LIFE IN SPACE AND TIME 1 7 



4 and 20 per cent of that under which we find ourselves. 

 The atmosphere is clear, and usually permits a good view 

 of the planet's surface, especially if the observations are 

 made with yellow or red hght. Clouds, though occasionally 

 observed, are very rare, and permanent surface markings 

 are numerous. With the aid of these, the rotation period 

 has been accurately determined as 24*" 37"" 22^ 58, only 

 a little longer than the Earth's, while the equatoi is inchned 

 to the orbit by 25°, which leads to a sequence of seasons 

 very Hke our own. Noteworthy seasonal changes have been 

 known ever since the planet was first observed telescopically. 

 The most conspicuous of these is the alternate waxing 

 and waning of the white polar caps, which shrink in summer 

 and form again during the winter night, exactly as snow- 

 caps might be expected to do. Unhke the Earth's polar 

 snows, however, they become very small in late summer, 

 the northern cap diminishing to 200 miles in diameter, and 

 the southern (which has its summer when Mars is nearest the 

 Sun) sometimes vanishing altogether. That these caps 

 are formed by some substance which, Hke snow, is pre- 

 cipitated from the atmosphere in cold weather, has never 

 been doubted; and there is no longer any question that 

 they are actually composed of frozen water. 



Radiometric measures of temperature, dealing not only 

 with the planet as a whole, but with separate regions of 

 the disk, were made extensively during the favorable 

 oppositions of 1924 and 1926. The results obtained inde- 

 pendently at Flagstaff and Mt. Wilson show that the 

 temperature rises at noon in the tropics to 10° or i5°c. 

 (50° to 70°F.), and gets almost as high at the poles in the 

 latter part of the long summer day (which endures for 

 almost a year of our time). At night, even at the equator, 

 it falls below the freezing point, and the polar nights must 

 be cold indeed. 



This is exactly the right temperature range for the 

 appearance and disappearance of snow or frost; and the 

 conclusion that water exists on the planet's surface is put 

 beyond doubt by spectroscopic observations, which show 

 that there is water-vapor in the atmosphere, though only 

 to about 5 per cent of the amount present in the Earth's. 



