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[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.

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.

[1] Cryovolcanic resurfacing is a popular mechanism to explain relatively young surface units on icy satellites of Jupiter, Saturn, Uranus, and Neptune. Prior to the Galileo data acquired between 1996 and 2001, Europa was thought to have undergone significant cryovolcanic resurfacing, facilitated by a global ocean beneath the icy surface. However, close examination of Galileo data at resolutions much better than those of Voyager images show that many of the features previously thought to be cryovolcanic are commonly best explained by other formative mechanisms, including tectonism and diapirism. In this study, I present an examination of the characteristics of a variety of Europan surface features for which effusive cryovolcanism is a possible origin, including apparently lobate “flows,” certain elliptical to circular lenticulae, and low-lying, smooth, low-albedo surfaces. A review of cryovolcanic eruption theory, together with Galileo data analysis of Europan surface geology and composition, indicates that cryovolcanism is a viable, though not unequivocal, explanation for some of these features. Some constraints on cryomagma properties and lithospheric structure are offered for these cases. The presence of small-volume, low-viscosity effusions is supported by observations and modeling. Some positive relief lenticulae could represent more viscous effusions, although diapirism may be a preferable explanation. However, strong evidence is lacking for cryovolcanic resurfacing on a large scale. On the basis of our experience with Galileo images of Europa (and Ganymede), Voyager-era inferences for widespread cryovolcanism on icy satellites may be overstated and will need to be carefully reexamined in the light of new data from upcoming spacecraft missions.

Recent observations of Ceres, made using the Heterodyne Instrument for the Far Infrared on board the European Space Agency's Herschel Space Observatory, have shown that water vapor is emanated from the low-albedo midlatitude regions. Based on the supposition that some dark spots on Europa may be caused by diapirism, we explored whether Ceres' environment also can induce compositional diapirism, and whether an upwelling diapir could explain the origin and distribution of the dark spots and vaporization of Ceres. If the density of the rock that constitutes Ceres is around 2700 kg m−3, the undifferentiated crust is stable over geological time periods at less than 180 K. At a surface temperature of 170 K, a diapir can reach the surface if the crust is a few tens of kilometers thick. Alternatively, if the surface temperature is 150 K, a large contrast in viscosity would prevent diapirs from reaching the surface. The relatively moderate surface temperature in the midlatitude regions of Ceres may be the reason for the formation of dark terrains occurring only in such areas. Our finite difference calculations do not show that diapirism melts the surface ice because the temperature gradient changes very little. However, if the observed vapor emission is caused by the sublimation of fresh ice, compositional diapirism could be the mechanism that transports the fresh ice to the surface. In addition to other mechanisms such as cryovolcanism, diapirism is a valid hypothesis for explaining the dark terrain and vapor emissions of Ceres.

Crosscutting relationships of tectonic lineaments on Europa record the history of surface deformation. We mapped the displacement and orientation of older features crosscut by two types of lineaments: bands and double ridges. These measurements allow us to determine both the strike-perpendicular and strike-parallel displacement along investigated features. Double ridges record both ridge-perpendicular contraction and expansion, with a mean of 0.16 ± 0.06 km (needs space) of contraction based on the analysis of 16 double ridges. Bands record expansion, with a mean of 3.33 ± 0.27 km for the six bands analyzed, but with perpendicular displacement less than their apparent morphologic widths of 3–24 km. The implied global surface strain for double ridges (including those that expand) and bands is 2.22 ± 0.76% contraction and 7.60 ± 3.7% expansion, respectively. Double ridges thus may accommodate part of the surface expansion recorded by bands. Most current models for double ridges do not predict contraction. The models that satisfy the observations for bands are “slow-spreading” models, cryovolcanism, and folding.

Galileo data enable the major geological units, structures, and surface features to be identified on Europa. These include five primary units (plains, chaos, band, ridge, and crater materials) and their subunits, along with various tectonic structures such as faults. Plains units are the most widespread. Ridged plains material spans a wide range of geological ages, including the oldest recognizable features on Europa, and appears to represent a style of tectonic resurfacing, rather than cryovolcanism. Smooth plains material typically embays other terrains and units, possibly as a type of fluid emplacement, and is among the youngest material units observed. At global scales, plains are typically mapped as undifferentiated plains material, although in some areas differences can be discerned in the near infrared which might be related to differences in ice grain size. Chaos material is composed of plains and other preexisting materials that have been severely disrupted by inferred internal activity; chaos is characterized by blocks of icy material set in a hummocky matrix. Band material is arrayed in linear, curvilinear, wedge-shaped, or cuspate zones with contrasting albedo and surface textures with respect to the surrounding terrain. Bilateral symmetry observed in some bands and the relationships with the surrounding units suggest that band material forms by the lithosphere fracturing, spreading apart, and infilling with material derived from the subsurface. Ridge material is mapped as a unit on local and some regional maps but shown with symbols at global scales. Ridge material includes single ridges, doublet ridges, and ridge complexes. Ridge materials are considered to represent tectonic processes, possibly accompanied by the extrusion or intrusion of subsurface materials, such as diapirs. The tectonic processes might be related to tidal flexing of the icy lithosphere on diurnal or longer timescales. Crater materials include various interior (smooth central, rough inner, and annular massif) and exterior (continuous ejecta) subunits. Structural features and landforms are shown with conventional symbols. Type localities for the units are identified, along with suggestions for portraying the features on geological maps, including colors and letter abbreviations for material units. Implementing these suggestions by the planetary mapping community would facilitate comparisons of maps for different parts of Europa and contribute to an eventual global synthesis of its complex geology. On the basis of initial mapping results, a stratigraphic sequence is suggested in which ridged plains form the oldest unit on Europa, followed by development of band material and individual ridges. Band materials tend to be somewhat older than ridges, but in many areas the two units formed simultaneously. Similarly, the formation of most chaos follows the development of ridged plains; although chaos is among the youngest materials on Europa, some chaos units might have formed contemporaneously with ridged plains. Smooth plains generally embay all other units and are late-stage in the evolution of the surface. C 1 craters are superposed on ridged plains but are crosscut by other materials, including bands and ridges. Most c2 craters postdate all other units, but a few c2 craters are cut by ridge material. C3 craters constitute the youngest recognizable material on Europa.

[1] Drainage networks on Titan, Earth, and Mars provide the only known examples of non-volcanic fluvial activity in our solar system. The drainage networks on Titan are apparently the result of a methane-ethane cycle similar to Earth’s water cycle. The scarcity of impact craters and the uneven distribution of fluvial dissection on Titan suggest that the surface may be relatively young. The purpose of this study is to assess the importance of erosion relative to other plausible mechanisms of resurfacing such as tectonic deformation, cryovolcanism, or deposition of aerosols. We present a new method, based on a measure of drainage network shape known as the width function, to estimate cumulative erosion into an initially rough surface. We calibrate this method with a numerical landscape evolution model, and successfully test the method by applying it to river networks on Earth with different exhumation histories. To estimate erosional exhumation on Titan, we mapped fluvial networks in all Synthetic Aperture Radar swaths obtained by the Cassini spacecraft through T71. Application of our method to the most completely imaged drainage networks indicates that for two of four regions analyzed, Titan’s fluvial networks have produced only minor erosional modification of the surface. For the best-constrained region in the northern high latitudes, we find that fluvial networks reflect spatially averaged erosion of more than 0.4% but less than 9% of the initial topographic relief. This result implies either a recent, non-fluvial resurfacing event or long-term fluvial incision rates that are slow relative to the rate of resurfacing.