150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



In every cubic inch of the air that we breathe there are some 500 

 million billion minute particles called molecules, flying about rapidly 

 in all directions and continually colliding with each other. The lighter 

 the molecule the higher is its average speed. The average speed of a 

 molecule of hydrogen is I14 miles a second, whereas the average speed 

 of a molecule of oxygen or nitrogen is not much greater than one- 

 fourth mile a second. Consider what is happening at the top of the 

 atmosphere; there will frequently be molecules that rebound after 

 a collision with a speed several times as great as the average speed. 

 Suppose hydrogen molecules are present : the speed of some of these 

 might well exceed 7 miles a second, the velocity of escape from the 

 Earth. If they happened to rebound in a direction away from the 

 Earth, they will escape from the Earth's gravitational pull, unless 

 they should happen to collide with some other molecule. A slow 

 escape of hydrogen from the outer layers of the Earth's atmosphere 

 would therefore be expected. The atmosphere of the Earth, in fact, 

 contains little or no hydrogen, the reason being that it has gradually 

 slipped away into space. For nitrogen and oxygen, on the other 

 hand, the velocity of escape is more than 20 times greater than the 

 average speed of the molecules. Velocities so much greater than the 

 average will rarely be reached, so that oxygen and nitrogen are held 

 prisoners by the invisible but powerful bonds of the Earth's 

 gravitation. 



THE ATMOSPHERE OF THE MOON 



Now contrast the Moon and Jupiter with the Earth. It is rather 

 more difficult for the Moon to hold an atmosphere of oxygen and 

 nitrogen than it is for the Earth to hold an atmosphere of hydrogen. 

 The Earth has existed long enough to lose its hydrogen and so we 

 may anticipate that the Moon has lost not merely hydrogen but 

 also oxygen, nitrogen, and water vapor and that it may be entirely 

 devoid of atmosphere. The velocity of escape from Jupiter, on the 

 other hand, is so high that it is impossible even for hydrogen, the 

 lightest of all substances, to be lost. We may accordingly expect to 

 find a very extensive atmosphere on Jupiter. 



These expectations are fully confirmed by observation. The start- 

 ling suddenness with which a star disappears when the Moon passes 

 in front of it, or occults it, proves that there can be no atmosphere. 

 If there were any atmosphere, the light from the star would be re- 

 fracted through it and the star would disappear gradually. The Moon 

 is the nearest to us of the heavenly bodies and its surface can be studied 

 in considerable detail. We see mountain ranges; many ring-shaped 

 mountains, both small and large ; great plains ; and narrow cracks or 

 fissures in its surface. But there are no oceans, lakes or rivers ; if these 

 ever existed on the Moon, they have gradually evaporated and the 



