LIFE IN OTHER WORLDS JONES 153 



to about 40° below freezing point. Venus has a long day; its length is 

 not known exactly but it is somewhere about 30 of our days. This 

 explains the great difference between the midday and midnight tem- 

 peratures on Venus. The temperatures on the surface of Venus, below 

 the permanent layer of cloud, are likely to be appreciably higher than 

 the measured temperatures. The temperature conditions on Venus are 

 certainly not such as to make life impossible. 



Wliat about her atmosphere ? No evidence of the presence of either 

 oxygen or water vapor has been found. The tests for oxygen are very 

 sensitive; those for water vapor are less sensitive, but the failure to 

 detect the presence of water vapor is surprising because the clouds on 

 Venus must almost certainly be clouds of water vapor, similar to the 

 clouds in our atmosphere. The explanation is probably that the atmos- 

 phere above the cloud layer is pretty dry. There may be a very humid 

 atmosphere below the clouds. One important fact has been estab- 

 lished — an abundance of carbon dioxide, very greatly in excess of the 

 amount in our own atmosphere, is present in the atmosphere of Venus. 



The scarcity of oxygen, combined with the abundance of carbon 

 dioxide, provides a clue to the conditions prevailing on Venus. On a 

 cooling planet of the size of Venus we should expect to find both 

 water vapor and carbon dioxide evolved from the molten rocky mass 

 as it cooled and solidified. We should not expect to find oxygen, 

 because oxygen is a chemically active element and does not like to 

 exist alone. The surprising thing is not the absence of oxygen in 

 the atmosphere of Venus, but the abundance of it in our own atmos- 

 phere. The oxygen is continually being depleted from our atmosphere 

 by combining with other substances and there must be a source of 

 replenishment. This is undoubtedly provided by the vegetation on 

 the Earth's surface, which extracts carbon dioxide from the air and 

 uses the carbon for building up the plant cells, giving out oxygen. 

 The supply of carbon dioxide is in turn replenished by processes such 

 as combustion, respiration, and the decay of vegetable matter. When 

 life started on the Earth there was probably plenty of carbon dioxide 

 but comparatively little oxygen in the atmosphere. We seem to see in 

 Venus a world where the conditions are somewhat similar to those that 

 the Earth passed through millions of years ago. Any life on Venus 

 can be, at the most, primitive plant life. It is possible that life may be 

 in process of gradual development and that in the millions of years 

 to come, when life on the Earth may be nearing extinction, Venus 

 may be the home of higher and higher types of life. 



Mars. — The last of the planets to be considered is Mars, the most 

 interesting of all the planets. In the telescope it appears as a beautiful 

 orange-colored object, on which misty dark markings can be seen. 

 These markings are permanent, and we see them carried round as the 



