1. Introduction

In 1537, the age old belief in an Earth-centered solar system was radically shaken when Nicolaus Copernicus published "De revolutionibus orbium coelestium". His 17th-century successor Galileo Galilei was the first to discover physical details about the individual bodies of the Solar System, advancing what became known as the Copernican revolution. He discovered that the Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it (named the Galilean moons in his honour). His remarkable observations were described in a book entitled "Sidereus Nuncius" (1610). Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn (Systema Saturnium 1659). And later, Giovanni Domenico Cassini discovered four more moons of Saturn and the Cassini Division in Saturn's rings.

In the early decades of this new age, solar exploration programs were dominated by scientific missions designed to simply discover the general nature of the Solar System, which encompassed the Mariner missions to Venus, Mercury, and Mars and the Pioneer (Ulivi & Harland 2007) and Voyager (Evens & Harland 2003) explorations of planets and satellites in the outer Solar System. A new phase of exploration began with the Viking missions to Mars, the Magellan mission to Venus (Ulivi & Harland 2008), the Galileo mission to Jupiter and its satellites (Harland 2007), and the Cassini-Huygens mission to Saturn and its satellites (Harland 2007). The goals of these missions reflected a new focus: more intensive exploration, including in situ measurements with landing and atmospheric probes.

Data returned from these recent series of successful missions have initiated an exciting new period of exploration, as we begin the process of learning about the vast diversity of environments that may support life. Consequently, solar system exploration has been structured around the concept of habitability - the ability of a planet or moon to sustain life for extended periods of time or for a planetary system to host a planet or moon that can in turn sustain life (Rampelotto 2010a). The habitability concept provides a unifying theme for an exploration program, which includes research and analysis and leads to a set of exploration objectives.

In this context, the next great step in space science will be the exploration of the outer solar system, where planetary bodies have been emerging as potential habitable worlds (Russell & Kanik 2010; Chela-Flores 2010; Schulze-Makuch 2010). These environments provide an opportunity to extend the concept of planetary habitability (which was previously limited to Earth-like planets) and improve our understanding about the nature and distribution of habitable environments in the universe (Lal 2010; Javaux and Dehant 2010).

The complexity of such endeavor requires significant focused technology development prior to mission start, and extended engineering developments, as well as extensive pre-decisional trade studies to determine the proper balance of cost, risk, and science return. For these reasons, they represent major national investments that must be selected and implemented in a strategic manner. The present work discusses the astrobiological potential of planetary bodies in the outer Solar System in order to establish priorities for future missions.

2. A Mission to Europa

Until recently, Europa was widely accepted as the most promising target for a future mission in terms of astrobiological potential (Greenberg 2005). Since the time of the Voyager, the existence of a water ocean below its surface ice crust has been suggested (Greenberg 2008). Data from the Galileo mission, which observed Europa from 1996 to 2002, indicated that such ocean may be warm and salty, greater by a factor of two to three than the volume of liquid water on Earth, what renewed the interest of the scientific community to explore this Jupiter’s satellite in more details (Pappalardo 2009).

The few large impact craters point to a young surface age (Zahnle et al., 2003) and geological models for the formation of surface features suggest that this moon has been geologically active (Figueredo & Greeley 2004) or was in a recent past. Europa is unique among the large icy satellites because its ocean may be in direct contact with its rocky mantle beneath (Kuskov & Kronrod 2005). Other satellites from outer solar system such as Ganymede, Callisto and Titan may also have subsurface oceans; however, those liquid layers are likely sandwiched between two ice layers of different phases (Sohl et al. 2010). The possible deep ocean environment within Europa may resemble that on Earth, where hydrothermal vents on the ocean bottom support a surprisingly diverse suite of organisms by providing a variety of energy-rich chemical compounds (Chyba & Phillips 2002; Rampelotto 2010c). The main point of interest is currently the thickness of the ice layer that exists on top of the aqueous ocean. Although it has been estimated to be around 20-50km, the thickness remains unknown, since there is no consensus among model results (Billings & Kattenhorn 2005; Ruiz et al. 2007). This topic is of great importance for astrobiology because it determines the extent of material exchange from the surface to the subsurface ocean and vice-versa. A thin ice crust permits direct communication of the ocean material with the surface, whereas a thick ice crust results in an ocean largely isolated from the surface. The latter possibility may not maintain a chemically and energetically rich environment, and consequently may not support living systems. Cratering and tectonic features on Europa raise the possibility that material from the deep ocean has been deposited on the surface (Kattenhorn 2009). However, it is important to note that the Europa surface is exposed to the harshest radiation environment in the Solar System since this planetary body presents a very tenuous atmosphere (Paranicas et al. 2007). This very high level of radiation from high-energy charged particles trapped in Jupiter’s magnetosphere can dissociate any organic molecules on the surface and may have a similar effect on prebiotic chemistry coming from the interior. Hence, the presence of biosignatures on the surface of this moon seems not feasible, or at least, their concentration may be very low.

Therefore, the astrobiological interest on Europa lies in the possible existence of life inside the internal ocean and its biosignatures in a subsurface environment (Chyba & Phillips 2001; Marion et al. 2003; Abbas & Schulze-Makuch 2008). Before sending a lander probe or penetrator to Europa, it is necessary to define astrobiological interesting landing sites, where liquid water from the ocean could have recently reached the surface or near surface. Whether large cracks represent these locals remains controversial and it is still unclear how to interpret features on Europa’s surface (Spencer et al. 2006). On the basis of current knowledge, it is difficult to determine with confidence where the potential landing sites may be. It will be first necessary to analyze results especially from the thickness of the ice crust and high resolution images from geological features, which means that sending in situ instruments before a better understanding of Europa may be premature and highly risky.

In this context, the proposed next flagship mission to Europa would be an orbiter restricted to producing images in various wavelengths as proposed to the Jupiter Europa Orbiter (JEO), the current NASA/ESA concept to explore Europa (Clark et al. 2010). Though new technology developments are not required, it is necessary to adapt current instrument designs into a single orbited able to perform within a harsh radiation environment. These challenges result in a mission launch date no earlier than 2020 with Europa orbital insertion around 2028. The data would bring some closure to arguments about the thickness of the crust and the existence of an aqueous internal ocean and could identify places where the subsurface can be more easily accessed. However, in the context of astrobiology, such orbiter would be only the precursor to the next mission - probes able to access the subsurface/internal ocean and provide in situ life signal measurements. Considering the complexity and cost for a mission to the outer solar system and the arguments discussed previously, such mission to detect life or biosignatures on Europa may probably take many decades to happen.

3. A Mission to Ganymede

The JEO mission is one component of the NASA/ESA Europa Jupiter System Mission (ESJM ESA working title: Laplace). The European-led component of ESJM is an orbiter to Ganymede (Jupiter Ganymede Orbiter - JGO) which has been considered a high priority target for astrobiology (Blanc et al. 2009). However, the habitability of this planetary body is based just on the presence of water. Considering that the subsurface ocean in this satellite is probably sandwiched between two layers of ice and that the subsurface ice crust may have around (or more than) 100km, the existence of sources capable of sustaining chemical complexity as well as effective sources of energy are highly improbable (Rampelotto 2010b). For these reasons, Ganymede seems not to be able to sustain life, and consequently, although not discarded, its astrobiological potential remains quite low.

4. A Mission to Titan

Titan, as revealed by Cassini-Huygens mission, is a complex world with diverse geophysical and atmospheric processes offering many similarities with the primitive Earth (Coustenis & Taylor 2008). In situ measurements indicate the presence of methane rain on Titan (Tokano et al. 2006; Lunine & Atreya 2008) and liquid methane-ethane lakes on Titan’s geologically active surface have been confirmed by the Cassini-Huygens mission (Stofan et al. 2007).

Despite its frigid temperature, the coupled atmosphere/surface Titan system is an active laboratory for synthesis of complex organic compounds. The abundance of methane and its organic products in the atmosphere, seas and dunes exceeds by more than an order of magnitude the carbon inventory in the Earth’s ocean, biosphere and fossil fuel reservoirs (Lorenz & Lunine 2009).

The initial step of this chemistry is relatively well understood (Lorenz & Mitton 2010). It starts with the dissociation of N2 and CH4 through electron and photon impacts. In these processes C2H2 and HCN play a key role (Imanaka et al. 2010). These key molecules are formed in the high atmosphere. They then diffuse down to the lower levels where they allow the formation of high molecular weight hydrocarbons and nitriles as well as complex organic aerosols (Waite et al. 2009). The molecular composition of these aerosols (also called tholins) has still to be determined. The current understanding of Titan’s organic chemistry has been simulated successfully in the laboratory. These experiments have been produced all types of organic species in the gas phase already detected in Titan’s atmosphere, with relative concentration for most of them (Raulin 2005). Moreover, it is very likely that Titan’s organic chemistry may not only be more complex in the atmosphere (up to and certainly beyond C7 hydrocarbons that the Cassini mass spectrometer was able to measure), but also in the hydrocarbon lakes (Brown & Lebreton 2009). The fraction of tholins deposited on Titan’s surface may be able to take the next evolutionary step by reacting with transient exposures of liquid water provoked by impact melt and cryovolcanism. In fact, a variety of cryovolcanic surface features can be attributed to Titan’s recent internal activity (Lopes 2007), e.g. the 180 km structure Ganesa Macula. Such geological structures would indeed permit aqueous chemistry to occur for centuries or longer (Neish et al., 2006). In addition, the possible presence of a water-ammonia ocean in the depths of Titan, as expected from models of its internal structure (Tobie et al., 2005; Lorenz et al. 2008), may also provide an efficient way to convert simple organics into complex molecules (Fortes, 2000). The temperature and pH dependence of the rates and yields of these reactions have been studied in laboratory (Neish et al. 2009). Oxygen incorporation into organic aerosols is remarkably fast, both in pure water and 13 wt. % ammonia-water. The reactions occur over time scales as short as days and weeks even at temperatures as low as 253 K, producing a product that is high percent oxygen by mass (Neish et al. 2008). These results indicate that even with an atmosphere free of oxygen, the production of compounds with oxygen in its molecular structure is possible on Titan (Neish et al. 2010). Additional laboratory experiments have shown that the interaction of water with organic aerosols can yield amino acids in substantial amounts (~ 1% by mass) (McDonald et al. 1994). Simpler nitriles have been detected in the gas phase on Titan, along with HC3N and possibly C4N2 in the solid phase (Anderson et al. 2009). These nitriles are deposited as condensate on the surface and also can react to form astrobiologically interesting material in water. Consequently, although the extent to which present-day Titan resembles the prebiotic Earth is not clear, Titan may provide key insights into planetary organic chemistry, and possibly insights into chemistry leading to life.

More exotic prebiotic-like processes, involving a non-aqueous solvent, may be occurring today on Titan (Rampelotto 2010d). The most abundant volatile organic product of Titan’s atmospheric chemistry is ethane which accumulates at the surface, in particular in the polar lakes discovered by Cassini (Stofan et al. 2007; Brown et al. 2008). These surface liquid bodies provide an environment where organics can accumulate and chemically differentiate (Raulin 2008). The possibility that this active fluid of methane and ethane on Titan could be a solvent, in which life operates, has been suggested by several authors (Baross et al. 2007). If living systems are flourishing in the ethane/methane lakes and consuming the hydrocarbons produced in the atmosphere, they would be widespread on the surface of Titan and have a global effect on the environment. In this case, some years ago, a study suggested a type of life on Titan, called methanogens (McKay & Smith 2005). According to the researches, microorganisms could react H₂ with organic material (like acetylene or ethane) to derive energy. The waste product in this case would be CH₄. The key conclusion of that study was "The results of the recent Huygens probe could indicate the presence of such life by anomalous depletions of acetylene and ethane as well as hydrogen at the surface"(McKay & Smith 2005). Recent studies seem to corroborate with this hypothesis. Based on a computer calculation, one of these studies predicts a strong flux of hydrogen into the Titan’s surface (Strobel 2010) and another paper reported depletions of acetylene at the surface (Clark et al. 2010). Moreover, it is known that ethane on the surface is lower than expected, while Ch4 is abundant on Titan (Rampelotto 2010a). Although these results are not evidence of life, they are extremely interesting.

Altogether, the arguments presented in this section indicate the potential of Titan for science and especially for astrobiological studies. It is a promising field of research with the potential to improve our knowledge in a full range of planetary science disciplines including geology, geophysics, chemistry, atmospheres and related areas. In addition, it may represent a local to understand prebiotic process of "life as we know it" as well as to search for exotic forms of life. Furthermore, such studies may improve the design of life-detection experiments on exoplanets, since titan-like planets may be common in the universe. Indeed, planets around the most common stellar type, the cool M dwarfs, at the distance of the Earth from the Sun will be as cold as Titan (Lunine 2009).

For these reasons, Titan is the highest priority for astrobiological studies in the solar system. Exploration of this complex world, with diverse geophysical and atmospheric processes, will require long-term orbiting and in situ exploration, at a season different than Cassini- Huygens. The primary architecture of such mission, the Titan Saturn System Mission (TSSM), has already been designed, reviewed and improved (TSSM Final Report 2009; Coustenis et al. 2009). While the Titan-dedicated orbiter would provides a set of mapping and profiling instruments for the surface, atmosphere, and ionosphere, a comprehensive in situ investigation would be accomplished via a hot-air balloon, which has been carefully studied by U.S. and European space agencies, for surface high resolution imaging and atmospheric measurements, and a probe with the capability to land on the surface of a hydrocarbon lake to study its composition and search for potential exotic forms of life.

5. A Mission to Enceladus

Although the main focus of TSSM will be Titan, this Flagship Mission would perform several flybys on Enceladus, a Saturn’s satellite that has attracted considerable attention from the scientific community when the Cassini spacecraft identified a water plume venting from near its south polar area (Porco et al. 2006). The source is a series of parallel fissures, nicknamed the "tiger stripes", which propels the plumes 80 km above the surface. The water ice particles present in the plumes appear to be the source of the E ring of Saturn (Kempf et al. 2010) indicating that the activity has been occurring for a considerable period of time. Such cryovolcanic activity suggests that there is ample heat available to drive chemistry in Enceladus’ interior (Matson et al. 2007; Abramov & Spencer 2009), and the regions around the fissures of these cryo-volcanoes have been extensively resurfaced through time (Schenk et al. 2009; Helfenstein 2010). Although the physical mechanism for production of the heat is being debated, there is no question that a significant and persistent heat source is present (Ingersoll et al. 2009).

The observed chemistry of Enceladus is remarkable in the context of astrobiology. Organics, methane, carbon dioxide, carbon monoxide, and possibly several nitrogen species have been observed on the surface or in the plume material (Waite et al. 2006; Brown et al. 2006; Hedman et al. 2009). Part of the material ejected may return and deposit on the surface (Tobie et al. 2010). Since the Enceladus’ surface may not be heavily exposed to radiation, the compound deposited on the surface may remain unaltered and easily detected by an orbiter. The probable presence of nitrogen and carbon-based compounds in the plume and organics along the tiger stripes suggests that the prebiotic synthesis of amino acids could take place in Enceladus’ interior. The main question regarding this moon is the presence of a subsurface liquid layer. In this quest, recent studies have provided support that Enceladus has a subsurface ocean, either global or possibly localized (Collins and Goodman 2007; Tyler 2009), beneath an ice shell between 80 to 100 km thick. The possibility that this subsurface ocean may be in contact with a rocky core of radius ~ 150-160 km has been also suggested (Barr and McKinnon 2007; Schubert et al. 2007). Such rocky/water contact may provide a warm and chemically rich environment that may facilitate complex organic chemistry and biological processes in a similar way proposed to Europa (Parkinson et al. 2007). Unlike the icy Galilean satellites, whose geological activity may have occurred millions of years ago, Enceladus is active today and presents a clear geochemical cycle (Spencer et al. 2009). The existence of a complex chemistry in an aqueous solvent, sustained by cryovolcanic activity and tectonic resurfacing, associated with a persistent source of free energy, provides the fundamental requirements for the origin and persistence of life on Enceladus (McKay et al. 2006; Parkinson et al. 2006). For these reasons, Enceladus has been ranked as the second most promising target in the outer solar system for astrobiological studies, ahead of the Galilean satellites (Rampelotto 2010b).

What is really interesting in the case of Enceladus is that the search for life on it may not require a lander or ice penetrator. The plume provides an opportunity to sample its deep interior. Hence, an orbiter properly equipped will be able to collect or analyze samples from the plumes in the search for biosignatures. The capacity of the TSSM will greatly exceed that of Cassini by providing superior mass spectroscopy of the Enceladus plume, higher resolution infrared imaging and spectroscopy of the surface, and radar altimetry of the active regions at the South Pole. Altogether, these measurements would give a complete understanding about the habitability of Enceladus.

6. Concluding Remarks

The present work discussed the astrobiological potential of planetary bodies in the outer Solar System in order to establish priorities for future missions. According to the arguments previously pointed out, Titan and Enceladus represent the highest priorities for future missions in the context of astrobiology. Considering the proximity of these planetary bodies, a single Flagship Mission for both targets would be the best option. The TSSM is clearly justified and should be performed in terms of its current concept. The complexity of this endeavor poses technical challenges requiring additional studies and technology development, specially improved technology for the balloon and lake lander. However, considering the fundamental contribution to science, a mission to Titan is worthy of all the financial and technical support that will be necessary. It may be succeeded worldwide, with the participation of more research institutes and universities with scientific and technological experience in space exploration.

The EJSM is not justified in terms of its main scientific goal (The emergence of habitable worlds around gas giants) since Ganymede has low priority in terms of astrobiological potential. Furthermore, the current concept of the EJSM will not be able to fully address the habitability of Europa, which may take many decades to happen. For these reasons, an alternative architecture for the EJSM, with focus on Europa and including in situ instrumentation, should be studied.