148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1942 



on summer afternoons in the equatorial regions. The atmosphere 

 contains at most only small amounts of oxygen and carbon dioxide, 

 perhaps none at all, so that there can be no vegetation comparable 

 with that of the earth. The surface, in so far as it can be tested by 

 a study of its powers of reflection and polarization, appears to con- 

 sist of lava and volcanic ash. To us it may not seem a promising 

 or comfortable home for life, but life of some kind or other may be 

 there nevertheless. 



Being at the same average distance from the sun as the earth, the 

 moon has about the same average temperature, but the variations 

 around this average temperature are enormous, the equatorial tem- 

 perature varying roughly from 120° C. to —80° C. The telescope 

 shows high ranges of mountains, apparently volcanic, interspersed 

 with flat plains of volcanic ash. The moon has no atmosphere and 

 consequently no water; it shows no signs of life or change of any 

 kind, unless perhaps for rare falls of rock such as might result from 

 the impact of meteors falling in from outer space. A small town 

 on the moon, perhaps even a large building, ought to be visible in 

 our largest telescopes, but, needless to say, we see nothing of the 

 kind. 



Venus, the planet next to the earth, presents an interesting prob- 

 lem. It is similar to the earth in size but being nearer the sun is 

 somewhat warmer. As it is blanketed in cloud we can only guess as 

 to the nature of its surface. But its atmosphere can be studied and 

 is found to contain little or no oxygen, so that the planet's surface 

 can hardly be covered with vegetation as the surface of the earth is. 

 Indeed, its surface is probably so hot that water would boil away. 

 Yet no trace of water vapor is found in the atmosphere, so that the 

 planet may well be devoid of water. There are reasons for thinking 

 that its shroud of clouds may consist of solid particles, possibly 

 hydrates of formaldehyde. Clearly any life that this planet may 

 harbor must be very different from that of the earth. 



The only planet that remains is Mercury. This always turns the 

 same face to the sun and its temperature ranges from about 420° C. 

 at the center of this face to unimaginable depths of cold in the eternal 

 night of the face which never sees the sun. The planet is too feeble 

 gravitationally to retain much of an atmosphere and its surface, in 

 so far as this can be tested, appears to consist mainly of volcanic ash 

 like the moon and Mars. Once again we have a planet which does 

 not appear promising as an abode of life and any life that there may 

 be must be very different from our own. 



Thus our survey of the solar system forces us to the conclusion 

 that it contains no place other than our earth which is at all suitable 

 for life at all resembling that existing on earth. The other planets 



