LIFE ON OTHER WORLDS — JEANS 147 



of Jupiter, where it seems beyond the bounds of possibility that canals 

 could have been constructed, as well as on Venus, on which real canals 

 could not possibly be seen since its solid surface is entirely hidden 

 under clouds. It may be significant that E. E. Barnard, perhaps the 

 most skilled observer that astronomy has ever Imown, was never 

 able to see the canals at all, although he studied Mars for years through 

 the largest telescopes. 



A more promising line of approach to our problem is to examin( 

 which, if any, of the planets is physically suitable for life. But we 

 are at once confronted with the difficulty that we do not know what 

 precise conditions are necessary for life. A human being transferred 

 to the surface of any one of the planets or of their satellites, would 

 die at once, and this for several different reasons on each. On Jupiter 

 he would be simultaneously frozen, asphyxiated, and poisoned, as 

 well as doubly pressed to death by his own weight and by an atmos- 

 pheric pressure of about a million terrestrial atmospheres. On Mer- 

 cury he would be burned to death by the sun's heat, killed by its ultra- 

 violet radiation, asphyxiated from want of oxygen, and desiccated 

 from want of w\ater. But this does not touch the question of whether 

 other planets may not have developed species of life suited to their 

 own physical conditions. When we think of the vast variety of con- 

 ditions under which terrestrial life exists on earth — plankton, soil 

 bacteria, stone bacteria, and the great variety of bacteria which are 

 parasitic on the higher forms of life — it would seem rash to suggest 

 that there are any physical conditions whatever to which life cannot 

 adapt itself. Yet as the physical states of other planets are so dif- 

 ferent from that of our own, it seems safe to say that any life there 

 may be on any of them must be very different from the life on earth. 



The visible surface of Jupiter has a temperature of about — 138° C, 

 which represents about 248 degrees of frost on the Fahrenheit scale. 

 The planet probably comprises an inner core of rock, with a surround- 

 ing layer of ice some 16,000 miles in thickness, and an atmosphere 

 which again is several thousands of miles thick and exerts the pressure 

 of a million terrestrial atmospheres which we have already mentioned. 

 The only known constituents of this atmosphere are the poisonous 

 gases methane and ammonia. It is certainly hard to imagine such 

 a planet providing a home for life of any kind whatever. The planets 

 Saturn, Uranus, Neptune, and Pluto, being farther from the sun, 

 are almost certainly even colder than Jupiter and in all probability 

 suffer from at least equal disabilities as abodes of life. 



Turning sunward from these dismal planets, we come first to Mars, 

 where we find conditions much more like those of our own planet. 

 The average temperature is about —40° C, which is also —40° on the 

 Fahrenheit scale, but the temperature rises above the freezing point 



501591— 4n 11 



