Life on Exomoons

Although entirely in the realm of speculation it is nonetheless interesting to consider what life might have to deal with on exomoons. For example, tidal heating may be episodic on geological timescales if driven by moons wandering in and out of resonance conditions. If a moon has a sufficiently massive silicate/metal core then in certain situations radiogenic heating, combined with the insulating properties of the outer ice crust, could maintain a partial sub-surface liquid water environment, close to the core. Upon re-entering a period of tidal heating this liquid zone would expand until a new equilibrium is reached. Biota might be capable of dealing with these extended "deep-freezes".

If a combination of tidal and radiogenic heating maintain a heat flow between a silicate/metal core and a body of liquid water then the opportunity for hydrothermal systems akin to those on Earth arises (Lowell & DuBose (2005), and see Fig. 11.8). Biota associated with such systems do not rely on photosynthetic chemical pathways (such as considered for the upper ocean environment on Europa, Sect. 11.1) -although there is intriguing evidence that some terrestrial species can indeed harvest the infrared photon flux from deep ocean vents (Beatty et al., 2005). Given the possibility that even on Earth these systems could play a key role in the origin of life (e.g. Gold, 1992) it is reasonable to speculate that they could be of central importance to life on moons.

It is also important to remember that moons inhabit a rather unique orbital architecture. Based on our own Solar System it appears that they will often share their environment with other nearby satellites. This raises some interesting possibilities for systems containing multiple habitable objects. There is likely to be (especially during early epochs) significant transfer of material between moons due to spallation-like impacts (Melosh, 1984). The short dynamical timescale of the system can result in quite rapid transfer, and the possibility of extensive exchange of both organic chemistry and even active biological material.

Moons will in general also be subject to diurnal total shadowing, or eclipse, events by their host planet. Although such periods would often be short (depending on orbital geometry), they could act to constantly perturb any surface climate. This is particularly relevant for high mass moons that could best represent a terrestriallike habitable environment. While some limited studies have been made on the climatic effect of strong annual variations in stellar flux (i.e. for planets on eccentric

Fig. 11.8. A terrestrial hydrothermal vent system located at an ocean depth of approximately 2km on the Endeavor Ridge in the Pacific Ocean. Superheated (300°) water, rich in minerals, is being forced up from the ocean floor. On cooling, mineral deposition forms the "chimney" structures seen in the center of this picture. Surrounding these is an oasis of life, from extremophilic microbial species to the remarkable tube-worm colony on the left (Photo: V. J. Tunnicliffe, University of Victoria).

Fig. 11.8. A terrestrial hydrothermal vent system located at an ocean depth of approximately 2km on the Endeavor Ridge in the Pacific Ocean. Superheated (300°) water, rich in minerals, is being forced up from the ocean floor. On cooling, mineral deposition forms the "chimney" structures seen in the center of this picture. Surrounding these is an oasis of life, from extremophilic microbial species to the remarkable tube-worm colony on the left (Photo: V. J. Tunnicliffe, University of Victoria).

orbits, Williams & Pollard, 2002), little is known about the impact of rapid "on/off" insolation.

The magnetosphere of a giant planet may represent a difficult radiation environment for life on moons. However, it may also serve to alter the internal dynamics of a moon with consequences for life. The case of Ganymede's intrinsic magnetic field is intriguing in this respect, and it has been suggested that the moon's liquid core dynamo could in fact have been "spun up" by Jupiter's magnetosphere (Kivelson et al., 1998). Here then is another route to maintaining, or encouraging, a molten interior and all that follows in terms of potential habitability. It is also true that a giant planet's magnetic field may vary considerably. Saturn's field strength is some 30 times weaker than that of Jupiter (although still 580 times that of the Earth). The degree to which charged particles are trapped and accelerated may therefore vary significantly from planet to planet.

11.6 Summary

Are moons of exoplanets likely habitats for life? Certainly within our own Solar System there are at least two moons (Europa and Enceladus) that appear to meet some minimum criteria to encourage our interest in searching for evidence of life, and there may be more (Ganymede and Titan). Our expanding view of the diversity of life on Earth has created a plethora of variations on the classical ideas of hab-itability - from hot to cold, and chemically reactive to inert environments. On the basis of these facts alone it would seem reasonable to consider that moons elsewhere could be equally as interesting as terrestrial-type planets. The added temptation is that moons are likely to exist in a multitude of different physical, chemical, and thermodynamical configurations, including those resembling that of the Earth -potentially all within a single planetary system. It would be prudent to allow for the possibility that our terrestrial-centric view of life needs yet another revision.

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