1. Introduction

Europa, Ganymede, Titan, Enceladus Icy satellites of the gaseous giants, Jupiter and Saturn, at orbits beyond the ice-line, constitute extremely interesting planetary bodies due to – among other - their unique geological (Prockter et al. 2010) and surface composition characteristics (Dalton, 2010; Dalton et al. 2010; Fortes & Choukroun, 2010), the possible internal water ocean underneath their icy crust and the habitability potential (Schubert et al. 2010; Tobie et al. 2010; Hussmann et al 2010; Sohl et al. 2010). Two of the largest Jovian satellites, Ganymede and Europa, as well as Saturn's Enceladus and Titan, show not only surface features similar to the terrestrial planets and especially the Earth, but also internal heating and occasionally volcanism. Hereafter we summarize some of the characteristics of these moons.

In the neighborhood of Jupiter, two moons are good candidates for the internal liquid water ocean and together form today the main targets of the Europa Jupiter System Mission (EJSM), a concept studied by ESA and NASA for a launch in 2020.

Europa hosts one of the smoothest surface topographies in the Solar System, composed essentially of water ice and other (Dalton, 2010; Dalton et al. 2010), and displays structures like linear chains (lineae) (e.g. Figueredo & Greeley, 2004), which most likely are crack developments caused primarily by diurnal stresses (Prockter et al. 2010) and display the main characteristics of ridges. Other structures are domes (pits that are surface depressions), dark spots and very few craters (Pappalardo et al. 1998a) as the Galileo mission showed. Internal stratigraphic modeling suggests that the satellite is primarily composed of silicate rock (olivine-dominated mineralogy) (e.g. Schubert et al. 2004; Sohl et al. 2010) and most probably an iron-rich core (Anderson et al. 1998). It is also suggested that the icy crust is decoupled from the moon's deeper interior due to the presence of a subsurface liquid ocean (e.g. Schenk &Mckinnon, 1989; Zimmer et al. 2000; Schenk et al. 2004). The cracks, faults and lineae observed on the surface, have most probably been formed by endogenic processes that lead to tectonic movements and imply the presence of a subsurface ocean that interact with the crust. Also, Europa suffers heavily bombardment by charged particles from the Jovian magnetosphere that lead to surface material decomposition and form the tenuous atmosphere composed mainly of oxygen (Shematovich & Johnson, 2001; Coustenis et al. 2010).

On the other hand, Ganymede, the largest satellite of the Solar System, is the only known moon to possess a magnetosphere (Kivelson et al. 2002). Most of Ganymede's surface coverage displays dark and brighter regions. The former are filled with impact craters while the latter are covered by terrains curved by tectonic ridges and grooves (e.g. Pappalardo et al. 2004; Patterson et al. 2010). Its internal structure possibly consists of a relatively small ironrich core, overlain by silicate rocky material, which is covered by an icy crust. It is believed that a liquid ocean exists within the mantle almost 200 km deep (e.g. McCord et al. 2001). The satellite possesses a thin oxygen atmosphere (Hall et al. 1998; Coustenis et al. 2010 and references therein).

The aforementioned information for Europa and Ganymede has been provided mostly from ground-based but also from in situ missions such as Pioneer 10 and 11, Voyager and more recently and efficiently Galileo. Taking into account the geological and structural elements of both satellites, they readily become promising worlds of astrobiological potential. The forthcoming Europa Jupiter System Mission (EJSM) which is composed of two orbiters, one dedicated to Europa and one to Ganymede, will provide detailed investigation of these satellites (e.g. Blanc et al. 2009).

Orbiting at circa 10 AU, the Kronian satellites Titan and Enceladus, stand as intriguing objects as the Jovian satellites. The Cassini-Huygens mission has unveiled the multivariable Earth-like geology of Titan since its arrival in 2004. Furthermore, Titan is the only one planetary object except the Earth that possesses a unique nitrogen and full of organics atmosphere (e.g. Coustenis & Taylor, 2008). Recently, a subsurface liquid ocean was suggested at Titan based on thermal and orbital calculations (e.g. Tobie et al. 2005) spin rate measurements and SAR reflectivity observations (Lorenz et al. 2008; Stiles et al. 2010; Hussmann et al. 2010). The surface investigation brought to light many geological expressions such as extensive mountains, ridges, dendritic networks, dunes, lakes, channels, canyons and riverbeds (Lopes et al. 2010). Furthermore, there is the possible existence of active zones on the satellite due to past or recent cryovolcanic and tectonic activity (e.g. Soderblom et al. 2007; Lorenz et al. 2008; Nelson et al. 2009a; 2009b).

Another Saturnian satellite, Enceladus, has been shown by Cassini to be one of the most active objects in the Solar System despite its minor size. Indeed, jets from its southern polar region consisted mainly of water ice particles that reach over 435 km in height, enrich the Ering of Saturn (Dougherty et al. 2006; Porco et al. 2006; Waite et al. 2006). The source of such activity could be a large internal liquid deposit, most likely an underground liquid ocean (e.g. Porco, 2008; Collins et al. 2007; Postberg et al. 2009). The surface expressions observed on Enceladus include craters and smooth terrains that consist of extensive linear cracks, scarps, troughs and belts of grooves.

Due to their complex surfaces and intriguing interiors, both Saturnian satellites, require new long-term missions with advanced instrumentation and possibly in situ elements. Such a concept was proposed within the Titan Saturn System Mission (TSSM), studied by ESA and NASA and could be launched after EJSM, around 2025. The mission consists in an orbiter, a Titan montgolfière hot air balloon and a Titan lake lander for simultaneous in situ and remote exploration of Titan and Enceladus.

A number of evidences, either surficial expressions or geophysical factors, endorse the existence of internal oceans beneath the four aforementioned satellites. The consideration of the possible implications from geological features on the surface and their possible relation to the interior is presented in the review hereafter.

2. Geology: Surface features and their implications

The diversity of geological features on Europa and Ganymede Without doubt, the surface expressions observed on a planetary body are the signatures of both external and internal processes as occurred with time. Hence, although the environmental conditions and the working material are different from the terrestrial case, in the presence of similarities in the surface features observed, the heritage of Earth science can be used as a tool for their study on other bodies.

Europa and Ganymede present a major diversity in terms of appearance and surface geological structures and therefore in terms of the surface-shaping forces. Europa seems to be subject to active tectonism and cryovolcanism since it displays a young, smooth and active surface. On the contrary, Ganymede is heavily cratered on most of its surface and internal processes like cryovolcanism seem to have played only a minor role in the surface modification since there is little indication of resurfacing. In general, erosion as well as mass movement and landform degradation seem to play an important role in resurfacing as it reduces the topographic relief by moving surface materials to a lower gravitational potential (Moore et al. 2010).

The most distinct and characteristic morphotectonic features on Europa are the lineations that intersect the entire upper part of the satellite's crust (Fig. 1). These formations are probably the major resurfacing mechanism since their genesis is based on the intersection of any two parts of the surface by bedding or/and cleavage. Figure 2 displays a scheme of intersect lineae formation as seen on the Earth (Park, 1997). The tectonic activity that forms these edifices along with the heating from the subsurface diapirs (mobile material that was forced into more brittle surrounding rocks and follows an upward direction) composes the dominant dynamic potential of the satellite.

Geissler et al. (1998) proposed four different classes of lineaments that vary with age and wipe out Europa's geologic history through time. Their size ranges from 1 to 20 km and they are separated into (a) incipient-simple colorless cracks, (b) ridges that are wider than the cracks, (c) multiple ridged triple bands and (d) ancient bands (Fig. 3). A distinct intersecting 1,500 km feature, Agenor Linea, is a candidate active region as found in photometric observations (Hoppa et al. 1998; Geissler et al. 1998).

Other geological expressions seen on Europa by Galileo are features that are called ‘lenticulae' and ‘chaos'. In terrestrial geology, a lenticulae feature is a depositional body that is thick in the middle and thin at the edges, resembling a convex lens in cross-section. Both features have probably formed by rising diapirs that produced partial melt. On Europa, the lenticulae are ovoidal features ranging from 5 to 20 km in diameter (Pappalardo et al. 1998) while the chaos structures, like Conamara Chaos (8°N, 274°W), are larger features presenting blocky material (Carr et al. 1998; Spaun et al. 1998; Sotin et al. 2002). Another hypothesis for the genesis of lenticulae features suggests a correlation between them and the chaos formations. This hypothesis by Greenberg (2008) assumes that the lenticulae are chaos formations in smaller dimension that low-resolution Galileo imaging interpreted wrongfully as distinct structures.

In contrast to Europa's flattened topography and homogeneous surface, Ganymede possesses two distinct types of terrain. The dominant terrain comprises the brighter regions marked with geological expressions such as extensive ridges, grooves and faults while its age considers as the youngest in comparison with the rest of the surface (Pappalardo et al. 1998) (Fig. 5).

The other geological terrain-type is the dark terrain (Prockter et al. 1998) (Fig. 6), which is the oldest one in terms of age, heavily cratered with astrobiological interest due to the existence of organic materials (McCord et al. 1998). Considering the fact that the satellite is heavily cratered and the dark terrain comprises one of the oldest terrains of the Galilean satellites (Zahnle et al. 1998), this surface is an indicator of the system's cratering history (Pappalardo et al. 1998).

The regions that are particularly interesting in terms of geology are the transitional regions (Fig. 7). Such regions correspond to surface areas that constitute a transition from the dark terrain to the bright grooved terrain of Ganymede. Even though cratering is present on both types of terrain, the dark ones seem heavily and more extensively bombarded (Showman & Malhotra, 1999) suggesting that they represent the oldest preserved (non-resurfaced) surfaces on Ganymede. Pappalardo et al. (2004) suggested that the modification of the dark terrain's material due to tectonic and cryovolcanic resurfacing formed the primary bright terrain.

Another significant region on Ganymede is the Galileo Regio (Fig. 8), which has likely been formed during an active geologically period (Casacchia, 1984). The Galileo Regio is a heavily cratered area but not an impact crater. It seems to have been shaped under the influence of tectonic processes and young and bright material that arose from the interior (Harland, 2000).

Both moons, Europa and Ganymede illustrate their own fascinating surface geology. Many factors contribute to their formation with the most influencing ones being the internal dynamics like tides, volcanism and tectonics as well as external factors like impact cratering.

Active worlds: Titan and Enceladus and their habitability Even though Titan and Enceladus' surface expressions are very different in terms of composition, materials and size, they highly resemble the Earth's geomorphology. Titan's surface consists of structures like mountains, ridges, faults and canyons (Fig. 9), formed most probably by tectonic processes, as discovered by the Cassini-Huygens mission (Brown et al. 2009; Lopes et al. 2010; Mitri et al. 2010). Titan, other than its atmospheric uniqueness, is also the only among outer planet satellites where aeolian and fluvial processes operate to erode, transport, and deposit material (Moore et al. 2010).

The phenomenon that most probably formed terrestrial-like volcanic structures like calderas, flows and domes on Titan, is cryovolcanism. Currently, there are three possible cryovolcanic regions. These are Tui Regio (20°S, 130°W), Hotei Regio (26°S, 78°W) and Sotra Facula (15°S, 40°W). The latter is considered as the most promising one with obvious surface expressions of peaks, flows and one caldera structure (Fig. 10).

Furthermore, aeolian and fluvial processes acting on Titan's surface create edifices such as lakes, seas, riverbeds, sand dunes, shorelines, and dendritic drainage networks (Fig. 11). External impact phenomena like impact craters are rarely observed on the surface due to the resurface activity.

Similarly to Titan, Enceladus presents major tectonic features and active cryovolcanism. The most fascinating phenomenon occurs on Enceladus currently due to its tremendous internal dynamic convective forces that cause Geyser-like fountains at its southern pole that could reach more than 400-km in height (Porco et al. 2005). The rest of Enceladus' surface is covered by smooth and cratered terrains, rifts, ridges, grooves, escarpments and extensive linear fractures (Johnson, 2004). The geology of this tectonized moon is a field of active scientific research awaiting for new observations. Up to date, the observations and analysis showed two types of tectonic terrains. The north pole consists of heavily cratered landforms while the central region and southern pole of tectonic molded terrain with cryovolcanic features.

3. Interior models and liquid water subsurface oceans in giant planets' satellites

Our current knowledge of the icy moons' internal stratification and their composition is being built on a combination of spacecrafts data, laboratory experiments, and theoretical geophysical modeling. Resembling Earth's moon in terms of structure, icy moons consist of a core, a mantle, and a crust, with the specificity of the existence of a liquid ocean lying within the icy mantle (Fig. 13).

According to current models of internal structure, the existence of subsurface oceans is expected for most of the icy moons of the Outer planets (e.g. Sohl et al. 2010; Schubert et al. 2010 and references therein). Even if an ocean is not currently hidden within the interior, it is suggested that a liquid layer was present in the past but cooled to ice over time. An example of such case is Neptune's moon Triton.

Evidence for hydrated sulfate salts on the surfaces of Europa and Ganymede from spectroscopic data support the possible existence of subsurface oceans (McCord et al. 1998; 2001; Cassidy et al. 2010; Fortes et al. 2010 and references therein) suggesting the deposition of minerals following internal hydrothermal events. In addition, Galileo's magnetometer (e.g. Khurana et al. 1998; Kivelson et al. 2002), detected induced magnetic fields at Europa and Ganymede that imply the presence of an electrically conductive subsurface layer (e.g. Sohl et al. 2010). Furthermore, the detection of a low viscosity layer underneath the icy crust again endorses the presence of a subsurface liquid ocean inducing recent geological activity (Stern & McKinnon, 1999; Ruiz & Fairen, 1999).

The common properties that need to be satisfied on all bodies in order to sustain a liquid subsurface ocean are:

The radioactive decay cannot provide the amount of heat required to modify entirely the satellite surface, as observed on Europa. In this case, the surface temperature is limited to 110K at the equator and 50K at the poles; such cold temperatures make the ice locally as hard as terrestrial igneous granite (McFadden et al. 2007). On the other hand, hydrothermal activity has the potential to reshape the surface crust. Recent studies suggested that the influence of Jupiter on Europa due to its small but non-zero obliquity probably generates large tidal waves that keep the ocean warm (Tyler, 2008).

If the heat propagation and the buoyant oceanic currents are not intense then the ice shell will be thick and a warm ice layer will be formed at the bottom of the shell (Fig.15b). During the hydrothermal processes this warmer ice will rise and slide like the terrestrial glaciers. Such movements can cause surface modification and produce structures like the ‘chaos terrain'. Other features supporting the existence of a thick ice layer are the large impact craters surrounded by concentric rings filled with ice. Geophysical models, associating the mere presence of these structures to the amount of heat generated by the tides, suggest an icy crust 10- 30 km thick with a warm ice layer at the bottom and a liquid ocean probably 100 km thick (Schenk et al. 2004). Additionally, a scenario that consists of a thick icy crust of almost 15 km suggests a stratigraphic model with a 100 km deep ocean, which may be extremely deep, about10 times deeper than the deepest point of Earth's oceans, the Challenger Deep on the Mariana Trench, which is 11 km below sea level and the lowest elevation of the surface of the Earth's crust. The potential ocean of Europa would contain an amount of water twice the entire terrestrial surface hydrological system.

Alternatively to the previous model, if the heat flow and the plumes are intense then the ice shell is expected to be thin (Fig. 15a). The fragmentation of such a thin crust is most probably expressed with tectonic-like formations like the Conamara region. The debate regarding the thickness of Europa's crust gave birth to an alternative hypothesis regarding the thin ice model, which suggests a thin upper crust layer, almost 200-m thick, that behaves elastically and is in contact with the surface through zones of weakness like multiple linear ridges (Billings & Kattenhorn, 2005).

Europa's case supports the existence of a stagnant lid underneath its crust (Fig. 16) (Showman & Han, 2004). The stagnant lid is a relatively cold and stiff conductive layer covering the warmer convective icy interior (Schubert et al. 2004). Since the expected activity within Europa's interior is upwelling thermal diapirism, the stagnant lid can prevent cold nearsurface icy material from sinking towards the ocean.

Compared to the Earth's oceans, the composition of Europa's hidden ocean should be significantly different. The Earth's ocean major component is sodium chloride while Europa's should be magnesium sulfate as indicated by Galileo data (e.g. Fanale et al. 2001). Indeed, Europa's weak magnetic moment is induced by the varying part of the Jovian magnetosphere (Schilling et al. 2007) and requires a highly conductive subsurface ocean. Such conductive materials candidates are the magnesium sulfate (e.g. McCord et al. 1998) or sulfuric acid hydrate (Carlson et al. 2005).

Similarly, Ganymede consists of a four-layer interior, based on measurements regarding its gravity field, mass, density and size. Geophysical models suggest two kinds of internal stratigraphy, one of an undifferentiated mixture of rock and ice and one of a differentiated body consisting of a rocky core, an extended icy mantle and an icy crust. We think the latter model is the most plausible as it is compatible with the gravity field measurements made by Galileo (e.g. Sohl et al. 2002). Such intrinsic magnetic field supports the existence of an iron-rich core (Hauk et al. 2006). The thickness of the layers in the interior depends on both the fraction of olivine and pyroxene and the amount of sulfur in the core (e.g. Sohl et al. 2002) (Fig. 17). Thus, the 2,634-km-radius Ganymede consists of an 800 km iron sulfide core, an almost 900- km thick outer ice mantle and a silicate-rich mantle 700 km thick (e.g. Kuskov et al. 2005).

Although the mechanisms that formed Ganymede's complex surface are still unidentified, multiple scenarios have been proposed. Most of the scenarios agree on a general mechanism that uses tectonism as the main chisel that formed and shaped the existing structures, especially the grooved terrain (Sohl et al. 2002). On the other hand, and in contrast with the other three aforementioned satellites, cryovolcanism does not seem to participate actively in the geodynamic processes. The radiogenic heating from within the satellite as well as tidal heating from past events are considered as the main forces that generate the stresses that lead to tectonic movements and eventually to tectonic structures.

Contrary to Ganymede, probably Titan but especially Enceladus provide evidence of past and current cryovolcanism that shapes their surfaces. The mechanisms that formed Titan's surface by endogenic factors are still unknown, although the central idea is focused on cryovolcanism and morphotectonism, with the latter being the short- and long-term surficial expressions of any tectonic activity originated from endogenic processes (Solomonidou et al. 2010). However, it is possible that Titan underwent a period of tectonism resembling those on Europa's and Ganymede's.

According to geophysical models, Titan's differentiated interior consists of a serpentinite core (~1,800 km), a high-pressure ice mantle (~400 km), a liquid layer of aqueous ammonium sulphate (50 to 150 km wide), and an externally heterogeneous icy few kilometers wide (Tobie et al. 2005; Fortes et al. 2007).

The subsurface instability due to the interactions within an interior liquid ocean causes the modification of extended features on Titan's surface, whether they derive from cryovolcanic or morphotectonic dynamic processes. Currently, all the geophysical models that try to explain the geodynamics of Titan support the existence of an oceanic layer that decouples the mantle from the icy crust. Additionally, the identification of a small but significant asynchronicity in Titan's rotation from Cassini SAR data favors the aforementioned decoupling (Lorenz et al. 2008). Internal geodynamic activity can transport effusively the explosive material from the oceanic layer to the surface and form the cryovolcanic structures like the lobate flows in Sotra Facula.

Tobie et al. (2006) suggested a cryovolcanic model for Titan's case to solve the mystery of Titan's methane replenishment since it is should vanish in 100 Ma. These authors suggested that episodic methane outgassing events occurred through three distinct episodes covering a chronological period from 2000 Ma ago until 500 Ma ago (Fig. 18). The internal ocean provides the ‘magma' chamber by means of material, while convective processes are the triggering mechanisms that initiate the dynamic activity. A convective model of a stagnant lid is capable to explain such activities (Solomatov 1995) as explained for Europa's case earlier. However, the ice shell for Titan's case is expected to be thicker. In opposition, several studies suggest that Titan is not currently convective (Mitri et al. 2010; Nimmo and Bills, 2010).

The uniqueness of Titan's tectonism - even though not yet confirmed - lies in that the tectonic processes are contractional rather than extensional, setting Titan out as the only planetary body in the Solar System other than the Earth, where contractional deformation occurs. Mitri et al. (2010) proposed an internal thermal model, focused on the changes in volume of a potential underground ocean caused by heat flux variations during freezing or melting. The authors suggest that the continuing cooling of the moon can develop global volume contraction, as described by Tobie et al. (2005; 2006).

Cassini's data proved that, despite its small size (about 505 km in diameter), Enceladus is an active planetary body that spews material through hydrothermal vents resembling terrestrial geysers. This moon is not in hydrostatic equilibrium (Schubert et al. 2007; Schubert et al. 2010), thus a simple and very general stratigraphic interior is being suggested: it consists of a 169 km rocky core overlain by an icy 82 km mantle (Barr and McKinnon, 2007; Fortes, 2007; Schubert et al. 2007). The subsurface ocean is lying between the two layers, and supplies the fountains observed at the south polar region through cracks called ‘Tiger Stripes' (e.g. Porco, 2008; Postberg et al. 2009). The moon's tidal dissipation is proposed as the triggering mechanism as well as the cause of the dynamic phenomena like tectonics (convection-conduction, expansion contraction) and cryovolcanism (e.g. Mitri and Showman, 2008; Mitri et al. 2010) that formed the surface expressions described in the previous section.

4. Discussion: Internal activity and surface modification

The possible existence of subsurface liquid oceans underneath the crusts of the icy moons of Jupiter (Europa and Ganymede) and Saturn (Titan and Enceladus), places them in a potential group of planetary bodies where life could emerge and evolve. Other than data processing that provide evidence and information about the internal liquid layers, the surficial expressions that are related to the hydrothermal and dynamic processes occurring within these layers are the surface evidence that could lead to their identification. Specifically, the cryovolcanic and morphotectonic structures seen on the aforementioned satellites are the surface expressions of the internal activity while they are formed by modification of the crustal layer and deposition of material coming from the subsurface ocean. Therefore, investigating these surface exposures and associating them to terrestrial features where water is involved could shed some light on the investigation of internal liquid water oceans in the icy moons. Trying to model the triggers of internal active phenomena, basic geophysical models usually propose liquid water reservoirs while the geodynamic models point at both radioactive decay and tidal stresses, caused by the giant planets Jupiter and Saturn. We shall herein try and reconcile both views.

The major and most significant structures on Europa's surface are the bunch lineae (e.g. Prockter et al. 2010) (Fig. 3). Data analysis showed that the geometry and the spectral properties vary depending on age, which indicates an evolutionary sequence (Geissler et al. 1998; Dalton, 2010). This means that persistent and drastic processes occur in order to form these features. Such processes seem to be different types of cryovolcanism in which multiple eruptions of ice (warmer than crustal ice) emerge through ‘tectonic' crustal weaknesses (Figueredo & Greeley, 2004). This tectonic formation resembles the terrestrial mid-ocean ridges (MOR), which are the extensive opening seafloor terrains, and considered to be a global rather than a local phenomenon. MOR are dynamic and volcanically active structures that constantly provide and deposit new material from the mantle to create new oceanic crust. As seen in Figure 4, the red streaks, as well as the red spots called lenticulae, are most possibly evidence of upwelling warmer material emerging from the liquid layer while colder ice near the surface sinks downwards. After spectroscopic analysis, the red streaks are thought to be rich in magnesium sulfate, another hint in favor of their internal origin (McCord et al. 1998; Dalton et al. 2010). Furthermore, the lineament formations like the dark and red streaks present the basic resurfacing system that dominate on Europa's surface. Their formation is an indicator for current geological activity. Additionally, the thermal diapirism that most likely formed these structures, as well as the red spots, would imply that convective upwelling thermal plumes originate in the lower boundary of the convective system and gets in contact with the cold stagnant lid with the icy plumes (Showman & Han, 2004). Such temperature ranges indicate possible habitability at the upper part of the ice shell. Consequently, Europa's surface morphology is directly connected to the internal dynamic processes and provides evidence of a subsurface interactive ocean.

Unlike Europa's conflicting oceanic hydrothermal system that originates in a rocky sea layer, Ganymede's ocean lays between two icy layers that decouple it from the mantle. Barr et al. (2001; 2004) suggested that large magmatic events due to convective plumes could occur at Ganymede's rock – ice boundary. The surface expressions that are most possibly connected to tectonism are the deep fault structures like the horst-and-graben (resembling terrestrial continental rifts) as well as many cracks that are observed in Ganymede's bright terrain (Showman & Malhotra, 1999). Tidal heating events, either past or current, could cause dynamic forces that modify the icy lithosphere. The main mechanism that deforms tectonically the lithosphere is most likely warm liquid plumes that rise from the upper mantle to the surface, following a pattern similar to the plume-lithosphere interactions at the Hawaiian Swell which cause thinning and instabilities at the crust layer (Moore et al. 1998). Even though the plume theory indicates cryovolcanic processes, little evidence of such activity has been detected on Ganymede. Notably, the extreme morphological difference between the bright and the dark terrain as described in a previous section suggests a massive geological event or set of events that caused such large-scale geo-terrain alteration. Hence, it is possible that the extensive tectonic forces that fractured the dark terrain partially affected the bright terrain as well. Such tectonic weaknesses could display pathways to small cryovolcanic events that lead to resurface processes. However, tilt-block faulting and shears (Head et al. 2002) are some of the structures that appear bright within the dark terrain suggesting that tectonic cracks functioned as path for warm internal oceanic material to pass through like what occurred in the bright terrain. In terms of tectonics, and similarly to the other icy moons, there is no evidence of compressional deformation (Showman et al. 1997). Since the deformational pattern is extensional, the question is whether it is a global or a local phenomenon. Collins et al. (1998) studied observations of grooved terrains of specific stratigraphic ages that have consistent directions over hundreds of kilometers, something that indicates global stressing phenomena. On Earth stressing phenomena could occur where severe forces cause convective currents in the ocean. In a similar way, the plume convection within Ganymede's oceanic layer could create enough turbulence and temperature-pressure instabilities to cause global stressing phenomena with an impact on tectonism. Nevertheless, Ganymede presents styles of tectonism different from Europa's.

Following a pattern similar to the one mentioned for Ganymede, Enceladus' internal tidal stresses and radioenergetic decay produce warm pockets of material, the plumes, that subsequently form the geyser formations at the south polar region. The major and most valuable evidence of Enceladus' cryovolcanic activity, supporting as well the ocean existence, is this geyser formation or geyser accumulation that form a fountain of more than 400 km, as observed by Cassini. The jets initiate from four sub-parallel linear depressions, which are tectonic in origin. Other surface expressions are scarps, ridges, and shields (Collins et al. 2009). On the other hand, Titan's surface structures related to its dynamic interior that also support the existence of a subsurface ocean are the three cryovolcanic candidate regions Tui Regio, Hotei Regio and Sotra Facula and many morphotectonic structures like mountains, ridges and canyons (e.g. Solomonidou et al. 2010 and references therein). Generally, in the case of both moons it is thought that their ice shells transit from a conductive to a convective state; since they probably overlay a pure liquid ocean (Tobie et al. 2005) this can have major effects on surface morphotectonics (Mitri and Showman, 2008). Indeed, thermodynamic oscillations within the ice shells may trigger repeated extensional and compressional events. In the presence of a subsurface ocean underneath Titan and as a consequence of local stress mechanisms, parts of the icy crust could behave like rigid ice floes due to lateral pressure gradients. If such floating occurs, many morphotectonic features like faults and canyons can relate their formation to this event. Furthermore, radial contraction of the internal high-pressure ice polymorphs could possibly amend the radial expansion caused during the cooling stage of the moon (Mitri and Showman, 2008), in which the existence of a liquid layer plays a significant role. As a result, the overall global contraction could form mountainous chains (Radebaugh et al. 2007). These four satellites are dynamic planetary bodies and the internal ocean most likely plays an important role on the formation and modification of their surfaces. The properties on which the presence of an internal ocean depends were mentioned earlier (section 3). Another issue that should be taken under consideration is the set of properties that determine the possible exchange of material between the subsurface and the surface. These are the mechanical properties of the lithosphere as proposed by Tobie et al. (2010).

The formation of the surface signature of any upwelling activity depends on:

5. Habitability issues for outer planet satellites

The existence of an internal liquid ocean underneath the icy crusts of the giant planet satellites could serve as a potential abode for life. The location of the ocean close to the surface provides food for thought on habitability zones, and conditions for life in general, in the Solar System.

Indeed, the discovery of hydrocarbon surficial lakes on Titan and the possible existence of subsurface liquid oceans in Europa, Ganymede, Titan, and Enceladus reveal alternative concepts to the classical definition of the habitable zone, and suggest the need for reconsidering its limits (Lammer et al. 2009). Currently, more and more studies regarding the planetary habitability propose the icy moons with subsurface oceans as potential worlds for initiating and/or sustaining some sort of life forms (Fortes, 2000; Raulin, 2008; Raulin et al. 2010; Coustenis et al. 2011 and references therein). Figure 19 presents the possible locations of the liquid layers within the icy satellites of Jupiter as well as their habitability potential.

For Europa, the assumed internal processes that trigger the geodynamic phenomena are the tidal stresses, which produce considerably less energy than radioenergetic decay. Europa is being compressed and stretched when affected by the gravitational forces of Jupiter and the other Galilean satellites and thus, the resulting tidal friction provides enough energy, as well as temperature, for an extensive ocean to exist underneath the crust. The provision of large amounts of energy at the bottom layer of an ocean can conceivably form hydrothermal vents like the ones seen on Earth. Notably, life could emerge around hydrothermal vents, which are important geological forms in terms of life propagation. Greenberg (2010) suggests that even ecology including complex organisms could exist on Europa. The author provided evidence of oxygen concentration within the ocean greater than that of the Earth's, which is suggested as an indicator for aerobic organisms. Nevertheless in the case where the concentration of salts is large, only extremophile organisms (like halophiles) could survive (Cooper et al. 2001; Marion et al. 2003).

Ganymede lays between structures 1 and 2 in the scheme shown on Fig. 19 indicating that it is much colder than Europa, a factor that lowers its habitability potential. On the other hand, reinforcing Ganymede's possibility for life to exist, Barr et al. (2001) imply that magmatic events could form pockets of liquid, which would then ascent buoyantly carrying nutrients to the internal ocean. Based on their calculations, a water plume could reach Ganymede's ocean and carry nutrient-rich material with an eruption time of 3 hours to 16 days. Additionally, Ganymede's system could provide the necessary tools to concentrate biological building block ingredients (Trinks et al. 2005) especially since it possesses a magnetic field that is able to protect life from harmful radiation and lies in a relatively quiet radio zone.

On Titan, terrestrial bacteria can absorb their energy and carbon needs from the tholins that exist in its thick atmosphere (Stoker et al. 1990). Furthermore, photochemically derived sources of free energy on the surface could maintain an exotic type of life, using liquid hydrocarbons as solvents (McKay & Smith, 2005). Other than the atmospheric properties that are favorable to life, the possible existence of an underground ocean might support terrestrial-type life that had been introduced previously or formed when liquid water was in contact with silicates early in Titan's history (Fortes, 2000). Furthermore, the possible amount of ammonia dissolved within the ocean is suggested to be ~10% (Lorenz et al. 2008), something that corresponds to a pH of 11.5, while at a depth of 200 km the pressure reaches ~ 5 kbars and hot plumes within the potential ocean could be generated (Coustenis et al. 2011 and references therein). The aforementioned conditions are not unfavorable to the emergence and maintenance of life (e.g. Fortes, 2000; Raulin, 2008).

Enceladus' extremely high temperatures at the south polar region are probably generated by the hydrodynamic processes that form the fountain, as previously described, and thus enhance the potential for habitability. The most convincing theory, after Cassini data analysis, suggests that a liquid ocean exists beneath the Tiger Stripes. There are standards for life that Enceladus' possible ocean is not consistent with: the sunlight, the oxygen compounds, and the organics produced on a surficial-crust environment. However, terrestrial regions like the deep oceans that do no satisfy the aforementioned prerequisites for life, still function as active ecosystems (McKay et al., 2008; Muyzer and Stams, 2008). There, sulfur-reducing bacteria consume hydrogen and sulfate, produced by radioactive decay. Notably, the comparison with the terrestrial ecosystems suggests that plume's methane may be biological in origin (McKay et al., 2008). Hence, Enceladus displays an internal warm and chemically rich ocean that may facilitate complex organic chemistry and biological processes (Coustenis et al. 2011).

In conclusion, the confirmation of the existence of a subsurface liquid ocean underneath the crust of the icy moons of Jupiter and Saturn will revolutionise our perspective regarding the habitability potentials of such planetary bodies. Indeed, liquid water may exist well outside the traditional habitability zone, which is merely based on the presence of liquid water on the surface. For planetary habitability, the principal criteria are the presence of liquid water anywhere on the body, as well as the existence of environments able to assembly complex organic molecules and provide energy sources: this can well be underneath the surface in some cases, if stability conditions are met. The four satellites described in this study seem to fulfill some or all of the above requirements. However, it is of high priority to revisit these bodies with new missions and advanced instrumentation (such as gravitational and magnetic field sounding systems and in situ element detectors) in order to obtain altimetry and in situ monitoring of tidally-induced surface distortion data (Sohl et al. 2010) that could unveil in detail the internal stratigraphy of the moons and the specificity of the subsurface oceans. Future large missions, or smaller dedicated ones, to the Galilean and Khronian systems would allow us to better understand the mechanics behind the astrobiological potential of worlds with subsurface oceans, and shed some light on the emergence of life on our own planet.

Acknowledgement. A.Solomonidou is supported by the "HRAKLEITOS II" project, co-financed by Greece and the European Union.