Fig. 9.20 The infrared spectrum of Titan observed by the SWS instrument on the ISO space observatory (top), and by the IRIS instrument on Voyager 1 (bottom)

After this discovery, Titan appeared as a unique laboratory for prebiotic chemistry, capable of providing us with information about the first stages in the emergence of life, whether on Earth or in other environments. Note, however, that despite the similarities with Earth, the two objects reveal enormous differences. The first is Titan's low temperature (93 K at the surface): its first effect must be to slow down all chemical reactions by a considerable amount. The second is the atmospheric composition: that of Titan is reducing, which resembles the conditions used in Miller and Urey's simulations. Finally, the internal structure of the Earth and Titan are radically different. On Titan, there is permanent degassing of methane and perhaps active cryovolcanism based on H2O and NH3.

On 14 January 2005, the Huygens probe successfully landed on the surface of Titan (Fig. 9.21). Its first images revealed a relatively flat surface, scattered with heavily eroded pebbles, which probably consist of water ice, and showing signs of fluvial outwash features, probably created by hydrocarbons (Fig. 9.22). The data from the Cassini orbiter and the Huygens probe are still currently being analyzed, and should give us a better understanding of the nature and origin this exceptional satellite.

Fig. 9.21 The surface of Titan as observed by the DISR descent camera on the Huygens probe, 14 January 2005. The rounded pebbles probably consist of water ice (image credit: courtesy ESA; NASA, JPL University of Arizona)

Fig. 9.22 Panorama of the surface of Titan, observed by the Huygens probe at an altitude of 8 km. Clearly visible are the traces of a dark fluvial network (ESA)

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