148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



structures of living cells would inevitably be broken up at these high 

 temperatures, so that no life could exist. Our search for the possi- 

 bility of the existence of life must therefore be directed to the cooler 

 bodies — the planets and satellites, associated as in our solar system 

 with a parent sun. But there we come up against the difficulty that 

 the stars are so distant that any planetary systems they may possess 

 are far beyond the reach of our most powerful telescopes. We cannot 

 hope to see them ; still less can we hope to learn anything about their 

 temperatures and atmospheres. All that is possible is to study in 

 detail the planets in the solar system and then to consider what gen- 

 eral conclusions may be drawn from this information, supplemented 

 by general considerations. 



ATMOSPHERES OP THE PLANETS 



The quest is not altogether hopeless, for we can measure the tem- 

 peratures of the planets by means of sensitive instruments, such as 

 the bolometer, and we can learn something about the constitution of 

 their atmospheres by spectroscopic examination. The planets are not 

 self-luminous : we see a planet by means of light from the Sun that 

 has fallen upon the planet and has been reflected or scattered back. 

 Suppose a planet has an atmosphere: the light by which we see it 

 started from the Sun, penetrated to a greater or less extent into the 

 planetary atmosphere, reaching possibly, though not necessarily, the 

 surface of the planet, and then came out again. This light will 

 differ from the light that we receive directly from the Sun, because 

 of the absorption of certain wave lengths in the atmosphere of the 

 planet. By comparing the spectrum of sunlight with the spectrum 

 of the planet's light, we may hope to identify the absorption produced 

 by the planet's atmosphere and to be able to assign its origin. But a 

 complicating factor is produced by the atmosphere of the Earth, 

 through which we view both the Sun and the planet. Absorptions 

 occur also in this atmosphere, produced by water vapor, oxygen, ozone, 

 carbon dioxide, and other substances. We can identify in two differ- 

 ent ways the absorptions in the spectrum of the Sun that originate 

 in the Earth's atmosphere. If we compare the spectra of the east 

 and west limbs of the Sun with that of the center of the disk, absorp- 

 tion lines originating on the Sun will be slightly displaced in the 

 spectra of the limbs, and in opposite directions for the two limbs, 

 with respect to the corresponding lines in the spectrum of the center 

 of the disk; this is merely an effect due to the rotation of the Sun. 

 Lines originating in the Earth's atmosphere will occupy the same 

 position in all three spectra. The second method is to compare the 

 spectra of the Sun photographed at different altitudes. The lower 

 the altitude, the greater the length of the path of the light through 



