Introduction

"What a wonderful and, amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths, and every one of them stock'd with so many Herbs, Trees and Animals, and adorn'd with so many Seas and Mountains!" "... even the little Gentlemen round Jupiter and Saturn "

Christiaan Huygens (1695, Cosmotheoros)

Huygens clearly felt strongly that life exists on any suitable body, including the "little Gentlemen". These are, of course, the major satellites of Jupiter and Saturn, and it would seem that he had no difficulty in placing them in a category whereby they too would be well "stock'd" with organisms. It was also true, however, that Huygens was aware that at this distance from the Sun, conditions on these bodies would be cold. A simple blackbody thermodynamic model for an illuminated spherical body yields an equilibrium effective temperature at its surface (Sect. 11.3). For the Galilean moons this implies surface temperatures of some ~ 100 K. At Saturn this drops to ~ 85 K. Over the three hundred years following Huygens' statements there was relatively little thought given to the possibility of the Galilean (or Satur-nian) moons being at all suitable for life. A notable exception to this was Proctor (1870), who speculated that Jupiter might be capable of heating its moons to temperate levels. However, the physics behind this assertion was incomplete, and he was motivated by a belief in the plurality of worlds. Interest in the moons of the giant planets began to increase again when Gerard Kuiper and others (e.g. Kuiper, 1957) were able to show, using infrared spectroscopy, that Europa's surface was composed primarily of water ice. Then, during the 1970's, flyby data from the Pioneer - and to a much greater extent the Voyager missions (Smith et al., 1979a,b) - not only confirmed Kuiper's observations but discovered some extraordinary properties for the icy crusts on many moons. In Fig. 11.1 Voyager data partially covering Europa is shown, together with an inset at higher resolution from the later Galileo mission.

Fig. 11.1. The Jovian moon Europa as observed by Voyager 2 in 1979 (Smith et al., 1979b). Very few impact features are seen on the water-ice surface. Extensive streaks and blushes indicate that an outer crust has been fractured, filled in from the interior, and re-frozen many times. Inset: Detailed image of the surface of Europa taken by the Galileo spacecraft in 1997. The area shown is approximately 34 by 42 kilometers in size, with a resolution of 54 meters. Crustal plates of ice are seen, which have broken and then "rafted" together into positions resembling those of terrestrial pack-ice. The size and shape of these plates has suggested a fluid or slush-like environment close to the surface during its breakup (Greeley et al., 1998b).

Fig. 11.1. The Jovian moon Europa as observed by Voyager 2 in 1979 (Smith et al., 1979b). Very few impact features are seen on the water-ice surface. Extensive streaks and blushes indicate that an outer crust has been fractured, filled in from the interior, and re-frozen many times. Inset: Detailed image of the surface of Europa taken by the Galileo spacecraft in 1997. The area shown is approximately 34 by 42 kilometers in size, with a resolution of 54 meters. Crustal plates of ice are seen, which have broken and then "rafted" together into positions resembling those of terrestrial pack-ice. The size and shape of these plates has suggested a fluid or slush-like environment close to the surface during its breakup (Greeley et al., 1998b).

The surface of this moon is not only almost devoid of impact craters (indicating a young age), but is criss-crossed by a remarkable network of shallow ripples, crack-like features, plate-like features, and distinct blemishes of the surface water ice (e.g. Greeley et al., 1998a). Galileo's Near-Infrared Mapping Spectrometer (NIMS) detected evidence for hydrated salts at various surface locations - suggesting evaporation from a globally mixed water layer (McCord et al., 1999). Detailed imaging also indicates regions consistent with low viscosity surface flows (now frozen), and anomalously shallow impact craters (i.e. filled in from the interior, Moore et al., 1998). While all of this evidence points towards the presence of extensive subsurface liquid, the most compelling result comes from the detection of an induced magnetic field (Kivelson et al., 2000; Zimmer et al., 2000). The magnitude and response of this field (to Europa's position relative to the powerful Jovian magnetosphere) indicates the presence of a near-surface, global, conducting layer - consistent with a salty water ocean of at least 10 km thickness. Figure 11.2 illustrates the possible internal structure of Europa in this case.

Cassen et al. (1979) predicted that Europa could be experiencing tidal heating sufficient to maintain a subsurface ocean. The tides arise from its eccentric orbit (e — 0.01) produced by the mean-motion 4:2:1 Laplacian resonance between Io, Europa, and Ganymede. Heating results from flexure between periapsis and apoapsis around Jupiter (Sect. 11.3.1). Following the Voyager data there were significant investigations of the implications of tidal heating for Europa (e.g. Squyres et al., 1983; Melosh et al., 2004), and the implications for habitability within subsurface oceans and episodic fracture zones enabling photosynthetic (Greenberg et

Fig. 11.2. Cutaway illustration of the possible internal structure of Europa (NASA/JPL). The presence of a metallic core and rocky interior is inferred from Europa's mean density, radius, and measurement of its gravitational field during spacecraft flybys. The presence of a liquid water ocean is strongly suggested but not yet confirmed.

Fig. 11.2. Cutaway illustration of the possible internal structure of Europa (NASA/JPL). The presence of a metallic core and rocky interior is inferred from Europa's mean density, radius, and measurement of its gravitational field during spacecraft flybys. The presence of a liquid water ocean is strongly suggested but not yet confirmed.

al., 2000) or non-photosynthetic biospheres to exist (Chyba, 2000). These investigations demonstrated that moons could represent an entirely new class of habitable environment - potentially less dependent on stellar insolation and driven largely by dynamical energy dissipation - which could furthermore influence climate in cases where atmospheres exist (Reynolds et al., 1987). The dynamically dense nature of moon systems, and their evolution due to moon-planet tides, appears to lend itself to situations of moon-moon orbital resonances. External perturbations could also be significant (see below). Furthermore, if the core-accretion model for planet formation is correct (e.g. Lissauer, 1993), then giant planets will form preferentially beyond the so-called "snow line" in a system Hayashi (1981). This may lead to moons naturally accumulating significant icy mantles, thereby circumventing many of the present uncertainties in "dry" versus "wet" formation for terrestrial planets (e.g. Raymond et al., 2006)).

Other moons in our Solar System, which would otherwise be inert, also show evidence for what may be (at least partially) dynamically driven heating. For example, the recent detection by Cassini of water "geysers" on Enceladus in the Saturnian system (e.g. Porco et al., 2006) points towards a remarkably active geology, even on such a small moon, only 500 km in diameter (Fig. 11.3). The precise origin of this activity is currently unresolved (see however Nimmo et al., 2007; Hurford et al., 2007), but is of enormous interest since it may indicate that there are reservoirs of subsurface liquid water. Even the small Uranian moon Ariel shows some evidence for tidally induced heating and cryovolcanism (Melosh et al., 1989).

Fig. 11.3. The Saturnian moon Enceladus back-lit by the Sun as imaged by the Cassini orbiter. A series of discrete, fountain-like sprays (or "geysers") are seen above the southern polar region of the moon. It is likely that these are erupting from subsurface, pressurized pockets or reservoirs of water at temperatures above 273 K (Image credit: NASA/JPL/Space Science Institute, 2005).

Fig. 11.3. The Saturnian moon Enceladus back-lit by the Sun as imaged by the Cassini orbiter. A series of discrete, fountain-like sprays (or "geysers") are seen above the southern polar region of the moon. It is likely that these are erupting from subsurface, pressurized pockets or reservoirs of water at temperatures above 273 K (Image credit: NASA/JPL/Space Science Institute, 2005).

Some moons are large enough to exhibit characteristics that are generally considered to be the province of planets. For example, Titan harbors a thick atmosphere dominated by nitrogen (98.4%) and methane (1.6%) (with a 1.5 atmosphere pressure at its surface), and a rich and complex low-temperature (90-95 K) hydrocarbon surface chemistry. Such an environment may represent conditions that not only were at one time present on the early Earth (although Titan is far colder), but also those which could conceivably allow for alternative forms of life (Lunine et al., 1998). Finally, Ganymede evidently has an extensive icy mantle (although it is slightly larger than Mercury it is only half the mass) and has an internally generated magnetic field (Kivelson et al., 1998), indicating that it may have a molten iron or iron-sulfur core.

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