65 



In any case, the water present produced its own demise. Dissolv- 

 ing atmospheric C0 2 caused weathering of the rocks that led to the 

 formation of carbonates, thereby reducing the amount of C0 2 in the 

 atmosphere. This negative feedback would ultimately lower the sur- 

 face pressure and the surface temperature. Abetted by other factors, 

 such as the loss of nitrogen by escape, this process led to the point 

 where liquid water could no longer exist. 



But we must now consider our other basic planetary characteris- 

 tic: size. What if Mars were a larger planet? We can return to our 

 hypothetical experiment of moving the Earth to the position of 

 Mars. In that case, more carbon dioxide could have been released 

 initially, and the greater amount of tectonic activity associated with 

 the larger heat engine in the bigger planet would provide a much 

 more efficient means of recycling that gas. Perhaps a planet the size 

 of Earth or slightly larger would be able to maintain a reasonably 

 warm climate at the Mars distance from the Sun, provided that it 

 could maintain a sufficiently large amount of carbon dioxide in its 

 atmosphere. This in turn might heat up the tropics of a planet still 

 largely frozen. Weathering would proceed very slowly, since much of 

 the crust would be protected by ice. Whether life could originate and 

 persist on such a planet is a matter of speculation. But these consid- 

 erations do show that an Earth-like planet could exist at a greater 

 range of distances from its central star than we would have con- 

 cluded had we required that the planet have an atmospheric history 

 identical with our own. 



Still farther from the Sun, we must consider other liquids and, 

 hence, other kinds of life. Ammonia is often suggested as an alterna- 

 tive to water as a medium for alien types of biology. We should 

 expect to find such environments on the surfaces of satellites of the 

 outer planets, since the planets themselves do not have solid surfaces 

 that would allow ammonia (or any other liquid) to collect. 



The best example of an object that might meet these criteria is 

 Titan, the largest satellite of Saturn. This is the only satellite in our 

 solar system known to have a substantial atmosphere; this atmo- 

 sphere contains both nitrogen and methane. The problem with Titan 

 is that it is too cold to be very interesting. The surface temperature 

 has been shown to be below -175° C by both spacecraft and Earth- 

 based measurements. If there is a liquid on Titan's surface, it is liquid 

 methane or liquid nitrogen, not ammonia; ammonia would be solid. 



