THE MOON, PLANETS, AND COMETS 255 



to the fact that Mars is farther from the sun and receives per 

 unit area only 44 per cent as much light and heat from the 

 sun as are received by the earth. 



The general surface of Mars is described as a dull brick 

 red. There are on it, however, large irregular areas (fig. 127) 

 that have a brownish and sometimes a greenish cast. There 

 is certainly no large body of water upon the planet, and con- 

 sequently the climatic conditions must be very different from 



A series of photographs by which the rotation of Mars is shown. Only a few 



seconds intervened between the three photographs of each vertical row. An hour 



and twenty-two minutes intervened between the upper left-hand and the lower 



right-hand photograph. Photographs by the Yerkes Observatory 



those of the earth. It is extremely probable that there 

 is some water on Mars, for during a winter of one of its 

 hemispheres there appears around its pole -a white mantle 

 or cap, often covering an area approximately as large as the 

 United States. This white mantle generally appears rather 

 quickly ; that is, in the course of two or three days. It 

 remains during tne long winter of Mars, and as spring 

 approaches and " the sun mounts daily higher in its sky," 



