QUEST FOR LIFE BEYOND THE EARTH — SAGAN 303 



weed pollen. It is not known, however, whether all the inclusions can 

 be similarly explained away. There is no evidence for viable micro- 

 organisms in meteorites. 



At first sight, the Jovian planets (Jupiter, Saturn, Uranus, and 

 Neptune) seem far too hostile to support life. Their measured 

 temperatures range to several hundred degrees below zero Fahrenheit, 

 and their atmospheres are mixtures of methane, ammonia, and other 

 ordinarily poisonous gases. The low temperatures, however, refer to 

 the top of the visible cloud layers on these planets ; as on Earth, the 

 temperatures should be much higher below the clouds. Furthermore, 

 rather than being unambiguously poisonous, the atmospheres of the 

 Jovian planets are similar to the primitive atmosphere of the Earth 

 in which living organisms first arose. Even today, there are many 

 microorganisms which do well in hydrogen-rich, anaerobic environ- 

 ments. It has recently been shown that water condensation is to be 

 expected at moderate temperatures below the visible cloud layers. 

 Organic molecules are likely being synthesized today, by ultraviolet 

 light and electrical discharges, in the Jovian atmospheres. The 

 amounts of organic material probably produced there over the last 5 

 billion years are enormous. The Jovian planets may eventually prove 

 to be immense and invaluable planetary laboratories on the origins of 

 life. 



The most Earthlike of the other planets in our solar system is Mars. 

 There is a detectable atmosphere, composed mainly of nitrogen and 

 carbon dioxide and smaller amounts of water vapor. The polar ice 

 caps wax and wane with the seasons, so that the amount of water 

 vapor in the atmosphere varies with time and place. The highest 

 temperatures measured on Mars are about 80° F. ; but every night, the 

 temperature falls 150° or so, and the average over the entire planet is 

 about 40° below zero. To round things out, there is no detectable 

 oxygen and ozone, and solar ultraviolet radiation harmful to terrestrial 

 organisms may reach the surface. 



Tentative identifications have been made of very small amounts of 

 nitrogen dioxide (NO2) on Mars. Since large amounts of NO2 are 

 injurious to many familiar organisms, a few scientists have concluded 

 that life on Mars is impossible. It is of interest to note that the 

 amount of NO2 in the air of smog-fiJled Los Angeles often exceeds 

 the amount on Mars. Life in Los Angeles may be difficult, but it is 

 not yet impossible. 



Freezing kills in two ways : it produces ice crystals which disrupt 

 cellular structure, and it makes liquid water unavailable, an effect 

 especially deleterious in microorganisms. Food technologists have 

 long known, however, that microorganisms can survive freeze-thaw 

 cycles comparable to those on Mars. Eecently, a number of labora- 

 tories have tested the survival and growth of terrestrial microorga- 



7ee-746— 65 21 



