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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



The planets naturally divide into two equal groups whose members 

 differ significantly in density. The four inner planets, Mercurj 7 , 

 Venus, Earth, and Mars are not very unlike in this respect. Of the 

 four outer planets, Jupiter, the most dense, is only 1.3 times as heavy 

 as an equal volume of water. We have hardly any earth, stones, or 

 metals so light as this. It is most probable that these four planets 

 are mainly gaseous. For this reason alone it would be unreasonable 

 to think of them as proper abodes of intelligent life, but we have 

 also to consider the temperature conditions which probably prevail 

 in the outer planets. 



The following table gives the principal data on which tempera- 

 ture estimates (given in the last column) have been based with re- 

 gard to the moon and to all the planets. The estimated temperature 

 of the sun depends upon the distribution of its radiant energy in 

 the spectrum ; that of the earth is well known from numerous ther- 

 mometric measurements. The temperature of the sunlit surface of 

 the full moon, which has been investigated by Langley, Very, and 

 others, appears to be probably between freezing and boiling points 

 of water. The dark side appears to be exceedingly cold. The esti- 

 mates here given of the temperatures of the other planets are based 

 upon the approximately known temperatures of the earth and moon 

 with consideration of the relative reflecting powers and solar dis- 

 tances of the planets as compared with those of the earth and moon. 

 The values given, although rough, are yet sufficiently indicative of 

 the conditions in the other planets. 



From this we see that if the sun is the sole source of heat and 

 light, the supposed inhabitants of Neptune, Uranus, Saturn, and 

 Jupiter would probably be much more unfavorably situated than the 

 Eskimo in their climatic conditions. The mass and volume of the 

 planets is not such as to warrant us in the belief that they can still 

 have, at their immense ages, sources of heat within themselves which 

 can supplement to a useful degree the sun as the supporter of life, 

 unless possibly such might be the case with Jupiter. On the whole, 



