THE ATMOSPHERES OF THE PLANETS 559 



almost exactly six times the mean molecular velocity of hydrogen at o° c. 

 Hence the atmosphere of the Earth should be immune from loss of 

 hydrogen and all other gases. 



At the present time the amounts of hydrogen and of helium in the 

 Earth's atmosphere are very small. The spectrum of the aurora does not 

 contain the lines of helium, an indication that the high regions of the 

 atmosphere cannot contain very much helium. The total helium content 

 of the atmosphere has been estimated to be about five parts in a million. 

 The supply is being gradually replenished by the weathering of the 

 igneous rocks of the earth's crust, which contain uranium and thorium 

 and also, consequently, helium. Yet the atmosphere does not now 

 contain more than a fraction of the amount of helium that it has gained in 

 geological times in the process of the formation of sedimentary rocks as 

 a result of the weathering of the igneous" rocks. We may therefore say 

 that there is direct observational evidence that helium is being lost from 

 the atmosphere at the present time. It is believed that there may be a 

 state of equilibrium between the rate of supply and the rate of loss. 



Even if the Earth had remained hot, in the early stages of its existence, 

 for a sufficient time for the hydrogen and helium then present in its 

 atmosphere to escape entirely, it still remains to explain how helium 

 continues to be lost when, according to the theoretical results, which are 

 based on the accepted principles of the kinetic theory of gases, it should 

 be immune from loss. There is one process by which the escape of 

 helium can be brought about. It is well known that the night sky is 

 faintly luminous. In addition to the light from the stars there is a faint 

 luminescence from the upper atmosphere, whose brightness seems to 

 vary with the sun-spot cycle, being greater at sun-spot maximum than at 

 sun-spot minimum. Lord Rayleigh has termed this the non-polar 

 aurora. In the spectrum of the night sky the characteristic green auroral 

 line, as well as the two red lines, are always present. These lines are 

 emitted by oxygen atoms that are in what the physicists term a metastable 

 state. An atom, when excited or loaded up with energy, usually unloads 

 its energy, with the emission of radiation, within a short interval of time 

 of the order of one hundred-millionth of a second. But a metastable 

 state is characterised by the peculiarity that the atoms in that state have a 

 very slight tendency to unload their energy. They may remain for an 

 average time of a second or longer in that state before emitting their 

 energy in the form of radiation. There is a high probability that before 

 this occurs the atom will have collided with another atom. When a 

 collision of a metastable oxygen atom with another atom occurs, the energy 

 of the oxygen atom will be unloaded and converted into kinetic energy. 

 The two atoms will rebound with a greatly increased speed. By such a 

 collision an atom of helium could acquire a speed of more than 12 km. /sec, 

 which is greater than the velocity of escape from the Earth. Hydrogen 

 atoms would acquire a still higher speed, but heavier atoms, such as those 

 of nitrogen or oxygen, would not by this process acquire sufficient speed 

 to escape. They would receive an equal amount of energy but, being 

 heavier, they would not move so fast. The loss of hydrogen and helium 

 from the atmosphere of the Earth is thus made possible by the fact that 

 free oxygen is present in the atmosphere. 



