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reminder that the early history of the Earth was more turbulent than 

 a simple volcanism model would suggest. 



We have seen how interesting and important it is to find rocks 

 from the earliest possible times in the Earth's history. Yet, it has 

 been much easier to find ancient rocks on the Moon, where many 

 samples have been dated at 4 b.y. or more. Unfortunately, these 

 ancient lunar rocks do not tell us about the Earth's primitive atmo- 

 spheric conditions, since the Moon evidently possessed no long- 

 enduring, substantial atmosphere, even in those early times. The 

 lunar rocks are grossly deficient in volatile elements and compounds 

 compared with the Earth, suggesting that they were thoroughly 

 degassed but that the gases escaped rapidly into space. 



There are large regions on the surface of Mars that exhibit crater 

 densities similar to those found on the lunar highlands, the oldest 

 region of the lunar surface. Although there is still some dispute about 

 absolute chronologies, this similarity in the distribution of impact- 

 craters has led several investigators to suggest that these regions of 

 the Martian surface are probably as old as the comparable areas on 

 the Moon. Thus, it seems reasonable to expect that on Mars as on the 

 Moon, rocks with ages greater than 4 b.y. should be reasonably abun- 

 dant, provided one were to go to the right region of the planet to 

 look for them. 



The great difference between Mars and the Moon is that Mars 

 has an atmosphere. This atmosphere has apparently developed from 

 an inventory of volatiles very similar in composition to the one that 

 formed the atmosphere of Earth. Indeed, there is every reason to 

 expect that the first steps in the development of the atmospheres on 

 these two bodies were essentially identical. Although we would 

 expect hydrogen to escape more rapidly from the low gravitational 

 field of Mars, this may have been partially compensated for by the 

 lower temperature of the exosphere on this more distant planet. 

 Thus, we can suggest that if Earth ever had a strongly reducing atmo- 

 sphere, Mars probably did too. 



This probability lends a special piquancy to the search for 

 ancient rocks on Mars. If we could find such rocks in a suitably pro- 

 tected setting, we would have an opportunity to test the possibility 

 that the early Martian atmosphere was strongly reducing by examin- 

 ing the mineral assemblages that the rocks contain. Our conclusions 



