Conclusions

Although exotic life, such as methanogenic life in liquid methane, cannot be fully ruled out (McKay and Smith, 2005), the presence of extant or extinct life on Titan seems very unlikely. Nevertheless, with the new observational data provided by the Cassini-Huygens mission, the largest satellite of Saturn looks more than ever like a very interesting object for astrobiology. The several analogies of this exotic and cold planetary body with the Earth and the complex organic chemical processes which are going on now on Titan provide a fantastic means to better understand the prebiotic processes which are no longer reachable on the Earth, at the scale and within the whole complexity of a planetary environment.

The origin and cycle of methane on Titan illustrates the whole complexity of Titan's system. Methane may be stored in large amounts in the interior of the satellite, in the form of clathrates (methane hydrates) trapped during the formation of the satellite from the Saturnian subnebula where it was formed by Fisher-Tropsch processes (Sekine et al., 2005). It may also be produced through high-pressure processes, like serpentinization, allowing the formation of H2 by reaction of H2O with ultramafic rocks, or by cometary impact (Kress and McKay, 2004). Interestingly, those processes have rarely been considered in the case of the primitive Earth, although they may have contributed to a possible reducing character of the primordial atmosphere of our planet, as mentioned earlier in this chapter. This is an example of how Titan's study is indeed providing new insights into terrestrial chemical evolution.

In Titan's atmosphere, methane is photolysed by solar ultraviolet radiation, producing mainly ethane and tholin-like organic matter. The resulting lifetime of methane in Titan's atmosphere is relatively short (about 10-30 My). Thus methane stored in Titan's interior may be continuously replenishing the atmosphere, through degassing induced by cryovolcanism. This volcanic activity at low temperature, where terrestrial lava is replaced by water, has been clearly evidenced from the first images of Titan's surface provided by the visual and infrared mapping spectrometer (VIMS), ISS, and radar instruments on Cassini (Sotin et al., 2005). It may also be released episodically to the atmosphere, as suggested by Tobie et al. (2006). In any case, the methane cycle should result in the accumulation of large amounts or complex organics on the surface and large amounts of ethane, which mixed with the dissolved atmospheric methane should form liquid bodies on the surface or

Fig. 14.9. One of the largest impact craters (about 80 km diameter) observed on Titan's surface by two Cassini instruments: VIMS infrared (left), radar image (centre) and false-colour image (right). The faint halo, slightly darker than the surrounding parts, is probably somewhat different in composition. Since it is made of material excavated when the crater was formed, this indicates that the composition of Titan's upper crust varies with depth, Image courtesy of NASA/JPL/University of Arizona.

Fig. 14.9. One of the largest impact craters (about 80 km diameter) observed on Titan's surface by two Cassini instruments: VIMS infrared (left), radar image (centre) and false-colour image (right). The faint halo, slightly darker than the surrounding parts, is probably somewhat different in composition. Since it is made of material excavated when the crater was formed, this indicates that the composition of Titan's upper crust varies with depth, Image courtesy of NASA/JPL/University of Arizona.

in the near subsurface of the satellite. It is possible that the dark feature seen in Figure 14.3 is one of these expected liquid bodies.

The Cassini-Huygens mission is far from complete. It will continue its systematic exploration of the Saturnian system up to 2008, and probably to 2011 if the extended mission is accepted. Numerous data of paramount importance for astrobiology are still expected from several of its instruments (Table 14.1). The CIRS spectrometer should be able to detect new organic species in the atmosphere during the future limb observation of Titan, especially at the pole. ISS and VIMS should provide a detailed picture of Titan's surface revealing the complexity but also the physical and chemical nature of this surface and its diversity, as evidenced by the discovery of a5-|im bright spot (Barnes et al., 2005). Radar observation will also continue the systematic coverage of Titan's surface which shows contrasted regions of smooth and rough areas, suggesting a possible shoreline. The coupled observation of the same regions by these instruments, which has already started (Figure 14.9), will surely bring essential new information to our understanding of this new, exotic and astonishing world.

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