. Because the impactors did not completely differentiate, the disk contained rocky material from their crusts, explaining the moons’ different densities and compositions. Long-lived radionuclides in Charon, assisted by ammonia antifreeze in the ice, melted ice and created a subsurface ocean that eventually refroze ≈ 1.7 − 2.5 Gyr ago. The freezing of this ocean would have created extensional stresses that possibly created Serenity Chasma, and could have led to widespread resurfacing, explaining the apparently younger age of Vulcan Planum. Buildup of radiogenic heat then created a second, smaller ocean that refroze 0.5–1.7 Gyr ago. As it froze, cryovolcanism would have been enabled, possibly creating Kubrick Mons. Charon’s “moated mountains” such as Kubrick Mons have a natural explanation as cryovolcanoes depressing a thin lithosphere over a cryomagma chamber. We offer further predictions about other aspects of Charon’s surface. Our previous predictions that Charon is a world shaped by geological activity have been largely borne out by New Horizons observations.

Craters on Ceres, such as Haulani, Kupalo, Ikapati, and Occator show postimpact modification by the deposition of extended plains material with pits, multiple lobate flows, and widely dispersed deposits that form a diffuse veneer on the preexisting surface. Bright material units in these features have a negative spectral slope in the visible range, making it appear bluish with respect to the grey-toned overall surface of Ceres. We calculate the drop height-to-runout length ratio of several flow features and obtain a coefficient of friction of < 0.1: The results imply higher flow efficiency for flow features on Ceres than for similar features on other planetary bodies with similar gravity, suggesting low-viscosity material. The special association of flow features with impact craters could either point to an impact melt origin or to an exogenic triggering of cryovolcanic processes.

We identified a large-scale lobate feature that is proximal to a double ridge on Ariel. We analyzed the morphology of this feature to investigate whether it was formed by cryovolcanism or mass wasting. Our results show that the head of the lobate feature is adjacent to a topographically elevated dome on the double ridge, which may have formed via extrusion and emplacement of cryolava in this location. We find that the coefficient of friction of the material that formed the lobate feature is more consistent with a cryovolcanic flow than either a dry or liquid-aided mass wasting flow. Similarly, the estimated yield strength for the neck and terminus of this feature is similar to geologic features that contained some liquid during formation. Alternatively, upwelling of material, in an ascending diapir, could also explain the morphology of this lobate feature, in particular its topographically elevated terminus, which is higher standing than its neck. Higher resolution images are needed to assess the surface texture of the lobate feature to further investigate whether it formed via flowing cryolava or diapirism. Furthermore, without higher spatial resolution images, the possibility of a mass wasting origin for the lobate feature cannot be ruled out and warrants further investigation. The possible presence of cryovolcanic features on Ariel supports the interpretation that this moon is a candidate ocean world that has, or had, a subsurface liquid water layer beneath its icy exterior.

Modeling the evolution and internal structure of large- and mid-size Trans-Neptunian objects (TNOs) is a challenging task, owing to the strong dependence on the thermophysical and structural parameters involved, which are poorly constrained. Thus the outcome of evolutionary tracks for a given object may range from fully pristine to completely differentiated and need to be further constrained by detailed observations of TNOs’ physical and chemical properties. The key question remains, whether knowing surface and bulk properties is sufficient for inferring internal properties and more ambitiously, the evolutionary course of a TNO from formation to present day. Recent models of Charon involve a complex evolution, including the formation of subsurface oceans and past cryovolcanism. This perspective should trigger the development of new comprehensive models, which—combined with new observations—should lead to significant progress in our understanding of how the whole population formed and evolved.

International audience; The Virgil Fossae region on Pluto exhibits three spatially coincident properties that are suggestive of recent cryovolcanic activity over an area approximately 300 by 200 km. Situated in the fossae troughs or channels and in the surrounding terrain are exposures of H2O ice in which there is entrained opaque red-colored matter of unknown composition. The H2O ice is also seen to carry spectral signatures at 1.65 and 2.2 μm of NH3 in some form, possibly as a hydrate, an ammoniated salt, or some other compound. Model calculations of NH3 destruction in H2O ice by galactic cosmic rays suggest that the maximum lifetime of NH3 in the uppermost meter of the exposed surface is ~109 years, while considerations of Lyman-α ultraviolet and solar wind charged particles suggest shorter timescales by a factor of 10 or 10000. Thus, 109 y is taken as an upper limit to the age of the emplacement event, and it could be substantially younger. The red colorant in the ammoniated H2O in Virgil Fossae and surroundings may be a macromolecular organic material (tholin) thought to give color to much of Pluto's surface, but probably different in composition and age. Owing to the limited spectral range of the New Horizons imaging spectrometer and the signal precision of the data, apart from the H2O and NH3 signatures there are no direct spectroscopic clues to the chemistry of the strongly colored deposit on Pluto. We suggest that the colored material was a component of the fluid reservoir from which the material now on the surface in this region was erupted. Although other compositions are possible, if it is indeed a complex organic material it may incorporate organics inherited from the solar nebula, further processed in a warm aqueous environment inside Pluto. A planet-scale stress pattern in Pluto's lithosphere induced by true polar wander, freezing of a putative interior ocean, and surface loading has caused fracturing in a broad arc west of Sputnik Planitia, consistent with the structure of Virgil Fossae and similar extensional features. This faulting may have facilitated the ascent of fluid in subsurface reservoirs to reach the surface as flows and as fountains of cryoclastic materials, consistent with the appearance of colored, ammoniated H2O ice deposits in and around Virgil Fossae. Models of a cryoflow emerging from sources in Virgil Fossae indicate that the lateral extent of the flow can be several km (Umurhan et al., 2019). The deposit over the full length (> 200 km) of the main trough in the Virgil Fossae complex and extending through the north rim of Elliot crater and varying in elevation over a range of ~2.5 km, suggests that it debouched from multiple sources, probably along the length of the strike direction of the normal faults defining the graben. The source or sources of the ammoniated H2O are one or more subsurface reservoirs that may or may not connect to the global ocean postulated for Pluto's interior. Alternatives to cryovolcanism in producing the observed characteristics of the region around Virgil Fossae are explored in the discussion section of the paper.

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.

This discovery is the latest in a growing stack of evidence pointing to the presence of an ammonia-rich water ocean beneath Pluto’s icy crust.

Evidence for long-lived sources of cryovolcanism on the nucleus of the Comet 29P/Schwassmann–Wachmann has been found from a study of its times of outburst ( t 0 ) and the morphological development of inner coma structures. Analysis of data from the Minor Planet Center observations archive spanning 2002–2014 and other observations have yielded 64 outburst times of mainly well-observed events with a median timing uncertainty of 0.40 d. Outbursts comprise largely (i) isolated explosive events; or (ii) multiple outbursts occurring typically within 5–15 d of each other. On rare occasions, a form of continuous or gradually increasing activity is manifest, appearing to be the result of a series of mini-outbursts. Quasi-periodicity in t 0 is manifested as an excess of outbursts every 52–60 d, along with a paucity of events every ∼30 d and ∼90 d. Seasonal changes in activity are evident from the temporal analysis of the outburst data. An unambiguous periodicity of 57.6 ± 0.4 d has been found in the times of 26 outbursts during 2010–2014, with all active sources at that time localised within a longitude span of ∼135–150°. Cluster analysis of t 0 data for 2002–2010 and 2010–2014, and HST imaging from 1996 confirm and refine the apparent periodicity, indicating that outbursts appear to be grouped in longitude centred on at least 6 circumferential locations. Sources of activity generally persist for at least 10–20 yr, and some appear discrete in nature, able to re-outburst after a single day–night cycle. Given that outbursts are triggered by solar heating, the analysis yields a value for the mean solar day of 57.71 ± 0.06 d, equivalent to a sidereal rotation period of 57.09 ± 0.06 d, assuming the more probable prograde direction of spin. A novel outburst mechanism is outlined in which some cometary ices, principally solid CH 4 , confined under pressure (>12 kPa) beneath a stabilisation crust, begin to melt and absorb supervolatile gases, mainly CO and N 2 . These gases liberate considerable heat (5–7 kJ mol −1 ) via their enthalpy of solution inducing further melting deep within the nucleus where direct insolation heating is absent. This gas-solute process is most active near the solid–liquid interface, where the solvent temperature is lowest and gas solubility is highest. An outburst occurs when insolation heating of the crust above a gas-laden subsurface reservoir softens paraffinic hydrocarbons and causes a crustal plate to dislodge under the accumulated gas pressure, the sudden release of which provokes the explosive ex-solution of dissolved gases, principally CO, propelling entrained dust and debris into space. Fissures reseal as the plate sinks back under the gravitational influence of the large nucleus and the adhesive, waxy hydrocarbon fraction solidifies, permitting a new outburst cycle to begin. A detailed account of the gas ex-solution mechanism is the subject of a partner paper (Miles, R. [2015]. Icarus).

Topographic moats are found at the edges of Charon's Vulcan Planitia, both where the plains meet the higher elevation terrain in Oz Terra, and surrounding isolated mountains within the smooth plains. The curved edges of the moats have been hypothesized to represent either the front of a cryolava flow from the emplacement of the plains material, or the result of elastic flexure of the plains surface due to the load of adjacent mountains or sinking crustal blocks. We tested both of these hypotheses by taking topographic profiles across the moats and comparing the observations to theoretical predictions from two lava flow models and two plate bending models. The algorithm we used returns a correlation coefficient as well as the parameter values for the theoretical model necessary to match the topography. Our results show that the plate bending models provided the best fits for the topography, though the lava flow models provided good matches for a few profiles. Our results for elastic thickness of ~1 km for a continuous plate or ~3 km for a broken plate are similar to results previously derived at Serenity Chasma. The values obtained for Bingham yield strength of ~10 kPa are also consistent with previous results from hypothesized cryovolcanic material on Ariel and Charon.

The paper presents an analysis of the real change in brightness of comet 174P/Echeclus (60558 Echeclus), using the cryovolcanism mechanism. The relationship between the mass ejected during the outburst and the active surface during quiet sublimation is also presented. It has been shown that the mass ejected is one of the key parameters determining the change in the brightness of a comet. This assumption was also confirmed by many years of observations of comets during their outbursts.

In the Earth’s regions with cold climate, cryovolcanism is widespread. This phenomena is manifested as eruptions of material due to freezing of closed-type or open-type water-bearing systems which is accompanied by generation of effusive topographic forms, such as «pingo». The Patom cone is a typical structure created by cryovolcanism in fractured bedrocks of the Proterozoic age. The cone was shaped a result of the long-term, possibly multistage freezing of the hydrogeological structure during continuous and complicated phase of cryo- and speleo-genesis. The ice-saturated breccia containing limestone, sandstone and shale, which composed the cone, was subject to slow spreading due to its plastic properties; the top of the mound developed into a subsidence cone bordered by ring-shaped ramparts and a knoll in the middle, while the longitudinal profile took on an asymmetric form. The absence of soil and vegetation cover on the surface of the cone, and a relatively weak degree of weathering of the rudaceous deposits bear no evidence that the geological object is young. The question as to the age of the cone is still open.

Explosive extrusion of cold material from the interior of icy bodies, or cryovolcanism, has been observed on Enceladus and, perhaps, Europa, Triton, and Ceres. It may explain the observed evidence for a young surface on Charon (Pluto’s surface is masked by frosts). Here, we evaluate prerequisites for cryovolcanism on dwarf planet-class Kuiper belt objects (KBOs). We first review the likely spatial and temporal extent of subsurface liquid, proposed mechanisms to overcome the negative buoyancy of liquid water in ice, and the volatile inventory of KBOs. We then present a new geochemical equilibrium model for volatile exsolution and its ability to drive upward crack propagation. This novel approach bridges geophysics and geochemistry, and extends geochemical modeling to the seldom-explored realm of liquid water at subzero temperatures. We show that carbon monoxide (CO) is a key volatile for gas-driven fluid ascent; whereas CO_2 and sulfur gases only play a minor role. N_2, CH_4, and H_2 exsolution may also drive explosive cryovolcanism if hydrothermal activity produces these species in large amounts (a few percent with respect to water). Another important control on crack propagation is the internal structure: a hydrated core makes explosive cryovolcanism easier, but an undifferentiated crust does not. We briefly discuss other controls on ascent such as fluid freezing on crack walls, and outline theoretical advances necessary to better understand cryovolcanic processes. Finally, we make testable predictions for the 2015 New Horizons flyby of the Pluto-Charon system.

[1] The existence of cryovolcanic features on Titan has been the subject of some controversy. Here we use observations from the Cassini RADAR, including Synthetic Aperture Radar (SAR) imaging, radiometry, and topographic data as well as compositional data from the Visible and Infrared Mapping Spectrometer (VIMS) to reexamine several putative cryovolcanic features on Titan in terms of likely processes of origin (fluvial, cryovolcanic, or other). We present evidence to support the cryovolcanic origin of features in the region formerly known as Sotra Facula, which includes the deepest pit so far found on Titan (now known as Sotra Patera), flow-like features (Mohini Fluctus), and some of the highest mountains on Titan (Doom and Erebor Montes). We interpret this region to be a cryovolcanic complex of multiple cones, craters, and flows. However, we find that some other previously supposed cryovolcanic features were likely formed by other processes. Cryovolcanism is still a possible formation mechanism for several features, including the flow-like units in Hotei Regio. We discuss implications for eruption style and composition of cryovolcanism on Titan. Our analysis shows the great value of combining data sets when interpreting Titan's geology and in particular stresses the value of RADAR stereogrammetry when combined with SAR imaging and VIMS.

The geologically active south pole of Enceladus generates a plume of micron-sized particles, which likely form Saturn's tenuous E-ring extending from the orbit of Mimas to Titan. Interactions between these particles and satellites have been suggested, though only as very thin surficial phenomena. We scrutinize high-resolution images with a newly developed numerical shape model of Helene and find that the leading hemisphere of Helene is covered by thick deposits of E-ring particles, which occasionally collapse to form gully-like depressions. The depths of the resultant gullies and near-absence of small craters on the leading hemisphere indicate that the deposit is tens to hundreds of meters thick. The ages of the deposits are less than several tens of My, which coincides well with similar deposits found on Telesto and Calypso. Our findings as well as previous theoretical work collectively indicate that the cryovolcanic activity currently occurring on Enceladus is ephemeral., Comment: 23 pages 6 figures

Titan, Saturn's largest satellite, is subject to solid body tides exerted by Saturn on the timescale of its orbital period. The tide-induced internal redistribution of mass results in tidal stress variations, which could play a major role for Titan's geologic surface record. We construct models of Titan's interior that are consistent with the satellite's mean density, polar moment-of-inertia factor, obliquity, and tidal potential Love number k2 as derived from Cassini observations of Titan's low-degree gravity field and rotational state. In the presence of a global liquid reservoir, the tidal gravity field is found to be consistent with a subsurface water-ammonia ocean more than 180 km thick and overlain by an outer ice shell of less than 110 km thickness. The model calculations suggest comparatively low ocean ammonia contents of less than 5 wt % and ocean temperatures in excess of 255 K, i.e., higher than previously thought, thereby substantially increasing Titan's potential for habitable locations. The calculated diurnal tidal stresses at Titan's surface amount to 20 kPa, almost comparable to those expected at Enceladus and Europa. Tidal shear stresses are concentrated in the polar areas, while tensile stresses predominate in the near-equatorial, midlatitude areas of the sub- and anti-Saturnian hemispheres. The characteristic pattern of maximum diurnal tidal stresses is largely compliant with the distribution of active regions such as cryovolcanic candidate areas. The latter could be important for Titan's habitability since those may provide possible pathways for liquid water-ammonia outbursts on the surface and the release of methane in the satellite's atmosphere.

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 10(exp 7)-10(exp 8) 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.

We investigate the internal thermal evolution of Kuiper belt objects (KBOs), small (radii 2500 kg m − 3 ) bodies orbiting beyond Neptune, focusing on Pluto's moon Charon in particular. Our calculations are time-dependent and account for differentiation. We review evidence for ammonia hydrates in the ices of KBOs, and include their effects on the thermal evolution. A key finding is that the production of the first melt, at the melting point of ammonia dihydrate, ≈ 176 K , triggers differentiation of rock and ice. The resulting structure comprises a rocky core surrounded by liquids and ice, enclosed within a >100-km thick undifferentiated crust of rock and ice. This structure is especially conducive to the retention of subsurface liquid, and bodies the size of Charon or larger (radii >600 km) are predicted to retain some subsurface liquid to the present day. We discuss the possibility that this liquid can be brought to the surface rapidly via self-propagating cracks. We conclude that cryovolcanism is a viable process expected to affect the surfaces of large KBOs including Charon.

We present the first reported adaptive optics spectra of Charon in the H and K bands, which examine the anti-Pluto and sub-Pluto hemispheres. The ice temperature is estimated at 40-50 K, based on the 1.65 μm feature of crystalline water ice. We obtain the most accurate profiles of the 2.21 μm feature and confirm that the feature is due to hydrated ammonia. We attribute hemispheric differences in the feature's profile to different hydration states. We calculate the rate at which crystalline water ice is amorphized by solar UV/visible radiation, finding that at the depths probed by H and K observations (≈350 μm), the e-folding time to amorphize ice is (3-5) × 104 yr. This implies Charon's ice crystallized from a melt, or has been heated to 90 K, during the last ~105 yr. The extent of the crystalline water ice and the short timescales involved argue that surface renewal is necessary, a conclusion reinforced by the presence of ammonia hydrates. We investigate possible mechanisms for surface renewal and conclude that cryovolcanism is the most likely.

Cryovolcanism is defined as the extrusion of liquids and vapors of materials that would be frozen solid at the planetary surface temperatures of the icy bodies of the outer solar system. Active cryovolcanism is now known to occur on Saturn's moon Enceladus and on Neptune's moon Triton and is suspected on Jupiter's moon Europa, while evidence for past cryovolcanic activity is widespread throughout the outer solar system. This chapter examines the mechanisms and manifestations of cryovolcanism, beginning with a review of the materials that make up these unusual “magmas” and the means by which they might erupt and concluding with a volcanologist's tour of the farthest reaches of the solar system.

The surface of Mars is too cold and dry to permit stable liquid water, yet fresh, apparently water-carved gullies and seepage features have been identified in high-resolution imaging of canyon and crater walls by the Mars Global Surveyor spacecraft. Here, a model of nonequilibrium hydrological activity and liquid water cryovolcanism explains the paradoxical appearance and observed properties of these landforms: Aquifers within a porous crust are confined by growing ice and impermeable volcanic layers or basement rock. Freeze–thaw cycles driven by changes in heat flow or climate create high ambient pore pressures and lead to catastrophic expulsion of water through cracks to the surface. The gullies are thus indicators of the geologic and thermal state of the crust, not the present surface environment. Investigations of deposits formed by this cryovolcanism are a means of probing the deep subsurface of Mars for potential biological activity.

We model the chemical evolution of Titan, wherein primordial NH3 reacts with sulfate-rich brines leached from the silicate core during its hydration. The resulting differentiated body consists of a serpentinite core overlain by a high-pressure ice VI mantle, a liquid layer of aqueous ammonium sulfate, and a heterogeneous shell of methane clathrate, low-pressure ice Ih and solid ammonium sulfate. Cooling of the subsurface ocean results in underplating of the outer shell with ice Ih; this gravitationally unstable system can produce compositional plumes as ice Ih ascends buoyantly. Ice plumes may aid in advection of melt pockets through the shell and, in combination with surface topography, provide the necessary hydraulic pressure gradients to drive such melts to the surface. Moreover, contact between the magma and wall rock (methane clathrate) will allow some methane to dissolve in the magma, as well as eroding fragments of wall rock that can be transported as xenoliths. Upon rising to the clathrate decomposition depth (∼ 2 MPa, or 1700 m), the entrained xenoliths will break down to ice + methane gas, powering highly explosive eruptions with lava fountains up to several kilometers high. Hence we predict that Titan is being resurfaced by cryoclastic ash consisting of ice and ammonium sulfate (or its tetrahydrate), providing an abundance of sedimentary grains, a potential source of bedload for fluvial transport and erosion, and of sand-sized material for aeolian transport and dune-building. The infrared reflectance spectrum of ammonium sulfate makes it a plausible candidate for the 5 μm-bright material on Titan's surface. © 2006 Elsevier Inc. All rights reserved.

We present a model of the internal evolution of Ceres consistent with pre-Dawn observations and preliminary data returned by Dawn. We assume that Ceres accreted ice and both micron- and millimeter-sized rock particles and that micron-sized fines stayed suspended in liquid during differentiation. We conclude that aqueously altered grains were emplaced on Ceres's surface during the first tens of megayears of its evolution. Our geochemical simulations suggest that Ceres's unusual surface mineralogy is consistent with aqueous alteration of CM material, possibly by NH3-bearing fluid. Thermal evolution simulations including insulating fines yield present-day liquid at depth if Ceres has a small core or no core at all; otherwise, they yield temperatures at the core-mantle boundary of 240–250 K, just warm enough for chloride brines to persist and be freezing today. We hypothesize that ongoing freezing may overpressurize liquid or briny reservoirs, driving cryovolcanic outflow whose surface expression may have been observed by Dawn at Ceres's “bright spots.” These outflows may contribute to the water vapor being produced at Ceres.

Evidence for plumes of water on Europa has previously been found using the Hubble Space Telescope (HST) using two different observing techniques. Roth et al. (2014) found line emission from the dissociation products of water. Sparks et al. (2016) found evidence for off-limb continuum absorption as Europa transited Jupiter. Here, we present a new transit observation of Europa that shows a second event at the same location as a previous plume candidate from Sparks et al. (2016), raising the possibility of a consistently active source of erupting material on Europa. This conclusion is bolstered by comparison with a nighttime thermal image from the Galileo Photopolarimeter-Radiometer (PPR) which shows a thermal anomaly at the same location, within the uncertainties (Spencer et al. 1999). The anomaly has the highest observed brightness temperature on the Europa nightside. If heat flow from a subsurface liquid water reservoir causes the thermal anomaly, its depth is ~1.8-2 km, under simple modeling assumptions, consistent with scenarios in which a liquid water reservoir has formed within a thick ice shell. Models that favor thin regions within the ice shell that connect directly to the ocean, however, cannot be excluded, nor modifications to surface thermal inertia by subsurface activity. Alternatively, vapor deposition surrounding an active vent could increase the thermal inertia of the surface and cause the thermal anomaly. This candidate plume region may offer a promising location for an initial characterization of Europa's internal water and ice and for seeking evidence of Europa's habitability. ~, Comment: 14 pages, 2 figures

International audience; The existence of cryovolcanic features on Titan has been the subject of some controversy. Here we use observations from the Cassini RADAR, including Synthetic Aperture Radar (SAR) imaging, radiometry, and topographic data as well as compositional data from the Visible and Infrared Mapping Spectrometer (VIMS) to reexamine several putative cryovolcanic features on Titan in terms of likely processes of origin (fluvial, cryovolcanic, or other). We present evidence to support the cryovolcanic origin of features in the region formerly known as Sotra Facula, which includes the deepest pit so far found on Titan (now known as Sotra Patera), flow-like features (Mohini Fluctus), and some of the highest mountains on Titan (Doom and Erebor Montes). We interpret this region to be a cryovolcanic complex of multiple cones, craters, and flows. However, we find that some other previously supposed cryovolcanic features were likely formed by other processes. Cryovolcanism is still a possible formation mechanism for several features, including the flow-like units in Hotei Regio. We discuss implications for eruption style and composition of cryovolcanism on Titan. Our analysis shows the great value of combining data sets when interpreting Titan's geology and in particular stresses the value of RADAR stereogrammetry when combined with SAR imaging and VIMS.

Evidence of past cryovolcanism is widespread and extremely varied on the icy satellites. Some cryovolcanic landscapes, notably on Triton, are similar to many silicate volcanic terrains, including what appear to be volcanic rifts, calderas and solidified lava lakes, flow fields, breached cinder cones or stratovolcanoes, viscous lava domes, and sinuous rilles. Most other satellites have terrains that are different in the important respect that no obvious volcanoes are present. The preserved record of cryovolcanism generally is believed to have formed by eruptions of aqueous solutions and slurries. Even Triton's volcanic crust, which is covered by nitrogen-rich frost, is probably dominated by water ice. Nonpolar and weakly polar molecular liquids (mainly N2, CH4, CO, CO2, and Ar), may originate by decomposition of gas-clathrate hydrates and may have been erupted on some icy satellites, but without water these substances do not form rigid solids that are stable against sublimation or melting over geologic time. Triton's plumes, active at the time of Voyager 2's flyby, may consist of multicomponent nonpolar gas mixtures. The plumes may be volcanogenic fumaroles or geyserlike emissions powered by deep internal heating, and, thus, the plumes may be indicating an interior that is still cryomagmatically active; or Triton's plumes may be powered by solar heating of translucent ices very near the surface. The Uranian and Neptunian satellites Miranda, Ariel, and Triton have flow deposits that are hundreds to thousands of meters thick (implying highly viscous lavas); by contrast, the Jovian and Saturnian satellites generally have plains-forming deposits composed of relatively thin flows whose thicknesses have not been resolved in Voyager images (thus implying relatively low-viscosity lavas). One possible explanation for this inferred rheological distinction involves a difference in volatile composition of the Uranian and Neptunian satellites on one hand and of the Jovian and Saturnian satellites on the other hand. Perhaps the Jovian and Saturnian satellites tend to have relatively "clean" compositions with water ice as the main volatile (ammonia and water-soluble salts may also be present). The Uranian and Neptunian satellites may possess large amounts of a chemically unequilibrated comet-like volatile assemblage, including methanol, formaldehyde, and a host of other highly water- and ammonia-water-soluble constituents and gas clathrate hydrates. These two volatile mixtures would produce melts that differ enormously in viscosity The geomorphologic similarity in the products of volcanism on Earth and Triton may arise partly from a rheological similarity of the ammonia-water-methanol series of liquids and the silicate series ranging from basalt to dacite. An abundance of gas clathrate hydrates hypothesized to be contained by the satellites of Uranus and Neptune could contribute to evidence of explosive volcanism on those objects.

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 Stofan et al. (2006). 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.

Titan, Saturn's largest satellite, is subject to solid body tides exerted by Saturn on the timescale of its orbital period. The tide-induced internal redistribution of mass results in tidal stress variations, which could play a major role for Titan's geologic surface record. We construct models of Titan's interior that are consistent with the satellite's mean density, polar moment-of-inertia factor, obliquity, and tidal potential Love number k 2 as derived from Cassini observations of Titan's low-degree gravity field and rotational state. In the presence of a global liquid reservoir, the tidal gravity field is found to be consistent with a subsurface water-ammonia ocean more than 180 km thick and overlain by an outer ice shell of less than 110 km thickness. The model calculations suggest comparatively low ocean ammonia contents of less than 5 wt % and ocean temperatures in excess of 255 K, i.e., higher than previously thought, thereby substantially increasing Titan's potential for habitable locations. The calculated diurnal tidal stresses at Titan's surface amount to 20 kPa, almost comparable to those expected at Enceladus and Europa. Tidal shear stresses are concentrated in the polar areas, while tensile stresses predominate in the near-equatorial, midlatitude areas of the sub- and anti-Saturnian hemispheres. The characteristic pattern of maximum diurnal tidal stresses is largely compliant with the distribution of active regions such as cryovolcanic candidate areas. The latter could be important for Titan's habitability since those may provide possible pathways for liquid water-ammonia outbursts on the surface and the release of methane in the satellite's atmosphere. Key Points Interior models and amplitude patterns of diurnal tidal stresses are calculated The diurnal tidal stress pattern is compliant with cryovolcanic candidate areas A warm, low-ammonia ocean could increase Titan's habitable potential ©2014. American Geophysical Union. All Rights Reserved.