LIFE IN OTHER WORLDS JONES 149 



our atmosphere and the greater, therefore, the absorption produced 

 by the atmosphere. 



The spectrum of the planet may show some absorptions that are not 

 to be found in the spectrum of the Sun, and other absorptions that 

 are present in the spectrum of the Sun but that are known to be of 

 terrestrial origin. The former can be attributed at once to the atmos- 

 phere of the planet; the latter may or may not be produced by the 

 atmosphere of the planet as well as by our own atmosphere. We can 

 endeavor to distinguish between the two possibilities by photograph- 

 ing the spectra of the Sun and the planet when the two bodies are at 

 the same altitude and comparing the intensities of the absorptions. 



If they are stronger in the spectrum of the planet relatively to 

 absorptions of solar origin, there must be absorption in the planetary 

 atmosphere superposed on absorption in our atmosphere. The 

 principle is simple but its application is not always easy. 



VELOCITY OF ESCAPE 



The most detailed information about the constitution of planetary 

 atmospheres can be obtained by observations at an observatory on 

 a mountain or high plateau. Most of the information that we possess 

 has been derived from observations at the Mount Wilson Observatory, 

 and at the Flagstaff Observatory, Arizona. 



We can infer something, however, about the atmospheres of the 

 planets without making any observations at all. A planet can retain 

 an atmosphere only by virtue of its gravitational pull. If we could 

 imagine our globe to be suddenly annihilated, leaving the atmospheric 

 shell undisturbed, the atmosphere would rapidly dissipate into space. 

 It is the gravitational pull of the Earth that prevents this dissipation. 

 A rocket shot off from the Earth will be drawn back by the force of 

 gravitation unless the velocity of projection exceeds 7 miles a second. 

 This critical velocity is called the velocity of escape. For any other 

 planet the velocity of escape will have a different value, the square of 

 the velocity being proportional to the mass of the planet and inversely 

 proportional to the radius. Because, as we shall see, this velocity is 

 a very important criterion, the values for several bodies are tabulated : 



Miles per 

 second 



Moon 1. 5 



Mercury 2. 2 



Venus 6. 5 



Earth 7.0 



Mars 3.1 



Jupiter 38.0 



Saturn 23. 



Uranus 13.0 



Neptune 14.0 



Sun 386.0 



