67 



We can find in this example another important property of 

 water that makes it well-suited as a liquid medium for life. When 

 ammonia is photodissociated, the reaction products are nitrogen and 

 hydrogen, neither of which protects the ammonia from further 

 photodissociation. It thus seems problematical whether one can ever 

 have an environment in which liquid ammonia is stable. In contrast, 

 the oxygen produced by the breakup of water can act to shield the 

 water while also providing a potential source of chemical energy (far 

 more accessible than nitrogen) for evolving life. An alternative would 

 be to provide a UV-protective smog layer, such as Titan in fact seems 

 to possess. The trick is then to maintain a warm enough surface to 

 have liquid ammonia but a cold trap high in the atmosphere that pre- 

 vents the ammonia from diffusing up to altitudes where the smog 

 cannot protect it. On Titan, no such ammonia sea is present, but per- 

 haps it is present somewhere far away. 



The outer planets themselves are less promising. There is ample 

 evidence for chemical reactions, particularly on Jupiter where we see 

 a variety of colors among the clouds (fig. IV-6). The expected con- 

 densed ammonia, ammonium hydrosulfide, ammonium hydroxide, 

 and water all preclude white clouds. Hence, the existence of colors 

 indicates that more complex, nonequilibrium compounds are being 

 formed. With solar ultraviolet light, lightning storms, bombardment 

 by charged particles, and escaping internal heat all available as energy 

 sources, we have a giant natural laboratory in which experiments 

 bearing on the first stages of chemical evolution are continuously 

 being performed. At the present time, these colored substances have 

 not been identified. It is becoming increasingly evident that we shall 

 have to probe Jupiter to solve this problem. 



THE ANCIENT SURFACES 



The recent exploration of the planets has given us a model of 

 the early Earth, but what terrestrial evidence do we have of what our 

 Earth looked like in the past? The most ancient metamorphosed sedi- 

 mentary rocks now known are those at Isua in western Greenland. 

 They were deposited roughly 3.8 b.y. ago as sediments carried by 

 water into a volcanic basin to which volcanos contributed solids and 



