152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



no residual heat of their own. There is a balance between the heat 

 they receive from the Sim and the heat they radiate back into space. 

 The farther from the Sun a planet is, the less is the heat that falls on 

 unit area of its surface; the lower, therefore, must be the average 

 surface temperature. These three planets are all sufficiently massive 

 to have retained their hydrogen ; marsh gas is a prominent constituent 

 of all their atmospheres, but ammonia becomes less prominent as, with 

 an increasing degree of cold, it is frozen out of the atmosphere. The 

 temperature of Neptune is so low that nitrogen could only exist on it 

 in the solid state. We need not look for life on these distant frozen 

 wastes. 



Mercury. — Returning to the nearer planets, it will be noticed that 

 the velocity of escape from Mercury is not much higher than that from 

 the Moon. Mercury always turns the same face to the Sun, so that 

 one hemisphere has perpetual day and the other has ])erpetual night. 

 The sunlit face is intensely hot; where the Sun is overhead it is as 

 hot as a bath of molten zinc. This high temperature would facilitate 

 the escape of any atmosphere, because the higher the temperature the 

 faster the molecules move. We must conclude that Mercury, in com- 

 mon with the Moon, is devoid of any atmosphere and is a dead, arid 

 world. The surface of the planet shows nothing more than extremely 

 faint markings, only observable with great difficulty; it is probably 

 a uniform plain, with no very distinctive features. 



Venus. — Venus and Mars have special interest and require more 

 detailed consideration. Venus is the planet most nearly equal to the 

 Earth, both in size and in weight. The velocity of escape from Venus 

 being nearly the same as from the Earth, Ave expect to find an ex- 

 tensive atmosphere on Venus, though the hydrogen will have escaped. 

 The telescope confirms this, for we find Venus to be covered with a 

 dense permanent layer of clouds, which entirely obscures her surface. 

 Faint, ill-defined markings are sometimes seen, but they have no 

 permanence. 



There has been a great development in recent years in the use of 

 photographic plates sensitive to the infrared light, the light of long 

 wave length to which the eye is not sensitive, for the photography of 

 distant landscapes, because the infrared light is able to penetrate 

 haze or fog much more easily than the light of short wave length, the 

 blue, violet and ultraviolet light to which ordinary photographic plates 

 are sensitive. Venus has been photographed using these infrared-sensi- 

 tive, or haze-cutting plates in the hope that something of her surface 

 might be revealed. But such plates show no more than our eyes can 

 see. The cloud layer is too thick. 



The temperature of Venus has been measured. The temperature 

 of the sunlit face reaches 80° or 90° F., while that of the dark face falls 



