66 



The development of alien life at these low temperatures seems 

 unlikely in view of the slowness of chemical reactions under these 

 conditions. Yet some very interesting chemistry is taking place on 

 Titan, for we can detect traces of reaction products in the atmo- 

 sphere. In addition to CH 4 , C 2 H 2 , C 2 H 4 , and C 2 H 6 that had been 

 detected by Earth-based telescopes, the Voyager spacecraft suc- 

 ceeded in identifying N 2 , H 2 , HCN, C 2 N 2 , HC 3 N, C 4 H 2 , C 3 H 4 , and 

 C 2 H 8 . Furthermore, the atmosphere is charged with a brownish 

 photochemical aerosol that may include polymers of one or more of 

 these substances. Since hydrogen can escape from Titan, fragments 

 of hydrocarbons that are produced by UV irradiation or charged 

 particle bombardment in the satellite's atmosphere are free to com- 

 bine to form more complex substances. Here, we have a highly 

 evolved atmosphere that has remained reducing, since oxygen is 

 safely trapped as water ice in Titan's interior. 



This lack of liquid water makes the current chemistry on Titan 

 fundamentally different from the chemistry on the primitive Earth. 

 But the chemistry occurring in the Earth's atmosphere in its early 

 history may well have been very similar to what we find on Titan 

 today, making further investigation of this object particularly 

 appealing. It depends, of course, on how reducing our early atmo- 

 sphere was. At this stage of our ignorance, a mixture of CH 4 , N 2 , 

 and H 2 can't be excluded. 



The photochemical reactions are taking place in the satellite's 

 upper atmosphere, which is some 80° C warmer than the surface. 

 The reaction products will gradually settle out and be preserved in 

 this cold trap (or dissolve in liquid methane or liquid nitrogen!). The 

 early history of Titan may have been even more interesting, however. 

 The nitrogen we now find in the atmosphere is presumably the result 

 of the photodissociation of ammonia. In order for the ammonia to 

 get into the atmosphere, Titan must have been much warmer. This 

 would have led to much more methane in the atmosphere as well, 

 so we can imagine an early atmosphere even denser than the present 

 one, which has a surface pressure of 1.6 bars. How warm was the 

 satellite during this early period? How long did this time last? What 

 kinds of chemistry occurred? Here in this cold, alien environment, 

 we find ourselves confronting the same questions we have considered 

 on Mars. 



