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escape of hydrogen, the heavier mass of deuterium should have led to 

 a net enhancement of this isotope in residual hydrogen compounds 

 over geologic time. The alternative to this picture is that Venus had 

 no (or very little) water from the beginning, a result again of its 

 proximity to the Sun; the temperature at which Venus formed was 

 so hot that water could not have condensed. This alternative seems 

 less satisfactory in view of recent models that explain the bombard- 

 ment histories of the inner planets by impacts from meteorites and 

 comets that had formed in colder parts of the solar system. These 

 impacts would necessarily inject additional volatiles such as water 

 into the atmosphere of an evolving planet. While the visible results of 

 this bombardment seem only superficial, it is likely that even during 

 the early periods of planet formation, there was opportunity for 

 ample mixing of materials from various parts of the solar nebula. 



Thus, if the Earth were much closer to the Sun than its present 

 position, it would be too hot for liquid water to be stable on its sur- 

 face. And without water, life as we know it cannot survive, and 

 probably cannot even originate. 



What if Earth were farther from the Sun? This situation is more 

 complex. A change in the composition of our planet's atmosphere 

 could lead to an enhanced greenhouse effect, resulting in tempera- 

 tures above the freezing point of water, even at the distance of Mars. 

 Indeed, model calculations show that if Mars itself had an atmo- 

 sphere in which the partial pressure of C0 2 was equal to the total 

 pressure of our own atmosphere, the mean surface temperature on 

 Mars would be above 0° C. This result has been used to explain the 

 presence of sinuous channels and other landforms on Mars that seem 

 to provide evidence for the action of running water on the planet's 

 surface at some time in the distant past - perhaps 3.5 b.y. ago. 



If Mars had a sufficiently dense C0 2 atmosphere during its early 

 history, it would have been warm enough to allow liquid water to 

 exist, and that water could have cut the channels we observe today. 

 Thus, it seems possible that Mars may have had an early history 

 much like that of the Earth. Yet we do not see evidence that there 

 were ever lakes or oceans on Mars. It looks more as though the water 

 came out in periodic floods, but never accumulated in large basins. 

 Perhaps conditions were simply not that stable. Was the atmosphere 

 never dense enough to do more than raise the temperature closer to 

 the freezing point than we find it today? 



