THE ATMOSPHERES OF THE PLANETS 563 



The investigation of the atmosphere of Venus has given no certain 

 evidence of the presence of oxygen. Observations with the 100-inch 

 telescope, in conjunction with the high-dispersion coude spectrograph, 

 have led to the conclusion that the amount of oxygen must be less than 

 one-thousandth part of that above an equal area of the Earth. It must 

 be remembered, however, that the observations refer only to the portion 

 of the atmosphere above the permanent layer of cloud and this layer 

 may be at a considerable height above the surface of Venus. More 

 surprising, perhaps, than the failure to detect oxygen is the failure to 

 detect the presence of water-vapour, even though the tests for water- 

 vapour are less sensitive than those for oxygen. It would seem that the 

 clouds on Venus must be clouds of water droplets, similar to the clouds 

 in the Earth's atmosphere ; the explanation of the apparent absence of 

 water-vapour may be that the atmosphere above the clouds is extremely 

 dry. 



The most interesting fact about the atmosphere of Venus is the great 

 abundance of carbon dioxide. In 1932 Adams and Dunham discovered 

 three well-defined bands in the infra-red region of the spectrum of Venus 

 which are not found in the spectrum of the Sun, even when setting. 

 They were evidently produced by absorption in the atmosphere of Venus. 

 These bands had not at that time been observed in any terrestrial spectrum. 

 Theoretical investigations indicated that they might be due to carbon 

 dioxide ; this was confirmed when Dunham succeeded in obtaining a 

 faint absorption, corresponding with the strongest of the bands, by 

 passing light through 40 metres of carbon dioxide at a pressure of 

 10 atmospheres. Later, Adel and Slipher reproduced the three bands 

 by passing light through 45 metres of carbon dioxide at a pressure of 

 47 atmospheres ; the absorptions so produced were less intense than the 

 corresponding absorptions in the spectrum of Venus. Adel and Slipher 

 concluded that the amount of carbon dioxide above the surface of Venus 

 is equivalent to a layer two miles in thickness at standard atmospheric 

 pressure and temperature. For comparison, it may be mentioned that 

 the whole atmosphere of the Earth is equivalent to a thickness of five 

 miles at standard pressure and temperature and that the amount of 

 carbon dioxide present in the path of sunlight, when the Sun is setting, is 

 equivalent to a thickness of only about thirty feet. Further confirmation 

 is thus obtained of an abundant atmosphere on Venus. 



The carbon dioxide will have a powerful blanketing efll^ect, the escape 

 of the long wave-length radiations being greatly impeded by the absorp- 

 tion by the carbon dioxide. It is not improbable that the temperature 

 at the surface of Venus may be as high as, or higher than, that of boiling 

 water. The high temperature, the lack of oxygen and the abundance of 

 carbon dioxide can be interpreted as indications that there cannot be 

 any great amount of vegetation on Venus and suggest that the planet 

 is not the abode of life. 



Mars occupies a position between Mercury on the one hand and 

 Venus and the Earth on the other, as regards size, mass and velocity of 

 escape. The velocity of escape is 5-0 km. /sec, about one-half of the 

 velocity of escape from Venus. It may be expected that Mars will 

 have a much thinner atmosphere than Venus or the Earth. The presence 



