1. Introduction

The field of astrobiology has received renewed attention in the last decade. This is partly due to the almost exponential increase in the number of detected exoplanets – planetary systems around other stars in our galaxy. However, increasing resolution of imaging, and quality of analysis of bodies within our own solar system have broadened our knowledge, and allowed speculation of astrobiological potential for life other than on Earth in our own solar system (Istock 2010; Lal 2010). Flyby imaging and analysis provided by missions such as Cassini-Huygens and the Galileo probe have revealed details of planets and their satellites in unrivalled detail (Porco et al. 2003; Belton et al. 1996).

Further, increasing research into the capabilities of life on Earth today and earlier in its history are assisting in understanding where and how life may be found elsewhere. In particular, it is shedding light on where in our solar system life may be supported.

2. Astrobiology and the Conditions for Life

Previous astrobiological searches, including the Viking landers on Mars and their dry chemistry experiments, and the ongoing SETI radio astronomy program, have yielded disappointing results. However, a new age of astrobiology takes a more anticipatory approach in trying to understand the habitability of various environments for different kinds of life, before launching ambitions and expensive exploratory missions. Notably, the forging of NASA’s Virtual Planetary Laboratory amongst other ventures, heralds a new approach in planetary modelling and a more mature understanding of astrobiological potential in our solar system and beyond. Astrobiology today involves looking at the past and present capabilities of life on Earth and extrapolating that out into cosmic settings.

An initial and critical question in the search for extraterrestrial life is: what exactly is life? NASA has concisely defined it as ‘a self contained, self replicating unit capable of Darwinian evolution’ (Luisi 1998). In real identification terms, the simplest life form that we can yet recognise is a membrane bound prokaryotic cell (Fleischaker & Margulis 1986). These cells have certain energy and metabolic requirements which restrict their distribution on Earth and, by extension, in the cosmos. A liquid solvent, an energy and electron source, and an electron donor all necessary. Given the presence of these, there are limits on the range of temperature, pH, water activity, salinity, radioactivity and other factors that life forms, even extremophiles, can withstand (Horikoshi 2010; Cavicchioli 2002).

3. Habitable Zones and Habitable Niches

When searching for life in the cosmos the simplest method would be to follow the critical requirements for life, that is the liquid solvent and the energy source. In particular, the classic search location has been in the ‘habitable zone’, defined as an area where liquid water can be maintained on the surface of a planet (Franck et al. 2000; Lammer et al. 2009). A distinction must be made between a Circumstellar Habitable Zone (CHZ) which is a favourable position relevant to the star, and a Galactic Habitable Zone (GHZ) which defines a favourable position relative to the galaxy (González et al. 2001; Lineweaver et al. 2004). The CHZ is a shell around a star centred at a distance proportional to the luminosity of that star. In our solar system, the habitable zone is thought to range between 0.725 and 0.95 AU for the inner edge (Dole 1964; Kasting et al. 1993), and 1.01 to 3 AU for the outer edge (Hart 1979; Fogg 1992). With this reconstruction, Mars lies in the more generous estimate of the outer edge, and Venus at aphelion lies inside the inner edge. However, no liquid water has been observed on these planets (although some geomorphological evidence for episodic water flow has been observed on Mars (Carr & Waenke 1991; Malin & Edgett 2000) and correspondingly, with the exception of the now critically disproved Martian meteorite fossil (McKay et al. 1996; Scott et al. 1997; Barber & Scott 2002; Becker et al. 1999), no signs of extra-terrestrial life has yet been identified. 5 Advances in our understanding of life and the capabilities of different metabolisms are helping to extend the defined boundaries of habitability.

Firstly, surface water may not be necessary for life. The discovery of bacteria deep in Earth’s demonstrates that the suns energy is not necessary to maintain a habitable environment (Mason et al. 2010). Instead, the thermal energy may be provided from an internal radioactive source or from tidal heating of a rocky body Secondly, the carbon based life that is found on Earth is not the only possibility that satisfies the NASA definition of life. Recent reports of arsenic substituted into the phosphatic backbone of DNA demonstrates that differing replication and chemistries may be possible (Wolfe-Simon et al. 2010). This is particularly relevant when considering the possibilities for life using solvents other than water.

While these possibilities considerably add to the range of habitable places, it is incorrect to simply extend the reach of the CHZ around our star. Instead, ‘habitable niches’ may be identified in specific places depending on the configuration and composition of planets around a star, and the satellites around those planets. CHZ’s may still yet be a useful tool in exploring potential habitability of extrasolar planets, where the fine detail of a distant planetary system is difficult to ascertain, and the presence of water on a planet is the best target for extraterrestrial life. For exploration of our own solar system however, we can look further than simply surface water into the richly diverse potential habitats afforded outside the traditional definition of a habitable zone.

4. An Analogy: Habitable niches on the Early Earth

The search for life in the fossil record of the early Earth can be compared to the search for extraterrestrial life, both being classified under the description of Astrobiology, and the analogy holds out when considering habitable zones and niches.

During the Proterozoic era (2500 – 542 Ma) simple prokaryotic life forms, which has appeared during the preceding Archaean (3800 – 2500 Ma), started to diversify into the complex eukaryote and ultimately multicellular forms that were the precursors to animals (Knoll et al. 2006; Javaux et al. 2001; Javaux et al. 2003). While some fossil evidence for this exists, it is limited, microscopic, and often poorly preserved because of its great age. In general, the search for these early eukaryotic organisms has taken place in rocks that were deposited in Proterozoic oceans. Recent research, however, has uncovered fossils of diverse Eukaryotic organisms in rocks that were laid down in lacustrine settings 1000 Myr ago (Strother et al. 2010). These fossils, from the Torridonian rocks of northwest Scotland, are exceptionally preserved in cryptocrystalline sedimentary phosphate, a medium not previously reported, and the assemblage contains morphotypes that have not been observed in marine rocks. The discovery of eukaryotic fossils in lakes is remarkable, as certain adaptations to fresh water and the risks of desiccation are unexpected in organisms of this age. The lakes may be considered a ‘habitable niche’ for specific, well adapted organisms, when compared to the overall ‘habitable zone’ of the Proterozoic ocean.

5. Searching for life on Satellites in our solar system

Various features of a planetary or satellite body are likely to be conducive to life as we know it. While not as restrictive as the necessity for liquid surface water in a habitable zone, these general factors will affect the type and location of life on these bodies.

Firstly, the presence of a liquid is essential, although this does not necessarily have to be located on the surface of the body. All life as we understand it requires a liquid solvent for its metabolic processes to function. In the classic habitable zone, this is maintained on the surface by energy directly from the sun, but can be located elsewhere and sustained by other forces, as seen in the Jovian and Saturnian systems. Water may be maintained deep beneath the surface of a planet by heating from within, either caused by intrinsic radioactive decay, or by tidal heating of the body, induced by orbital resonance with other nearby masses (Mashhoon 1978; Reynolds et al. 1987). This water may form between different crystallising forms of ice - as in Ganymede (Spohn & Schubert 2003), or beneath a thick homogenous icy crust - as in Europa (Carr et al. 1998), and possibly Callisto (Spohn & Schubert 2003) and Titan (Lunine & Stevenson 1987). Life may be supported in these ice-covered oceans through a variety of intrinsic and extrinsic energy and matter exchanges (Gaidos et al. 1999) Further, water may be prevented from freezing at temperatures below 0°C by the presence of ammonia or salts in small quantities. On the surface of the satellites, temperatures are too low, even given internal heating, to sustain liquid water. However, surface solvents may exist as liquid forms of other compounds, for example methane and ethane on Titan, where they form lakes in the solid surface of water ice (Stofan et al. 2007).

Given that a liquid solvent exists, the nature of that solvent will fundamentally determine the metabolism and nature any potential life within it. In cases where liquid water exists below the surface, the same energy fixing metabolism seen in terrestrial organisms is likely to still apply, whereas in a methane solvent such as that found in the lakes on the surface of Titan, the electron donors, acceptors and final products will differ considerably. For example H₂ may be used in place of O₂, acteylene in place of glucose, releasing CH₄ in place of CO₂ in a standard respiring metabolism. Ultimately, the type of metabolism prevalent may be detectable through remote sensing in the mixing ratio of the atmosphere (McKay & Smith 2005).

At an average distance (of the planets themselves) of 9.58 Au and 5.2 Au for Saturn and Jupiter respectively, the primary thermal energy from the sun is not sufficient to raise the temperature of the surface bodies much above -150 °C. A combination of a minor internal radioactive heat source, (in the case of bodies with an iron core (Io, Ganymede, Europa (Kuskov & Kronrod 2001), and an internal heating driven by orbital resonance with other nearby bodies (Mashhoon 1978) can help to raise the temperature below the surface however. For example, Io, Europa and Ganymede are linked in a 4:2:1 orbital resonance, leading to considerable tidal heading in these bodies (Peale 1999). Where this heat is great enough to melt ice to create liquid water oceans (assisted in some satellites by the minor anti-freezing properties of salts and ammonia) it is reasonable to assume that it is enough to sustain the metabolism of a carbon based life form. Microorganisms on earth have been found living at temperatures as low as –15°C, so this further extends the potential range (Morita 1975).

As with the solvent, the presence and composition of gaseous compounds in any atmosphere present can also give clues to the nature of any potential metabolic processes acting at the surface. The presence of an atmosphere is usually considered a bonus to life as it protects delicate organic structures from the deleterious effects of UV radiation, and may also provide an extra heat source through greenhouse heating (as is the case with Titan(Kondratyev & Moskalenko 1985). The composition of an abiogenic atmosphere may be modified by any metabolising organism present as electron donating compounds may be exploited and then become depleted if used in sufficient quantity. Similarly, electron acceptors and waste products of the metabolic process may exceed their equilibrium values. Even if disequilibrium is not detected, the kinds of compounds found in any atmosphere can give a clue to the types of metabolism that may be present and operating at levels lower than the threshold of detection. For example, the patchy sulphur-rich atmosphere of Io could support a sulphate reducing form of life (Schulze-Makuch 2010).

The Earth’s magnetic field protects the planet from the intense solar wind, which would strip away our atmosphere in its absence. The solar wind is not as strong at the greater distances of Jupiter and Saturn, and so the magnetosphere of less importance, but it will still offer some protection to the often tenuous and thin atmospheres of the satellites. In this particular case also, the satellites are almost all well protected by the magnetospheres of their host planet (excluding Callisto) with their own sometimes induced in a solid iron core (as in Europa (Kuskov & Kronrod 2001), or self generated with a convecting liquid iron core (as in Ganymede (Kuskov & Kronrod 2001).

The UV radiation on Earth reaches an average of only 0.17 rem/day at sea level and it has been shown that irradiation above 500 rem can critically damage delicate organic structures (Tranvik & Bertilsson 2001). Thus areas with more than this amount would be very unlikely to sustain terrestrial carbon based life forms. Radiative levels are not so high, out as far as Jupiter and Saturn, but levels of 540 rem/day as found on Europa (Ringwald 2009), would still be damaging to any surface inhabiting organisms. Life must instead thrive where levels are lower (e.g. Callisto (Ringwald 2009), or deeper beneath the surface where the radiation cannot penetrate.

The basic organic molecules, chains of hydrocarbons, can form abiogenically and are common within the solar system, so their presence is not necessarily indicative of life. However, the presence of more complex organic molecules like those identified in Titan’s atmosphere (Raulin & Owen 2002) or a notably overabundance of different compounds in a particular location (e.g. Enceladus (McKay et al. 2008) could suggest their utilisation by life, if not their generation. In some cases, detrital surface material which is yet to recieve spectroscopic identification such as those seen in the moons of the Amalthea group of the Jovian system, may represent exciting cocktails of organic molecules.

Given these general indications and assistances to the origin and maintenance of life on bodies outside the CHZ, planets, satellites and other cosmic bodies may be assessed independently for their potential for extraterrestrial life. Tables 1 and 2 illustrate this principle applied to the Saturnian and Jovian moons respectively. The smaller moons have not been considered as their small size and chaotic rotations as well as extreme eccentricities and inclinations preclude many of the major life inducing factors.

From these data, it is possible to tease out those moons which have higher possibility for extraterrestrial life. These are discussed below.

6. Astrobiological Potential of Jovian Moons

a. Europa One of the more often discussed potential locations for extraterrestrial life in the solar system, Europa is considered a likely habitat because of the large water ocean (~100 km deep) interpreted to exist beneath a crust of ice kilometres to tens of kilometres thick (Carr et al. 1998; Chyba 2000; Chyba & Phillips 2002; Greenberg 2005; Zimmer et al. 2000). Cracks and offsets of the ice visible on the surface suggest deep permeating fissures through which exchange of energy and materials with the underlying ocean may periodically occur (Gaidos & Nimmo 2000). Tidal heating of the interior caused by the 4:2:1 orbital resonance with Io and Ganymede provides the thermal energy to keep the ocean liquid, but is also interpreted to create hot-vent systems at the bottom of the Europan sea, much like those seen at spreading ridge settings on Earth (McCollom 1999; Schulze-Makuch & Irwin 2001). The tenuous oxygenic atmosphere, while not biological in origin, nevertheless may provide an additional source of electron donors and acceptors for potential life metabolism. Sulphurous constituents of Europan surficial material have been identified, (Chela-Flores & Kumar 2008) and although their biological origins remain in dispute (Chela-Flores 2010a), they are nevertheless proposed as a target for future penetrative exploration instrumentation (Chela-Flores 2010b).

b. Ganymede Ganymede is not usually considered for astrobiological potential, although it possesses sufficient of the possible habitable characteristics to make it worth consideration here. A likely ocean of water between layers of ice 200km down in crust (Chyba et al. 1998) could provide a refuge for chemotrophic, pressure resistant life forms. A salt enrichment of this ocean is interpreted from magnesium and sodium salts found on the surface (McCord 2001) and while this would decrease the freezing point of water, limiting the thermal energy available for life, it would nevertheless offer a source of electron acceptors and donors for metabolisms.

Tidal heating due to orbital resonance with Io and Europa creates enough energy to maintain a liquid iron core, as inferred from the independent magnetosphere possessed by the satellite, and so may provide sufficient heating to the ocean to allow metabolic processes to advance. A variety of compounds have been detected at the surface including CO₂, SO2, possibly cyanogen, hydrogen sulphate and organic molecules (McCord 1998). Further work on the equilibrium mixing ratio of these compounds may be used to assess their source and the potential for a biological metabolism. Again, the spatial occurrences of these surficial compounds are suggested as a target for explorative penetrative instrumentation (Chela-Flores 2010b).

c. Callisto Callisto may be considered for astrobiological potential because of a possible ocean of water around 100km below the surface of ice (Fortes 2000). This ocean is inferrred from conductivity measurements of the planet as a whole (Zimmer et al. 2000). A lack of orbital resonance with the other satellites will limit the thermal energy created by tidal heating, and so it is likely that the ocean is maintained below 0°C by the presence of a small amount of ammonia (Ruiz 2001).

In general, the astrobiological potential of Callisto is notable, but less likely than for Europa. The surface of the satellite is heavily createred, indicating an ancient surface which has not been reworked by cryovolcanism or contact with the subsurface ocean. Also, a low likelihood that the ocean is in contact with the silicate rock of the core (Lipps 2004) further blights the chances for the significant heat and energy transfer necessary for life.

Some organic compounds have been found on the surface, along with ice, CO2, and silicates, although their presence does not require a biological origin. Of note is the low radiation level on Callisto’s surface, which would require no particular radiation resistance for any surface dwelling organisms to survive.

7. Astrobiological Potential of Saturnian Moons

a. Enceladus Enceladus has also received considerable attention as a potential habitat for life, after the discovery of the cryogenic plumes from the southern pole (Matson et al. 2007). These plumes of water and ammonia indicate a direct link between a subsurface reservoir of liquid water and the surface. This link, probably most prevalent in the southern area characterised by the ‘tiger stripes’ surface feature, would be very advantageous for a life form living within the reservoirs, as exchange of energy and material is frequent and ongoing. The ammonia detected in the plume is probably acting to keep the water liquid (in combination with tidal heating caused by orbital resonance with Dione (Showman 1997) and may be utilised in the metabolism of any inhabiting life. While a full ocean beneath the surface ice has been proposed (Roberts & Nimmo 2008), the localisation of the plumes and other cryogenic features make it much more likely that liquid water exists only in isolated reservoirs, thus also limiting the likely extent of life.

A wide variety of organic compounds have been observed in the plume and on the surface of the satellite, including simple and complex hydrocarbons, propane, ethane and acetylane (Waite et al. 2006; Waite et al. 2009). It is not yet clear whether they are of biologic origin, although abiogenic sources are quite possible.

b. Tethys It is unlikely that extraterrestrial life persists on Tethys today, as there is no clear evidence for a persisting liquid solvent. Geomorphological evidence from the surface, in the form of an ancient global resurfacing event, suggests that the satellite would have been tectonically active in the past. This may have been caused by a previous orbital resonance with Dione earlier in the history of the satellites’ formation, which has since changed, causing cooling and thus the inferred internal liquid ocean to freeze (Chen & Nimmo 2008). Life may have had time to establish in the moon’s early history, or could have been seeded from nearby satellites, but would now be preserved as a fossil in the icy moon, and would not be detectable except by direct sample.

c. Titan More than one possibility exists for the potential of life on Titan. Most famously, the moon is the only known object in the solar system, other than the earth, to have a liquid solvent at its surface. Titan has methane and ethane lakes (Brown et al. 2008; Lorenz et al. 2008; Naganuma & Sekine 2010), and any inhabiting organism within them would need a metabolism accordingly adapted (McKay & Smith 2005). The nitrogen rich atmosphere provides a greenhouse effect, serving to warm the surface and offering extra thermal energy to metabolic processes. The nitrogenous compounds in the atmosphere, as well as the methane and ethane in the clouds and lakes would provide electron donors and acceptors for a metabolism that is radically different to those seen on earth, but nevertheless plausible (Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Counci 2007).

A wide range of organic molecules have been detected in the atmosphere and on the surface of Titan, including hydrocarbons, tholins, and, potentially, the five nucleotide bases found in the genetic material of all living things on Earth (Horst et al. 2010). While it has been postulated that a disequilibrium of H2 observed in the upper atmospheric layers could indicate life (McKay & Smith 2005), other, abiogenic processes offer more likely explanations, and the case remains open (Strobel 2010). The potential for surface life on Titan is large, and being perhaps the easiest kind of life to identify, is accordingly worthy of attention in future space missions.

A further, less often discussed possibility for life on Titan is the slimmer chance of organisms surviving in a potential subsurface water ocean around 50 km beneath the icy surface of the moon (Fortes 2000; Lorenz et al. 2008). As with Ganymede and others, this life would need to contend with the low exchange of energy and materials with an external environment except for during times of rare cryovolcanism, but is nevertheless plausible.

d. Iapetus No evidence of liquid water or other solvent has been detected or surmised for Iapetus, however note must be made of the range of organic compounds found at the surface, including carbonaceous molecules and cyano- compounds (Cruikshank et al. 2008). A biological explanation for the presence of these is just one of many perhaps more likely abiogenic explanations.

e. Hyperion This irregular shaped moon with its chaotic rotation affected intrinsically by nearby Titan would seem an unlikely place to look for life. It is certainly not a body what could be said to provide a stable refuge for extraterrestrial life forms. No liquid is known at its surface, but the moon’s inferred porosity of 40% (Thomas et al. 2007) may offer interesting internal spaces that could contain strange unquantified chemistries. Reddish material on the surface has been detected, and spectroscopically determined to contain carbon-hydrogen chains, whose presence could have an abiogenic, intrinsic or extrinsic biological explanation.

8. Summary and Future of Search for Life

The above discussion of the astrobiological potential of Jovian and Saturnian moons provides a methodical way of thinking about life outside the classic circumstellar habitable zone.

While it would be tempting to erect a ‘circumplanetary’ habitable zone with which to increase the resolution of the search for extraterrestrial life on satellites around other planets, it is not this simple (Reynolds et al. 1987). Habitability will vary considerably not only with the distance of the host planet from the star, and the distance of the satellite from the planet, but also of the composition and differentiation of the satellite, and the number and proximity of other satellites. In the specific case of the satellites of the Jovian and Saturnian systems, the best approach is shown to be an independent classification of each satellite individually, and prioritisation of further investigation based on those factors.

This summary has shown that the best targets for astrobiological search in the region of Jupiter and Saturn are Titan, Europa, and Enceladus. Other satellites may be considered but the likelihood of them developing or maintaining extraterrestrial life, based on the few supporting characteristics they possess, is small. Correspondingly, several astrobiologically focussed missions to the Saturnian and Jovian systems are under serious consideration under the NASA decadal review 2009-2011, including the Titan lake lander and hot air balloon probe of the Titan-Saturn System Mission (TSSM) (Coustenis et al. 2009), and the Europa-Jupiter System Mission (EJSM) (Lebreton et al. 2009). The combination of a theoretical, anticipatory approach to astrobiology and an informed exploration of candidate planets, satellites and other solar system bodies is the best approach, and is no more than should be expected for the modern age of astrobiology.

Acknowledgements: The author would like to acknowledge the inspiring and informative support offered by Martin D. Brasier, and Lynn Margulis. NERC grant NE/G524060/1 supports this and other related work.