17 



forming a vast atmosphere surrounding the still in- 

 tensely hot globe."* 



STAGES OF CLIMATIC EVOLUTION. 



Various stages of climatic evolution are apparently 

 represented in the conditions of the planets Jupiter, the 

 Earth, and Mars. The former is of great size, and is 

 apparently shrouded in dense clouds, and appears to be 

 in the first era of climatic evolution. The cloud sphere 

 presents a surface of high albedo, and the heat from it 

 is of about that intensity which we should receive by 

 the reflection of solar energy. f 



The Earth, a much smaller mass, has passed through 

 the first era of climatic evolution, and has reached the 

 earlier epoch of the second era; during this epoch glacial 

 conditions are being removed and surface temperatures 

 are slowly rising. Since the rays of the violet end of 

 the spectrum are most easily trapped, the Earth must 

 reflect light in which red rays predominate, and it has 

 a low albedo. 



Mars, smaller and more distant from the sun than the 

 Earth, has apparently reached a still further stage of cli- 

 matic evolution. Polar snow caps form in winter and 

 melt off in summer,! thus indicating milder polar condi- 

 tions than prevail within our polar circles. The albedo 

 of Mars is low, and solar energy reflected from its sur- 

 face is deficient in violet rays, thus showing that the at- 

 mosphere of Mars, like that of the Earth, most readily 

 traps the rays of the violet end of the spectrum. Thus 

 Mars is apparently in a condition towards which the 



*Textbook of Geology, p. 33 (London, 1882). 

 tYoung's General Astronomy. 



|Dr. E. E. Barnard, Popular Astronomy, No. 20, 1895. 

 See also Young's General Astronomy, p. 337. 



