THE ATMOSPHERES OF THE PLANETS^ 



By Henry Nokkis Russell 

 Research Professor of Astronomy, Princeton University 



Two ways are open to the retiring president of this association 

 when he makes what small return he can for the honor of his 

 election. By a sound and time-honored custom, it is his duty and 

 privilege to speak of some topic, within his own technical field, 

 but of general interest. He may therefore either report on his own 

 researches — if he is fortunate enough to have recent or unpublished 

 results good enough to measure up to the standard of a presidential 

 address — or he may survey some section of his part of the field of 

 science in which important gains have lately been made, though his 

 own contribution to this advance may be small. Only the latter 

 course is open to the present speaker, and so, this evening, we may 

 devote a little time to the atmospheres of the planets. 



As soon as telescopes became good enough to give a tolerable view 

 of details on the planets, evidence began to accumulate that some 

 of them, at least, possessed atmospheres. Doubtless the first to be 

 noticed were the changes in the markings on Jupiter, which differ 

 radically from one year to the next, and often appear siiddenly 

 and last but a few Aveeks, though thousands of miles in diameter. 

 Only clouds forming and dissolving in a Jovian atmosphere can 

 account for such rapid and capricious changes. 



Evidence for an atmosphere on Mars is afforded by the polar 

 caps. The steady shrinkage of these during the summer, accom- 

 panied by the growth of the opposite cap during the long, cold polar 

 night, is explicable only by the melting or evaporation of deposits 

 of some snowlike substance, which is carried as invisible vapor to the 

 opposite pole, and there deposited. A permanent, noncondensible 

 atmosphere is required for the transport of this vapor. 



Venus, when she is considerably nearer to the earth than to the 

 sun, shows a crescent phase, like that of the moon, and for the 

 same reason. As she comes more nearly into line between us and 



* Address of the retiring president of the American Association for the Advancement of 

 Science, Pittsburgh, Dec. 31, 1934. Reprinted by permi.ssion, with some additions, from 

 Science, vol. 81, no. 2088, Jan. 4, 1935. 



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