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remarkable property of matter is indeed something that springs forth 

 from natural causes wherever the conditions are right — these goals 

 are surely worth the effort and expense. 



A few remarks of caution may be of value. Now that we have a 

 basis for comparison of the early history of the Earth's atmosphere 

 with the present state of neighbors Mars and Venus, the conventional 

 wisdom seems far from secure. Instead of the strongly reducing 

 atmosphere rich in hydrogen — the methane, water, and ammonia 

 mix of two decades ago — the best guesses now suggest a mixture still 

 clearly reducing, but much less strongly, under the dominance of 

 water with the oxides of carbon. No doubt the inferences are sen- 

 sible, but are they unique? We need to recall that the comparison 

 among planets must also include the history of the Sun. The solar 

 models almost certainly require that the Sun slowly brighten; in the 

 time since Earth formation the solar input has risen by a third or so, 

 the same effect as a change in Earth orbit by 15%. So the old Earth 

 and the present Earth are themselves two planets at different dis- 

 tances, so to speak. It is true that the solar inputs to Venus, Earth, 

 and Mars are in the ratio of about 4:2:1; this difference is much 

 larger than that from solar evolution. But is the input change to be 

 neglected? Did it make some difference to the origin of life, a differ- 

 ence less important once life is vigorous, perhaps because of some 

 feedback effect of life itself? The topic can serve to remind us that 

 the early history of the planets is still only inferential, hardly part of 

 a secure understanding. We should take care that the present best 

 view does not jell into dogma. 



SUGGESTIONS FOR FURTHER READING 



Billingham, J., Ed.; Life in the Universe, NASA CP-2156, 1981. 

 Goldsmith, D.; and Owen, T.: The Search for Life in the Universe, 



Benjamin/Cummings Co., Inc., Menlo Park, California, 1980. 

 Walker, J. C. G.: Evolution of the Atmosphere, Macmillan, 



New York, 1977. 



