LIFE IN OTHER WORLDS — JONES 151 



water vapor has dissipated into space. If there were a lunar inhabitant 

 equipped with a powerful telescope, he would be able to see many 

 signs of human activity on the Earth. No signs of the activities of 

 lunar inhabitants are to be seen on the Moon ; it is a dead world, with- 

 out any atmosphere and without any water and therefore entirely 

 devoid of life. 



THE PLANETS 



Jupiter. — The appearance of Jupiter is far different. We see mark- 

 ings in the form of bright patches and dark patches, arranged for the 

 most part in belts parallel to the equator. At first sight, these mark- 

 ings might be thought to be surface features. But further examina- 

 tion shows that they are continually changing and that the appear- 

 ance is never twice the same. All that we can see on Jupiter are cloud 

 formations ; we can never penetrate to the solid surface of the planet. 

 Analysis by the spectroscope of the light from Jupiter reveals an at- 

 mosphere very different from our own. We can find no trace of oxy- 

 gen, or of carbon dioxide, or of water vapor. The absence of water 

 vapor is not to be wondered at, for when we measure the temperature 

 of Jupiter we find it to be about —200° F., so that any moisture would 

 be precipitated as ice or snow. The prominent features of the spectrum 

 of Jupiter are strongly marked absorptions characteristic of ammonia 

 md of marsh gas, the poisonous gas that the miner knows as fire damp. 



Strange though an atmosphere containing pungent ammonia and 

 poisonous marsh gas may seem, it is precisely the type of atmosphere 

 that might have been predicted. Jupiter, in common with the other 

 planets, was formed from matter ejected by the Sun. The Sun con- 

 sists largely of hydrogen, and Jupiter, when it was first formed, must 

 also have contained a large proportion of hydrogen. The great gravi- 

 tational pull of Jupiter prevented this hydrogen from escaping. As 

 Jupiter cooled, the oxygen in its atmosphere combined with some of 

 the hydrogen to form water vapor and this, with further cooling, was 

 deposited on the surface as a thick layer of ice. Nitrogen and carbon 

 also combined with some of the excess hydrogen to form saturated 

 compounds. The most volatile compounds of hydrogen with nitrogen 

 and carbon are ammonia and marsh gas, respectively, and these are 

 the gases that we should expect to find in the atmosphere of Jupiter. 

 We infer also, though we cannot test it by observation, that there must 

 be a great deal of hydrogen in the atmosphere. A poisonous atmos- 

 phere, containing neither oxygen, carbon dioxide, nor moisture, com- 

 bined with an extremely low temperature, must make any form of 

 life quite out of the question. 



The other large planets, Saturn, Uranus, and Neptune, can be 

 briefly disposed of. Being more distant from the Sun than Jupiter, 

 their temperatures must be appreciably lower. For the planets have 



