The Satellites of the Outer Planets

Although too far from the Sun to support liquid water on the surface, the satellites of the outer planets may provide interesting niches for exobiology. In fact, the phase diagram for water shows that liquid water may exist at high pressure within the interiors of these objects, and models of the internal structure of the satellites predict this possibility.

(b)
Fig. 9.17 (a) A Martian network of branching valleys (near Chryse Planitia) compared with (b) a network of dry valleys in Yemen (After Forget et al., 2003)

9.5.2.1 Europa and the Galilean Satellites

The four Galilean satellites of Jupiter were first explored by the Voyager mission in 1979, and then by the Galileo orbiter between 1995 and 2003. With the exception of Io, which is very close to Jupiter and endowed with active volcanism, they are all covered in water ice. Europa (Fig. 9.18), the second Galilean satellite, is the most interesting from the point of view of exobiology. Like Io, but to a lesser extent, it is, in fact, subject to tidal forces produced by its proximity to Jupiter. It is also in resonance with Io and Ganymede. The result is a certain internal energy, sufficient, according to models of its internal structure, to keep water in liquid form, down to the silicate mantle.

The Voyager images, confirmed by those from Galileo, have shown that the surface of Europa consists of a complex network of linear, intersecting structures. The plates that they surround appear to have been displaced relative to one another on a viscous or even liquid medium. The best candidate for this medium is liquid water, probably saline (Fig. 9.19). Another sign appears to reinforce this theory: a magnetic field has been detected by Galileo; it could be generated by a dynamo effect within the medium.

How may the liquid ocean on Europa be explored? A first stage would consist of verifying its existence, by gravimetric measurements or radar sounding from an orbiter. It would then be necessary to evaluate the thickness of the layer of ice, which, according to the models, could be several tens of kilometres thick. The in-situ exploration of the interior of Europa therefore seems, from the outset, to be a long-term project.

Fig. 9.18 The satellite Europa as observed by the Galileo probe (NASA)
Fig. 9.19 An example of the features on the surface of Europa: left, an image from the Galileo probe, right, interpretation of these features in terms of moving plates

Beyond Europa, Ganymede and Callisto are probably too far from Jupiter for tidal effects to generate sufficient energy to produce a liquid ocean deep enough to reach the silicate mantle.

9.5.2.2 Enceladus and the Icy Satellites of Saturn

In Saturn's environment, the satellites that are most likely to possess an internal, liquid ocean are those that undergo sufficiently strong tidal forces. In this respect, Enceladus is a special case. Confirming the first images taken by Voyager, the observations made by the Cassini probe have shown that the region around the south pole is much younger and hotter than the rest of the surface; plumes have also been detected, showing evidence for active cryovolcanism.

Titan, the largest of Saturn's satellites, is exceptional in the outer Solar System: it is the only satellite to possess a thick atmosphere. In addition, that atmosphere, like the Earth's, primarily consists of molecular nitrogen, and its surface pressure (1.5 bars) is close to that of the Earth's atmosphere.

It was the Voyager mission that revealed, in 1980, the nature of Titan, its atmospheric composition, and thermal structure. It also discovered the presence of a large number of hydrocarbon molecules and nitriles (Fig. 9.20). The hydrocarbons (C2H2, C2H4, C2H6, etc.) arose from the photochemistry of methane, as in the case of the gas giants; the presence of nitriles (HCN, C2N2) results from the dissociation of molecular nitrogen by energetic particles from Saturn's magnetosphere. Subsequently, HC3N and CH3CN were also detected. These molecules are precisely those that were predicted from laboratory experiments simulating the synthesis of complex organic molecules from simple mixtures, in the presence of energy sources.

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