230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



generally with a gleaming white spot at the top and sometimes at the 

 bottom as well, while on the otherwise red background are grayish 

 patches. If the telescope is a very large one, we may also see two 

 specks of light, like small stars, moving rapidly around the planet; 

 these are its two moons or satellites. 



But what makes Mars the most interesting of all the planets is not 

 its appearance as seen at any time with a telescope, but the changes 

 which often take place there. The white spots at the top and the 

 bottom mark the north and south poles of Mars. We know that our 

 polar regions are distinguished for their low temperatures and de- 

 posits of ice and snow. Seen from space, from the moon, or from 

 Mars, the earth would have gleaming white spots at the top and at 

 the bottom, just as we see on Mars. It is generally believed that these 

 white patches at the poles of Mars are deposits of ice and snow, but 

 they also partly consist of high cirrus clouds. We know this because 

 in photographs of Mars taken in infrared light, which penetrates to 

 the surface, these patches are smaller than as seen in the telescope or 

 on photographs taken in ultraviolet light, which show only the higher- 

 level features. 



Confirmation of the belief that Mars has true polar caps is given 

 by the fact that they are largest in the winter time on Mars, begin 

 to dwindle as spring comes on, and are smallest in summer. The cap 

 at the south pole has even been known to disappear altogether, which 

 never happens with the northern one. This is easily understood be- 

 cause when it is summer in the southern hemisphere Mars is nearest 

 to the sun, and therefore receives the maximum amount of heat. It is, 

 however, strange that while the pole is at the center of the northern 

 cap, the south pole is 180 miles away from the center of the southern 

 cap. 



The grayish patches on the disk are permanent features, and for 

 a long time were believed to be seas, because water reflects the sunlight 

 less strongly than the land. It is now known that these patches are 

 not seas, although they may once have been so. They are faint in 

 the winter time, but darken as spring gives way to summer, that is to 

 say, as the polar caps melt and the ice and snow are converted into 

 water. This suggests that water has something to do with the darken- 

 ing, and the consensus is that the dark areas are tracts of vegetation. 

 What this is, whether the vegetation consists of plants, shrubs, or 

 trees, we cannot say ; we only know that it is vegetation of some sort. 



These grayish patches are more numerous in the southern than in 

 the northern hemisphere, and encircle the south pole. Many of them 

 run into or join up with each other, but there are isolated patches. 



The remaining parts of the disk are uniform yellowish-red and are 

 almost certainly sandy deserts. Indeed Mars appears to bjj a world 



