TEMPERATURE OF MARS. 



A DETERMINATION OF THE SOLAR HEAT RECEIVED. 

 By Pebcival Lowell. 



Presented December 12, 1900. Received January 15, 1907. 



Heat hitherto deduced from Distance only. 



Up to the present time the chief obstacle to crediting Mars with the 

 possibility of life has lain in accounting for sufficient heat on the surface 

 of the planet. So far the determination of this heat has been limited to 

 a consideration of distance from the sun. Thus Professor Young, who 

 feels the difficulty acutely, says in his " General Astronomy " : ^ " We 

 know that on account of the planet's distance from the Sun the intensity 

 of solar radiation upon its surface must be less than here in the ratio of 

 1^ to 1524'^." For the resulting temperature he seems to have assumed 

 either with Newton, that a body radiates heat in direct proportion to 

 its temperature, which would give for the mean temperature of Mars 

 223.6° Abs. (— 236°F.), or Dulong and Petit'slaw, which would make it 

 363° Abs., or —96° F. ; for he entertains the possibility that the polar 

 caps may be composed of solid carbonic acid, which freezes only at 

 —109° F. 



A better determination has recently been made by Moulton by taking 

 Stefan's law of radiation, that of the fourth power of the temperature. 

 Stefan's formula is not only the best experimentally to-day, but has 

 since its enunciation been deduced from theoretic considerations by 

 both Boltzmann and Galitzine. On this basis the mean temperature 

 comes out —33° F., the reasoning being this ; If a body remains at the 

 same temperature, it must radiate as much heat as it receives. Con- 

 sequently the temperature is as the fourth root of the amount received. 

 Absolute zero is minus 459° F. The mean temperature of the earth is 

 usually taken at 60° F. Therefore, to determine the mean temperature 

 of Mars we have, calling x its temperature on the absolute scale, the 

 following equation : 



X : 518 Abs. : : ^4 : >^9, 



which gives —33° F. for the mean temperature of the planet. 



1 Ed. 1898, p. 3G3. 



