QUEST FOR LIFE BEYOND THE EARTH — SAGAN 305 



If the hypothetical Martians had radio reception equipment, and chose 

 to scan Earth in narrow wave bands, tliey would certainly be re- 

 warded — if that is the word — by television transmission from Earth. 

 There would be an intensity maximum when the North American 

 continent faced Mars, and it would perhaps be possible to determine 

 that this radio frequency emission was not entirely random noise. 

 But barring such observations, the problem of life on Earth would 

 remain an open question. 



What evidence, then, do we have for life on Mars ? The green color- 

 ation and rectilinear markings on Mars were once interpreted, respec- 

 tively, as vegetation and the artificial w^aterways of intelligent beings. 

 It is now known that the dominant color of the dark areas of Mars 

 is gray, not green, and that the so-called "canals" resolve, under the 

 best seeing conditions, into disconnected fine detail. 



There are, however, more reliable observations which may be indica- 

 tive of life on Mars. As the polar ice caps recede each year, releasing 

 water vapor into the Martian atmosphere, a wave of darkening pro- 

 ceeds from the polar regions toward the dark areas near the equator. 

 The edges of the dark areas sharpen and delicate pastels of brown, 

 green, and blue appear. There is no doubt about the darkening, but 

 some dispute exists about the reality of the color changes. The bio- 

 logical interpretation of these phenomena is this: the Martian dark 

 areas are covered wdth organisms, perhaps plants, whose metabolism 

 is sensitive to the availability of water. During most of the year they 

 are in a dormant state. As the wave front of water from the vaporiz- 

 ing polar cap arrives, the organisms grow rapidly and proliferate. 

 The changes in darkness and color of the dark areas can be attributed 

 to these metabolic activities. As the water vapor wave front passes, 

 the organisms once again fall into dormancy. 



It has also been proposed that the Martian dark areas are covered 

 with crystals which change their color and darkness when the avail- 

 ability of water increases. The polarimetric evidence, however, shows 

 that the dark areas cannot contain large amounts of such crystals. 



Analysis of the polarization of light reflected from Mars indicates 

 that the dark areas are covered with fine dark grains which change 

 in size and darkness with the seasons. These particles could be organ- 

 isms which grow to maximum size and proliferate as the wave front 

 of water arrives. But it may also be possible that the polarization 

 changes can be explained by a redistribution of sizes of inorganic 

 grains. Perhaps winds which accompany the w^ater vapor front dis- 

 turb the surface dust, which in the absence of winds has settled with 

 the very large and very small particles deepest. 



When the Martian dark areas are observed with an infrared spec- 

 trometer, three features are observed which are possibly produced 

 by the absorption of infrared radiation by organic molecules. The 



