71 



would then be very useful in helping us to evaluate the composition 

 of the atmosphere on the primitive Earth. 



But we can go further than this. The surface of Mars is also 

 marked by many different examples of fluvial erosion. Once again 

 the timing of the events that led to these landforms is controversial. 

 But it is probably conservative to say that the youngest of the 

 numerous floods took place at least 3.5 b.y. ago. In other words, 

 liquid water was evidently available on some kind of intermittent 

 basis for the first billion years of Martian history. The controversy 

 that still exists centers on the issue of how much later than this there 

 might have been epochs when water flowed on Mars. For our imme- 

 diate purpose, it doesn't matter; 1 b.y. is long enough! 



As already stated, our current view of the development of 

 inner-planet atmospheres suggests that Mars, Earth, and Venus all 

 began with a very similar volatile inventory. Models for the early 

 Martian climate indicate that a dense C0 2 atmosphere could have 

 melted ice by a greenhouse effect. The morphological evidence that 

 liquid water once flowed on Mars seems to substantiate both of these 

 points. The special significance of this picture of primitive Martian 

 history is apparent as soon as we ask what was happening during the 

 first billion years of our own planet's history. There is almost no 

 direct evidence available to answer this question. But the recent 

 discovery of stromatolites dated by three different methods to be 

 3.5 b.y. old indicated that life had originated, evolved, and become 

 firmly established on our planet within the first billion years. If this 

 happened on Earth, why not on Mars? 



There seems no way to exclude this possibility. We might be 

 more comfortable if there were a record of ancient seas and lakes on 

 Mars — proof that the presence of water was more than a series of 

 very transient events. But the evidence for such smooth landforms is 

 much more likely to disappear under the shifting sands of the wind- 

 blown Martian terrain than is the high relief associated with individ- 

 ual stream beds. In fact, there are many examples of craters with 

 diameters greater than 30 km that once contained standing water. 

 These "lakes" existed at the same time as the large fluid channels. 

 Periodic wetting and desiccation and/or freezing could help to con- 

 centrate prebiotic material, as has often been stressed in considera- 

 tions of the origin of life on Earth. We are thus confronted with the 

 arresting possibility that since life originated on Earth within the first 



