168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



other stars. It seems certain that an immense number of dark plane- 

 tary bodies may exist as companions to the stars without our being 

 able to detect their existence by the powerful spectroscopic and tele- 

 scopic means at our command. Indeed we can not conceive that any 

 increases of our observational apparatus will ever enable us to de- 

 cide whether or not such bodies exist. The probability is that they 

 do so, and in such immense numbers that among them there may well 

 be many suitable for abodes of intelligent life. This subject, of 

 course, opens the door wide for speculation, but this field lies so far 

 from the realms of certainty that it is not my purpose to enter upon 

 it here. 



ARE MERCURY, VENUS, AND MARS HABITABLE? 



To us the question has narrowed itself to this : Are Mercury, Venus, 

 and Mars fit for habitation? The answer requires us to consider 

 for a moment the most important requisites of life. Animals live 

 on plants. Plants require warmth, light, water, carbon compounds, 

 and certain inorganic salts. But are we justified in supposing that 

 in a climate where water would be changed to ice or steam, life would 

 be impossible ? It is difficult to conceive that water in the rigid form 

 of ice could serve a living being as a prime part of his make-up. 

 Where would be the flexibility of motion required to circulate the 

 food and carry on the functions of the body? It may, indeed, be 

 urged that other liquids might take the place of water. But the 

 properties of water are unique. An almost universal solvent, its 

 solutions possess electrical and chemical properties so far more won- 

 derful than any others that comparison is impossible. 

 ' Aside from water, one must insist on the element carbon as indis- 

 pensable to life. The spectroscope teaches us that all the heavenly 

 bodies are of the same chemical elements. Our earth has samples of 

 all of the star-building materials and we know well their combina- 

 tions. Among all these elements there is none that has the versatility 

 of carbon. Its compounds are innumerable and of the most bewilder- 

 ing complexity. It only can be the basis of life, which seems to 

 require the most complex of the mysterious intricacies of carbon 

 chain building for its simplest creatures. These complex life sub- 

 stances, however, are broken down by temperatures like that of 

 steam, and are mostly rigid at temperatures like that of ice. Within 

 this temperature range, from freezing to boiling, we must believe 

 lies all the theater of animal and vegetable life. 



Light, too, is necessary, but its requirements are more elastic. 

 Plants grow and animals thrive where the light is a thousandfold 

 less than daylight, and the full sun is far from being too strong for 

 most of them. All three of our planets would satisfy the require- 



