molecular absorptions and emissions originate. A strong laboratory effort must 

 therefore be carried out in conjunction with the observational program. 



Suggestions for Further Reading 



Encrenaz, T.: Primordial Matter in the Outer Solar System: A Study of 

 its Chemical Composition from Remote Spectroscopic Analysis. Space Science 

 Reviews, vol. 38, 1984, p. 35. 



Trafton, L.: The Atmospheres of the Outer Planets and Satellites. Reviews 

 of Geophysics and Space Physics, vol. 19, 1981, p. 43. 



3.4 Titan 



Some of the most interesting discoveries of the Voyager mission concern 

 Saturn's planet-sized moon, Titan. Titan, like the Earth, has an atmosphere 

 dominated by nitrogen with a surface pressure comparable to the terrestrial 

 value; its surface is strongly suspected to be (at least partly) liquid, possibly 

 composed of a mixture of methane and ethane. A number of organic molecules 

 have been identified on Titan: hydrogen cyanide (HCN), cyanoacetylene 

 (HC 3 N), and cyanogen (CN) 2 . These are postulated to be products of methane 

 (CH 4 ) and nitrogen (N 2 ) photodissociation reactions. Thus, Titan may serve as a 

 Miller-Urey-type model of a highly reduced early Earth atmosphere in which the 

 first stages of organic chemical evolution could take place in the atmosphere. 

 Indeed, laboratory simulations show that a Titan-like atmosphere, primarily a 

 N 2 /CH 4 mixture, under various energetic excitations forms not only HCN, 

 (CN) 2 , and HC 3 N (observed on Titan), but also CH 3 CN, (HCN) 4 , and finally, 

 adenine (HCN) S , a component of DNA. Titan thus appears as an object of 

 exceptional interest for exobiology. 



It is likely that Titan will be extensively explored by a space mission at the 

 end of the century: a joint ESA-NASA project, Cassini, is now under study. 

 Before this exploration, many observations need to be performed from Earth 

 orbit. Observing Titan is difficult since it is small and faint. Sensitive Earth-orbit 

 observations of Titan in the ultraviolet, visible, and infrared spectral ranges could 

 significantly improve our knowledge in the 10 to 15 forthcoming years and 

 provide valuable data complementing that which Cassini may provide. 



The ultraviolet and visible spectral ranges are suitable for studying Titan's 

 photochemistry. Two types of observations can be identified. 



The first is imaging Titan in the ultraviolet and visible regions. This imaging 

 can be accomplished in various bandpasses to within 0.1-arcsec resolution using 

 the Space Telescope wide-field/planetary camera (WF/PC) or the faint-object 

 camera (FOC). This camera would give about eight resolution elements across 



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