THE ATMOSPHERES OF THE PLANETS 561 



to the atmosphere. It seems probable that the present abundance of 

 oxygen in the atmosphere has been provided in this way and that if the 

 coal, oil and other organic deposits could be unburied and completely 

 burned, the whole of the oxygen in the atmosphere would be used up. 



The atmosphere of Venus is in marked contrast to that of the Earth. 

 Venus is the planet which, of all the planets, most closely resembles 

 the Earth in size, in mass, and in mean density. It is a little smaller 

 than the Earth, a little less massive and has a slightly lower mean density. 

 The velocity of escape from Venus is 10 -2 km. /sec, a little smaller 

 than the corresponding velocity from the Earth. This velocity is about 

 five times the mean molecular velocity of hydrogen, and it may therefore 

 be expected that Venus will have an atmosphere comparable with that 

 of the Earth in extent and density. The presence of an extensive at- 

 mosphere is confirmed by observation. Her disk shows faint ill-defined 

 transient markings, which are evidently cloud phenomena. No surface 

 details are shown, even on photographs with infra-red sensitive plates. 

 Photographs in ultra-violet light record cloudy markings which rapidly 

 change their form and are of short duration. 



The permanent cloud layer over Venus makes the determination of 

 the period of rotation difficult ; the cloud formations are not sufficiently 

 long-lived to give any more definite information than that the rotation 

 is not rapid. The spectroscopic determination of the period is difficult, 

 but the evidence is in favour of a period of not less than 20 or 30 days. 

 On the other hand, it is likely that the period is considerably shorter 

 than 225 days, the period of revolution of Venus round the Sun, because 

 it has been found by measurement that considerable heat is radiated 

 from the dark side of the planet. Although the measurements show 

 that the bright side sends us more heat than the dark side, the difference 

 can be to a large extent explained by the reflection of sunlight from the 

 cloud layer over the bright side. The small difference in temperature 

 between the bright and dark sides is to be expected on a planet that is 

 densely cloud-covered, the clouds acting as a blanket at night, provided 

 that the length of the day is not too great. If the planet turned always 

 the same face to the Sun, the difference in temperature between the 

 bright and dark faces would be greater than is found by observation. 

 Hence a rotation-period of several weeks seems probable. 



The method used for the determination of the temperatures of the 

 planets may be briefly described. The radiation received from the 

 planet, or from a portion of its surface, is measured with a sensitive 

 thermocouple or bolometer. This radiation consists of two portions : 

 reflected sunlight and long wave-length infra-red radiation from the 

 planet. By placing a small transparent vessel containing water in the 

 path of the rays the true planetary radiation of long wave-length is 

 absorbed and the amount of the radiation that is merely reflected sunlight 

 can be determined. Knowing, in this way, the amount of the true 

 planetary radiation, the temperature of the planet may be estimated 

 approximately ; this temperature refers to the radiating surface, and if 

 the planet has much atmosphere the actual surface temperature may be 

 considerably higher. 



The measured mean temperatures of the planets are in close general 



