Marvels of the Universe 



57 



THE FIRST FROST OF THE SEASON. 



Drawings of Mars made at the Lowell Observatory, Arizona. U.S.A.: that on the left by Professor Lowe]! 

 November 18, and that on the right by Mr. E. C. Slipher. on 1910 November 11. 



1910 



The essential quality of Lfe is the power which the organic body possesses of taking dead 

 matter into itself ; of assimilating it and breaking up its structure, and using it either for its own 

 growth or for the maintenance of its energy. The difference between the life of the plant and of 

 the animal hes in the greater directness with which the plant can lay hold on inorganic matter ; 

 while the animal requires for its sustenance matter that has already undergone organic change. 



Animal hfe, therefore, presupposes plant hfe, and plant life the presence of water, the one great 

 neutral solvent ; and of water as a liquid. Abundance of water, abundance of vegetation, are 

 necessary if the higher, more developed forms of life are to flourish. 



But water exists in the Hquid form only within comparatively narrow limits of temperature. 

 On the earth water freezes at 32°, and boils at 212°, but as the mean temperature of the earth 

 as a whole is about 60°, and for the equatorial regions about 80°, there is a wide margin of safety 

 on either side, and over the greater part of the earth's surface, water is normally in the liquid state. 

 In the two frigid zones the mean temperature is 32° or lower. Here, therefore, water is normally 

 found in the solid state as ice or snow. Life exists, indeed, in these two zones, but it exists because 

 for half the year the temperature is above freezing-point, and because there is a continual influx 

 of forms of life from the warmer zones. 



The first question, therefore, to be asked with regard to the habitability of Mars is " What 

 is the temperature of the planet ? " We know from its position in the solar system that it receives 

 from the sun, surface for surface, only three-sevenths of the light and heat that falls upon the earth. 

 We can judge roughly of the effect of this by noticing that a zone on the earth slightly outside the 

 Arctic Circle will, from the effect of its foreshortened presentation to the sun, receive for the unit 

 of surface about three-sevenths of the light and heat falling on the Equator. But the mean 

 temperature of such a zone will be fully 50° lower than that of the Equator. We may reasonably 

 suppose, therefore, that the mean temperature of Mars is 50° lower than that of the earth, or, in 

 other words, that it is about 10° instead of 60°. But a mean temperature of 10° is 22° below 

 freezing-point ; in other words, water on Mars is normally in the solid condition ; it exists chiefly 

 as frost, snow or ice. 



