lowell. — temperature of mars. 659 



Heat received by Earth and Mars. 

 We will now apply the argument from the albedo. 



Heat received at the Surfaces of Mars and the Earth. 



Per cent of Per cent of Heat received 



Whole Energy. to Whole Energy. 



Mars. Earth. 



Visual spectrum ... 32 73 23 



Infra-red G5 60 50 



Total 64 41.5 



The ultra-violet rays slightly increase the depletion by selective dis- 

 persion for both planets, and probably the more for Mars. 



Insolation. 



But this is not all. The above deduction applies only to such sky as 

 is clear. Now the earth is cloud-covered to the extent of 50 per cent of 

 its surface on the average ; Mars, except for about six Martian weeks, 

 at the time of the melting of the polar cap and over an area extending 

 some fifteen degrees from the pole, stands perpetually unveiled. The 

 surface thus fog-enveloped is 0.034 of its hemisphere, and the time 0.23 

 per cent of the half year, whence the total ratio of cloud to clear the 

 whole year through over the whole surface is less than 1 per cent. 



The albedo of cloud being .72, its transmission, including absorption 

 re-given out, cannot exceed .28, and may be taken as 20.^ Conse- 

 quently the effective heat received on this score by the earth is about 

 as 20 X 50 = 60 per cent, and for Mars 99 per cent, giving the ratio 

 that of .60 to .99. 



Taking now Stefan's law that the radiation of a body is as the fourth 

 power of its temperature, and remembering that, since the two planets 

 maintain their respective mean annual temperatures, they must ra- 

 diate as much heat as they receive, we have the following equation 

 from which to find the mean annual temperature of Mars, a:, in which 

 459.4° + 60° or 519.4°F. on the absolute scale denotes the mean an- 

 nual temperature of the earth : 



a; : 519.4°: : a/i' X .64 X .99 : V^l.524' X .415 X .60 

 or a:= 519.4° |?f 



giving X = 531.4° Abs. = 72° F. or 22° C. 



' This agrees with Arrhenius' estimate of the heat transmissibility of cloud. 



